Thinking In Java, 2nd Edition
Thinking in Java,
2nd Edition, Release 11
To be published by Prentice-Hall mid-June, 2000
Bruce Eckel, President,
MindView, Inc.
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Thinking
in
Java
Second Edition
Bruce Eckel
President, MindView, Inc.
Comments from readers:
Much better than any other Java book I’ve seen. Make that “by an order of
magnitude”... very complete, with excellent right-to-the-point examples
and intelligent, not dumbed-down, explanations ... In contrast to many
other Java books I found it to be unusually mature, consistent,
intellectually honest, well-written and precise. IMHO, an ideal book for
studying Java. Anatoly Vorobey, Technion University, Haifa,
Israel
One of the absolutely best programming tutorials I’ve seen for any
language. Joakim Ziegler, FIX sysop
Thank you for your wonderful, wonderful book on Java. Dr. Gavin
Pillay, Registrar, King Edward VIII Hospital, South Africa
Thank you again for your awesome book. I was really floundering (being a
non-C programmer), but your book has brought me up to speed as fast as
I could read it. It’s really cool to be able to understand the underlying
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attend your seminar in the not-too-distant future. Randall R. Hawley,
Automation Technician, Eli Lilly & Co.
The best computer book writing I have seen. Tom Holland
This is one of the best books I’ve read about a programming language…
The best book ever written on Java. Ravindra Pai, Oracle
Corporation, SUNOS product line
This is the best book on Java that I have ever found! You have done a
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Great book. Best book on Java I have seen so far. Jeff Sinclair,
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Thank you for Thinking in Java. It’s time someone went beyond mere
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only yours and Patrick Winston’s have found a place in my heart. I’m
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Other books cover the WHAT of Java (describing the syntax and the
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Thinking in Java is the only book I know that explains the WHY of Java;
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Thanks for writing a great book. The more I read it the better I like it. My
students like it, too. Chuck Iverson
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Most of the Java books out there are fine for a start, and most just have
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I wrote to you earlier about my favorable impressions regarding your
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Your examples are clear and easy to understand. You took care of many
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Germany
I’m a great fan of your Thinking in C++ and have recommended it to
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OK, I’ve only read about 40 pages of Thinking in Java, but I’ve already
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Just wanted to say what a “brilliant” piece of work your book is. I’ve been
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Group Leader, Ceedata Systems Pty Ltd, Australia
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I’ve *just* started Thinking in Java. I expect it to be very good because I
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better now. Anand Kumar S., Software Engineer,
Computervision, India
Your book stands out as an excellent general introduction. Peter
Robinson, University of Cambridge Computer Laboratory
It’s by far the best material I have come across to help me learn Java and I
just want you to know how lucky I feel to have found it. THANKS! Chuck
Peterson, Product Leader, Internet Product Line, IVIS
International
The book is great. It’s the third book on Java I’ve started and I’m about
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MTS, Lucent Technologies
Of the six or so Java books I’ve accumulated to date, your Thinking in
Java is by far the best and clearest. Michael Van Waas, Ph.D.,
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Java. Ray Frederick Djajadinata, Student at Trisakti University,
Jakarta
The mere fact that you have made this work free over the Net puts me into
shock. I thought I’d let you know how much I appreciate and respect what
you’re doing. Shane LeBouthillier, Computer Engineering
student, University of Alberta, Canada
I have to tell you how much I look forward to reading your monthly
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Dan Cashmer, B. C. Ziegler & Co.
Just want to congratulate you on a job well done. First I stumbled upon
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Not that I have anything against the concept—it is just that I thought this
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simonsez@smartt.com, Simon Says Consulting, Inc.
I must say that your Thinking in Java is great! That is exactly the kind of
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poor software design using Java. Dirk Duehr, Lexikon Verlag,
Bertelsmann AG, Germany
Thank you for writing two great books (Thinking in C++, Thinking in
Java). You have helped me immensely in my progression to object
oriented programming. Donald Lawson, DCL Enterprises
Thank you for taking the time to write a really helpful book on Java. If
teaching makes you understand something, by now you must be pretty
pleased with yourself. Dominic Turner, GEAC Support
It’s the best Java book I have ever read—and I read some. Jean-Yves
MENGANT, Chief Software Architect NAT-SYSTEM, Paris,
France
Thinking in Java gives the best coverage and explanation. Very easy to
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Choice, Inc., Pittsburgh PA
Your book is great. I have read lots of programming books and your book
still adds insights to programming in my mind. Ningjian Wang,
Information System Engineer, The Vanguard Group
Thinking in Java is an excellent and readable book. I recommend it to all
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University of Otago, Dunedin, New Zealand
You make it possible for the proverbial free lunch to exist, not just a soup
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software and books about it. Jose Suriol, Scylax Corporation
Thanks for the opportunity of watching this book grow into a masterpiece!
IT IS THE BEST book on the subject that I’ve read or browsed. Jeff
Lapchinsky, Programmer, Net Results Technologies
Your book is concise, accessible and a joy to read. Keith Ritchie, Java
Research & Development Team, KL Group Inc.
It truly is the best book I’ve read on Java! Daniel Eng
The best book I have seen on Java! Rich Hoffarth, Senior Architect,
West Group
Thank you for a wonderful book. I’m having a lot of fun going through the
chapters. Fred Trimble, Actium Corporation
You have mastered the art of slowly and successfully making us grasp the
details. You make learning VERY easy and satisfying. Thank you for a
truly wonderful tutorial. Rajesh Rau, Software Consultant
Thinking in Java rocks the free world! Miko O’Sullivan, President,
Idocs Inc.
About Thinking in C++:
Best Book! Winner of the
1995 Software Development Magazine Jolt Award!
“This book is a tremendous achievement. You owe it to yourself to
have a copy on your shelf. The chapter on iostreams is the most
comprehensive and understandable treatment of that subject I’ve seen
to date.”
Al Stevens
Contributing Editor, Doctor Dobbs Journal
“Eckel’s book is the only one to so clearly explain how to rethink
program construction for object orientation. That the book is also an
excellent tutorial on the ins and outs of C++ is an added bonus.”
Andrew Binstock
Editor, Unix Review
“Bruce continues to amaze me with his insight into C++, and Thinking
in C++ is his best collection of ideas yet. If you want clear answers to
difficult questions about C++, buy this outstanding book.”
Gary Entsminger
Author, The Tao of Objects
“Thinking in C++ patiently and methodically explores the issues of
when and how to use inlines, references, operator overloading,
inheritance, and dynamic objects, as well as advanced topics such as
the proper use of templates, exceptions and multiple inheritance. The
entire effort is woven in a fabric that includes Eckel’s own philosophy
of object and program design. A must for every C++ developer’s
bookshelf, Thinking in C++ is the one C++ book you must have if
you’re doing serious development with C++.”
Richard Hale Shaw
Contributing Editor, PC Magazine
Thinking
in
Java
Second Edition
Bruce Eckel
President, MindView, Inc.
Prentice Hall
Upper Saddle River, New Jersey 07458
www.phptr.com
Library of Congress Cataloging-in-Publication Data
Eckel, Bruce.
Thinking in Java / Bruce Eckel.--2nd ed.
p. cm.
ISBN 0-13-027363-5
1. Java (Computer program language) I. Title.
QA76.73.J38E25 2000
005.13'3--dc21 00-037522
CIP
Editorial/Production Supervision: Nicholas Radhuber
Acquisitions Editor: Paul Petralia
Manufacturing Manager: Maura Goldstaub
Marketing Manager: Bryan Gambrel
Cover Design: Daniel Will-Harris
Interior Design: Daniel Will-Harris, www.will-harris.com
© 2000 by Bruce Eckel, President, MindView, Inc.
Published by Prentice Hall PTR
Prentice-Hall, Inc.
Upper Saddle River, NJ 07458
The information in this book is distributed on an “as is” basis, without warranty. While every precaution
has been taken in the preparation of this book, neither the author nor the publisher shall have any liability
to any person or entitle with respect to any liability, loss or damage caused or alleged to be caused directly
or indirectly by instructions contained in this book or by the computer software or hardware products
described herein.
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Java is a registered trademark of Sun Microsystems, Inc. Windows 95 and Windows NT are trademarks of
Microsoft Corporation. All other product names and company names mentioned herein are the property of
their respective owners.
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ISBN 0-13-027363-5
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Dedication
To the person who, even now,
is creating the next great computer language
Overview
Preface
1
Introduction
9
1: Introduction to Objects
29
2: Everything is an Object
101
3: Controlling Program Flow
133
4: Initialization & Cleanup
191
5: Hiding the Implementation
243
6: Reusing Classes
271
7: Polymorphism
311
8: Interfaces & Inner Classes
349
9: Holding Your Objects
407
10: Error Handling with Exceptions
531
11: The Java I/O System
573
12: Run-time Type Identification
659
13: Creating Windows & Applets
689
14: Multiple Threads
825
15: Distributed Computing
903
A: Passing & Returning Objects
1013
B: The Java Native Interface (JNI)
1065
C: Java Programming Guidelines
1077
D: Resources
1091
Index
1099
What’s Inside
Preface 1
implementation.................37
Inheritance: reusing
Preface to the 2nd edition ....4
the interface...................... 38
Java 2 ............................................. 6
Is-a vs. is-like-a relationships ......42
The CD ROM....................... 7
Interchangeable objects
Introduction 9
with polymorphism .......... 44
Prerequisites .......................9
Abstract base classes
Learning Java.................... 10
and interfaces ...............................48
Goals ..................................11
Object landscapes and
Online documentation ...... 12
lifetimes............................ 49
Chapters............................ 13
Collections and iterators .............. 51
Exercises ........................... 19
The singly rooted hierarchy .........53
Multimedia CD ROM ........ 19
Collection libraries and
Source code .......................20
support for easy collection use.....54
Coding standards ......................... 22
The housekeeping dilemma:
Java versions.....................22
who should clean up? ................... 55
Seminars and
Exception handling:
mentoring .........................23
dealing with errors ............57
Errors ................................23
Multithreading ................. 58
Note on the cover design...24
Persistence........................ 60
Acknowledgements ...........25
Java and the Internet ....... 60
Internet contributors ................... 28
What is the Web?......................... 60
1: Introduction
Client-side programming .............63
Server-side programming ............70
to Objects
29
A separate arena:
The progress
applications .................................. 71
of abstraction ....................30
Analysis and design........... 71
An object has
Phase 0: Make a plan....................74
an interface .......................32
Phase 1: What are we making?..... 75
The hidden
Phase 2: How will we build it? .....79
implementation.................35
Phase 3: Build the core.................83
Reusing the
Phase 4: Iterate the use cases.......84
Phase 5: Evolution ....................... 85
Java program................... 115
Plans pay off................................. 87
Name visibility.............................115
Extreme programming .....88
Using other components .............116
Write tests first.............................88
The static keyword .....................117
Pair programming........................ 90
Your first Java program .. 119
Why Java succeeds............ 91
Compiling and running ...............121
Systems are easier to
Comments and embedded
express and understand................91
documentation ................122
Maximal leverage
Comment documentation .......... 123
with libraries ................................ 92
Syntax ......................................... 124
Error handling ............................. 92
Embedded HTML....................... 125
Programming in the large............ 92
@see: referring to
Strategies for transition ....93
other classes................................ 125
Guidelines .................................... 93
Class documentation tags........... 126
Management obstacles ................ 95
Variable documentation tags ..... 127
Java vs. C++? .................... 97
Method documentation tags ...... 127
Summary...........................98
Documentation example ............ 128
2: Everything is
Coding style .....................129
Summary .........................130
an Object
101
Exercises..........................130
You manipulate objects
with references................ 101
3: Controlling
You must create
Program Flow
133
all the objects .................. 103
Using Java operators.......133
Where storage lives.................... 103
Precedence.................................. 134
Special case: primitive types.......105
Assignment................................. 134
Arrays in Java..............................107
Mathematical operators ............. 137
You never need to
Auto increment
destroy an object............. 107
and decrement............................ 139
Scoping....................................... 108
Relational operators ....................141
Scope of objects.......................... 109
Logical operators ........................ 143
Creating new data
Bitwise operators........................ 146
types: class ...................... 110
Shift operators ............................ 147
Fields and methods.....................110
Ternary if-else operator...............151
Methods, arguments,
The comma operator .................. 152
and return values .............112
String operator + ...................... 153
The argument list........................ 114
Common pitfalls
Building a
when using operators ................. 153
Casting operators ........................154
Multidimensional arrays ............236
Java has no “sizeof”.....................158
Summary ........................ 239
Precedence revisited ...................158
Exercises......................... 240
A compendium of operators .......159
5: Hiding the
Execution control............ 170
Implementation 243
true and false...............................170
package:
if-else........................................... 171
the library unit................ 244
Iteration ......................................172
do-while.......................................173
Creating unique
for ................................................173
package names............................247
break and continue .....................175
A custom tool library.................. 251
switch ......................................... 183
Using imports to
Summary......................... 187
change behavior..........................252
Exercises .........................188
Package caveat............................254
Java access specifiers ......255
4: Initialization
“Friendly”....................................255
& Cleanup
191
public: interface access.............256
Guaranteed initialization
private:
with the constructor.........191
you can’t touch that!...................258
Method overloading........ 194
protected: “sort of friendly”.... 260
Distinguishing
Interface and
overloaded methods....................196
implementation...............261
Overloading with primitives .......197
Class access .................... 263
Overloading on
Summary .........................267
return values .............................. 202
Exercises......................... 268
Default constructors .................. 202
6: Reusing Classes
271
The this keyword....................... 203
Cleanup: finalization
Composition syntax......... 271
and garbage collection ...207
Inheritance syntax...........275
Initializing the base class ...........278
What is finalize( ) for?.............208
Combining composition
You must perform cleanup ........209
and inheritance ...............281
The death condition....................214
Guaranteeing
How a garbage
proper cleanup............................283
collector works ............................215
Member initialization ..... 219
Name hiding .............................. 286
Choosing composition
Specifying initialization ..............221
vs. inheritance ................ 288
Constructor initialization........... 223
Array initialization.......... 231
protected ........................ 290
Incremental
development ................... 291
Summary ........................ 346
Upcasting ........................ 291
Exercises......................... 346
Why “upcasting”?....................... 293
8: Interfaces &
The final keyword ..........294
Inner Classes
349
Final data ................................... 294
Interfaces........................ 349
Final methods ............................ 299
Final classes ............................... 301
“Multiple inheritance”
Final caution .............................. 302
in Java.........................................354
Initialization and
Extending an interface
class loading....................304
with inheritance..........................358
Initialization
Grouping constants ....................359
with inheritance .........................304
Initializing fields
Summary.........................306
in interfaces ................................ 361
Exercises .........................307
Nesting interfaces.......................362
Inner classes................... 365
7: Polymorphism
311
Inner classes and upcasting ...... 368
Upcasting revisited ..........311
Inner classes in
Forgetting the object type...........313
methods and scopes ...................370
The twist.......................... 315
Anonymous inner classes...........373
Method-call binding ...................315
The link to the outer class ..........376
Producing the right behavior......316
static inner classes ....................379
Extensibility ............................... 320
Referring to the
Overriding vs.
outer class object ........................ 381
overloading .....................324
Reaching outward from
Abstract classes
a multiply-nested class...............383
and methods ...................325
Inheriting from inner classes .... 384
Constructors and
Can inner classes
polymorphism.................330
be overridden?............................385
Order of constructor calls .......... 330
Inner class identifiers.................387
Inheritance and finalize( ) ...... 333
Why inner classes? .................... 388
Behavior of polymorphic
Inner classes &
methods inside constructors..... 337
control frameworks ....................394
Designing with
Summary ........................ 402
inheritance ......................339
Exercises......................... 403
Pure inheritance
9: Holding
vs. extension................................341
Your Objects
407
Downcasting and run-time
type identification ...................... 343
Arrays ............................. 407
Arrays are first-class objects ..... 409
Returning an array......................413
Choosing between Lists.............502
The Arrays class ........................415
Choosing between Sets ..............506
Filling an array........................... 428
Choosing between Maps........... 508
Copying an array........................ 429
Sorting and
Comparing arrays ......................430
searching Lists................ 511
Array element comparisons........431
Utilities ............................512
Sorting an array ......................... 435
Making a Collection
Searching a sorted array ............ 437
or Map unmodifiable................. 513
Array summary .......................... 439
Synchronizing a
Introduction to
Collection or Map ................... 514
containers .......................439
Unsupported
Printing containers .....................441
operations........................516
Filling containers ....................... 442
Java 1.0/1.1 containers ....519
Container disadvantage:
Vector & Enumeration ............... 519
unknown type .................450
Hashtable.................................... 521
Sometimes it works anyway....... 452
Stack ........................................... 521
Making a type-conscious
BitSet ..........................................522
ArrayList.................................. 454
Summary ........................ 524
Iterators ..........................456
Exercises..........................525
Container taxonomy .......460
10: Error Handling
Collection
functionality....................463
with Exceptions
531
List functionality............467
Basic exceptions ............. 532
Making a stack
Exception arguments..................533
Catching an exception .... 534
from a LinkedList.....................471
Making a queue
The try block ..............................535
from a LinkedList.................... 472
Exception handlers.....................535
Set functionality ............. 473
Creating your own
exceptions........................537
SortedSet ................................. 476
Map functionality...........476
The exception
specification ................... 542
SortedMap............................... 482
Hashing and hash codes ............ 482
Catching any exception ..............543
Overriding hashCode( ) .......... 492
Rethrowing an exception ...........545
Holding references..........495
Standard Java
exceptions....................... 549
The WeakHashMap ...............498
Iterators revisited........... 500
The special case of
Choosing an
RuntimeException.................550
implementation............... 501
Performing cleanup
with finally ...................... 552
Reading from standard input.... 603
What’s finally for? .................... 554
Changing System.out
Pitfall: the lost exception ............557
to a PrintWriter...................... 604
Exception restrictions.....558
Redirecting standard I/O .......... 604
Constructors....................562
Compression................... 606
Exception matching ........566
Simple compression
Exception guidelines.................. 568
with GZIP....................................607
Summary.........................568
Multifile storage with Zip.......... 608
Exercises .........................569
Java ARchives (JARs) .................611
11: The Java
Object serialization..........613
I/O System
573
Finding the class.........................618
Controlling serialization............. 619
The File class.................. 574
Using persistence ...................... 630
A directory lister ........................ 574
Tokenizing input ............ 639
Checking for and
StreamTokenizer ...................639
creating directories .................... 578
StringTokenizer .....................642
Input and output............. 581
Checking capitalization style......645
Types of InputStream..............581
Summary .........................655
Types of OutputStream.......... 583
Exercises......................... 656
Adding attributes
and useful interfaces.......585
12: Run-time Type
Reading from an InputStream
Identification 659
with FilterInputStream......... 586
The need for RTTI .......... 659
Writing to an OutputStream
The Class object.........................662
with FilterOutputStream...... 587
Checking before a cast................665
Readers & Writers.......589
RTTI syntax .....................674
Sources and sinks of data........... 590
Reflection: run-time
Modifying stream behavior.........591
class information.............677
Unchanged Classes .................... 592
A class method extractor............679
Off by itself:
Summary ........................ 685
RandomAccessFile..........593
Exercises......................... 686
Typical uses of
13: Creating Windows
I/O streams.....................594
& Applets
689
Input streams............................. 597
The basic applet.............. 692
Output streams .......................... 599
A bug?......................................... 601
Applet restrictions ......................692
Piped streams.............................602
Applet advantages ......................693
Standard I/O...................602
Application frameworks .............694
Running applets inside
a Web browser............................ 695
Pop-up menus ............................766
Using Appletviewer ...................698
Drawing ......................................768
Testing applets ...........................698
Dialog Boxes ............................... 771
Running applets from
File dialogs.................................. 776
the command line ...........700
HTML on
A display framework .................. 702
Swing components ..................... 779
Using the Windows Explorer..... 705
Sliders and progress bars .......... 780
Making a button..............706
Trees ........................................... 781
Capturing an event.......... 707
Tables..........................................784
Text areas.........................711
Selecting Look & Feel .................787
Controlling layout ........... 712
The clipboard..............................790
BorderLayout ..............................713
Packaging an applet
FlowLayout..................................714
into a JAR file..................793
GridLayout ..................................715
Programming
GridBagLayout............................716
techniques .......................794
Absolute positioning ...................716
Binding events dynamically .......794
BoxLayout ................................... 717
Separating business
The best approach? .....................721
logic from UI logic .....................796
The Swing event model... 722
A canonical form ........................799
Event and listener types............. 723
Visual programming
Tracking multiple events ........... 730
and Beans .......................800
A catalog of Swing
What is a Bean? ..........................801
components..................... 734
Extracting BeanInfo
Buttons....................................... 734
with the Introspector............. 804
Icons........................................... 738
A more sophisticated Bean......... 811
Tool tips...................................... 740
Packaging a Bean........................816
Text fields................................... 740
More complex Bean support ......818
Borders....................................... 743
More to Beans.............................819
JScrollPanes............................... 744
Summary .........................819
A mini-editor.............................. 747
Exercises......................... 820
Check boxes................................ 748
14: Multiple Threads
825
Radio buttons............................. 750
Responsive
Combo boxes
user interfaces ................ 826
(drop-down lists) ........................751
Inheriting from Thread ........... 828
List boxes ................................... 753
Threading for a
Tabbed panes ..............................755
responsive interface.................... 831
Message boxes............................ 756
Combining the thread
Menus......................................... 759
with the main class .................... 834
A more sophisticated
Making many threads ................ 836
example.......................................939
Daemon threads.........................840
Servlets ........................... 948
Sharing
The basic servlet .........................949
limited resources.............842
Servlets and multithreading.......954
Improperly accessing
Handling sessions
resources .................................... 842
with servlets................................955
How Java shares resources........848
Running the
JavaBeans revisited ................... 854
servlet examples ........................ 960
Blocking ..........................859
Java Server Pages ........... 960
Becoming blocked......................860
Implicit objects...........................962
Deadlock..................................... 872
JSP directives .............................963
Priorities ......................... 877
JSP scripting elements ...............964
Reading and
Extracting fields and values .......966
setting priorities......................... 878
JSP page
Thread groups ............................882
attributes and scope .................. 968
Runnable revisited ....... 891
Manipulating
Too many threads ......................894
sessions in JSP............................969
Summary.........................899
Creating and
Exercises ......................... 901
modifying cookies....................... 971
15: Distributed
JSP summary..............................972
RMI (Remote Method
Computing 903
Invocation) ......................973
Network programming ...904
Remote interfaces.......................973
Identifying a machine ................ 905
Implementing the
Sockets .......................................909
remote interface .........................974
Serving multiple clients ..............917
Creating stubs and skeletons......978
Datagrams.................................. 923
Using the remote object .............979
Using URLs from
CORBA ........................... 980
within an applet ......................... 923
CORBA fundamentals ................981
More to networking ................... 926
An example ................................ 983
Java Database
Java Applets and CORBA.......... 989
Connectivity (JDBC) ....... 927
CORBA vs. RMI ......................... 989
Getting the example to work.......931
Enterprise JavaBeans..... 990
A GUI version
JavaBeans vs. EJBs .................... 991
of the lookup program ............... 935
The EJB specification.................992
Why the JDBC API
EJB components.........................993
seems so complex....................... 938
The pieces of an
EJB component.......................... 994
further down a hierarchy..........1034
EJB operation ............................ 995
Why this strange design? ......... 1035
Types of EJBs............................. 996
Controlling
Developing an EJB..................... 997
cloneability ....................1036
EJB summary........................... 1003
The copy constructor................1042
Jini: distributed
Read-only classes ..........1047
services..........................1003
Creating read-only classes........1049
Jini in context .......................... 1003
The drawback
What is Jini? ............................ 1004
to immutability.........................1050
How Jini works ........................ 1005
Immutable Strings.................. 1052
The discovery process .............. 1006
The String and
The join process ....................... 1006
StringBuffer classes ..............1056
The lookup process .................. 1007
Strings are special...................1060
Separation of interface
Summary ...................... 1060
and implementation................. 1008
Exercises........................1062
Abstracting
B: The Java Native
distributed systems.................. 1009
Summary....................... 1010
Interface (JNI)
1065
Exercises ....................... 1010
Calling a
native method................1066
A: Passing &
The header file
Returning Objects
1013
generator: javah........................ 1067
Passing
Name mangling and
references around ......... 1014
function signatures...................1068
Aliasing......................................1014
Implementing your DLL...........1068
Making local copies........1017
Accessing JNI functions:
Pass by value .............................1018
the JNIEnv argument ..1069
Cloning objects..........................1018
Accessing Java Strings ............. 1071
Adding cloneability
Passing and
to a class ................................... 1020
using Java objects.......... 1071
Successful cloning.................... 1022
JNI and
The effect of
Java exceptions .............1074
Object.clone( )...................... 1025
JNI and threading .........1075
Cloning a composed object .......1027
Using a preexisting
A deep copy
code base .......................1075
with ArrayList........................ 1030
Additional
Deep copy via serialization ...... 1032
information ...................1076
Adding cloneability
C: Java Programming
Books ............................. 1091
Guidelines 1077
Analysis & design......................1093
Design ........................... 1077
Python.......................................1095
Implementation ............1084
My own list of books.................1096
D: Resources
1091
Index 1099
Software ........................ 1091
Preface
I suggested to my brother Todd, who is making the leap
from hardware into programming, that the next big
revolution will be in genetic engineering.
We’ll have microbes designed to make food, fuel, and plastic; they’ll clean
up pollution and in general allow us to master the manipulation of the
physical world for a fraction of what it costs now. I claimed that it would
make the computer revolution look small in comparison.
Then I realized I was making a mistake common to science fiction writers:
getting lost in the technology (which is of course easy to do in science
fiction). An experienced writer knows that the story is never about the
things; it’s about the people. Genetics will have a very large impact on our
lives, but I’m not so sure it will dwarf the computer revolution (which
enables the genetic revolution)—or at least the information revolution.
Information is about talking to each other: yes, cars and shoes and
especially genetic cures are important, but in the end those are just
trappings. What truly matters is how we relate to the world. And so much
of that is about communication.
This book is a case in point. A majority of folks thought I was very bold or
a little crazy to put the entire thing up on the Web. “Why would anyone
buy it?” they asked. If I had been of a more conservative nature I wouldn’t
have done it, but I really didn’t want to write another computer book in
the same old way. I didn’t know what would happen but it turned out to
be the smartest thing I’ve ever done with a book.
For one thing, people started sending in corrections. This has been an
amazing process, because folks have looked into every nook and cranny
and caught both technical and grammatical errors, and I’ve been able to
eliminate bugs of all sorts that I know would have otherwise slipped
through. People have been simply terrific about this, very often saying
“Now, I don’t mean this in a critical way…” and then giving me a
collection of errors I’m sure I never would have found. I feel like this has
1
been a kind of group process and it has really made the book into
something special.
But then I started hearing “OK, fine, it’s nice you’ve put up an electronic
version, but I want a printed and bound copy from a real publisher.” I
tried very hard to make it easy for everyone to print it out in a nice looking
format but that didn’t stem the demand for the published book. Most
people don’t want to read the entire book on screen, and hauling around a
sheaf of papers, no matter how nicely printed, didn’t appeal to them
either. (Plus, I think it’s not so cheap in terms of laser printer toner.) It
seems that the computer revolution won’t put publishers out of business,
after all. However, one student suggested this may become a model for
future publishing: books will be published on the Web first, and only if
sufficient interest warrants it will the book be put on paper. Currently, the
great majority of all books are financial failures, and perhaps this new
approach could make the publishing industry more profitable.
This book became an enlightening experience for me in another way. I
originally approached Java as “just another programming language,”
which in many senses it is. But as time passed and I studied it more
deeply, I began to see that the fundamental intention of this language is
different from all the other languages I have seen.
Programming is about managing complexity: the complexity of the
problem you want to solve, laid upon the complexity of the machine in
which it is solved. Because of this complexity, most of our programming
projects fail. And yet, of all the programming languages of which I am
aware, none of them have gone all-out and decided that their main design
goal would be to conquer the complexity of developing and maintaining
programs1. Of course, many language design decisions were made with
complexity in mind, but at some point there were always some other
issues that were considered essential to be added into the mix. Inevitably,
those other issues are what cause programmers to eventually “hit the
wall” with that language. For example, C++ had to be backwards-
compatible with C (to allow easy migration for C programmers), as well as
1 I take this back on the 2nd edition: I believe that the Python language comes closest to
doing exactly that. See www.Python.org.
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efficient. Those are both very useful goals and account for much of the
success of C++, but they also expose extra complexity that prevents some
projects from being finished (certainly, you can blame programmers and
management, but if a language can help by catching your mistakes, why
shouldn’t it?). As another example, Visual Basic (VB) was tied to BASIC,
which wasn’t really designed to be an extensible language, so all the
extensions piled upon VB have produced some truly horrible and
unmaintainable syntax. Perl is backwards-compatible with Awk, Sed,
Grep, and other Unix tools it was meant to replace, and as a result is often
accused of producing “write-only code” (that is, after a few months you
can’t read it). On the other hand, C++, VB, Perl, and other languages like
Smalltalk had some of their design efforts focused on the issue of
complexity and as a result are remarkably successful in solving certain
types of problems.
What has impressed me most as I have come to understand Java is what
seems like an unflinching goal of reducing complexity for the
programmer. As if to say “we don’t care about anything except reducing
the time and difficulty of producing robust code.” In the early days, this
goal has resulted in code that doesn’t run very fast (although there have
been many promises made about how quickly Java will someday run) but
it has indeed produced amazing reductions in development time; half or
less of the time that it takes to create an equivalent C++ program. This
result alone can save incredible amounts of time and money, but Java
doesn’t stop there. It goes on to wrap all the complex tasks that have
become important, such as multithreading and network programming, in
language features or libraries that can at times make those tasks trivial.
And finally, it tackles some really big complexity problems: cross-platform
programs, dynamic code changes, and even security, each of which can fit
on your complexity spectrum anywhere from “impediment” to “show-
stopper.” So despite the performance problems we’ve seen, the promise of
Java is tremendous: it can make us significantly more productive
programmers.
One of the places I see the greatest impact for this is on the Web. Network
programming has always been hard, and Java makes it easy (and the Java
language designers are working on making it even easier). Network
programming is how we talk to each other more effectively and cheaper
than we ever have with telephones (email alone has revolutionized many
Preface
3
businesses). As we talk to each other more, amazing things begin to
happen, possibly more amazing even than the promise of genetic
engineering.
In all ways—creating the programs, working in teams to create the
programs, building user interfaces so the programs can communicate
with the user, running the programs on different types of machines, and
easily writing programs that communicate across the Internet—Java
increases the communication bandwidth between people. I think that
perhaps the results of the communication revolution will not be seen from
the effects of moving large quantities of bits around; we shall see the true
revolution because we will all be able to talk to each other more easily:
one-on-one, but also in groups and, as a planet. I've heard it suggested
that the next revolution is the formation of a kind of global mind that
results from enough people and enough interconnectedness. Java may or
may not be the tool that foments that revolution, but at least the
possibility has made me feel like I'm doing something meaningful by
attempting to teach the language.
Preface to the 2nd edition
People have made many, many wonderful comments about the first
edition of this book, which has naturally been very pleasant for me.
However, every now and then someone will have complaints, and for
some reason one complaint that comes up periodically is “the book is too
big.” In my mind it is faint damnation indeed if “too many pages” is your
only complaint. (One is reminded of the Emperor of Austria’s complaint
about Mozart’s work: “Too many notes!” Not that I am in any way trying
to compare myself to Mozart.) In addition, I can only assume that such a
complaint comes from someone who is yet to be acquainted with the
vastness of the Java language itself, and has not seen the rest of the books
on the subject—for example, my favorite reference is Cay Horstmann &
Gary Cornell’s Core Java (Prentice-Hall), which grew so big it had to be
broken into two volumes. Despite this, one of the things I have attempted
to do in this edition is trim out the portions that have become obsolete, or
at least nonessential. I feel comfortable doing this because the original
material remains on the Web site and the CD ROM that accompanies this
book, in the form of the freely-downloadable first edition of the book (at
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www.BruceEckel.com). If you want the old stuff, it’s still there, and this is
a wonderful relief for an author. For example, you may notice that the
original last chapter, “Projects,” is no longer here; two of the projects have
been integrated into other chapters, and the rest were no longer
appropriate. Also, the “Design Pattens” chapter became too big and has
been moved into a book of its own (also downloadable at the Web site).
So, by all rights the book should be thinner.
But alas, it is not to be.
The biggest issue is the continuing development of the Java language
itself, and in particular the expanding APIs that promise to provide
standard interfaces for just about everything you’d like to do (and I won’t
be surprised to see the “JToaster” API eventually appear). Covering all
these APIs is obviously beyond the scope of this book and is a task
relegated to other authors, but some issues cannot be ignored. The biggest
of these include server-side Java (primarily Servlets & Java Server pages,
or JSPs), which is truly an excellent solution to the World Wide Web
problem, wherein we’ve discovered that the various Web browser
platforms are just not consistent enough to support client-side
programming. In addition, there is the whole problem of easily creating
applications to interact with databases, transactions, security, and the
like, which is involved with Enterprise Java Beans (EJBs). These topics
are wrapped into the chapter formerly called “Network Programming”
and now called “Distributed Computing,” a subject that is becoming
essential to everyone. You’ll also find this chapter has been expanded to
include an overview of Jini (pronounced “genie,” and it isn’t an acronym,
just a name), which is a cutting-edge technology that allows us to change
the way we think about interconnected applications. And of course the
book has been changed to use the Swing GUI library throughout. Again, if
you want the old Java 1.0/1.1 stuff you can get it from the freely-
downloadable book at www.BruceEckel.com (it is also included on this
edition’s new CD ROM, bound into the book; more on that a little later).
Aside from additional small language features added in Java 2 and
corrections made throughout the book, the other major change is in the
collections chapter (9), which now focuses on the Java 2 collections used
throughout the book. I’ve also improved that chapter to more deeply go
into some of the important issues of collections, in particular how a hash
Preface
5
function works (so that you can know how to properly create one). There
have been other movements and changes, including a rewrite of Chapter
1, and removal of some appendices and other material that I consider no
longer necessary for the printed book, but those are the bulk of them. In
general, I’ve tried to go over everything, remove from the 2nd edition what
is no longer necessary (but which still exists in the electronic first edition),
include changes, and improve everything I could. As the language
continues to change—albeit not quite at the same breakneck pace as
before—there will no doubt be further editions of this book.
For those of you who still can’t stand the size of the book, I do apologize.
Believe it or not, I have worked hard to keep it small. Despite the bulk, I
feel like there may be enough alternatives to satisfy you. For one thing,
the book is available electronically (from the Web site, and also on the CD
ROM that accompanies this book), so if you carry your laptop you can
carry the book on that with no extra weight. If you’re really into slimming
down, there are actually Palm Pilot versions of the book floating around.
(One person told me he would read the book in bed on his Palm with the
backlighting on to keep from annoying his wife. I can only hope that it
helps send him to slumberland.) If you need it on paper, I know of people
who print a chapter at a time and carry it in their briefcase to read on the
train.
Java 2
At this writing, the release of Sun’s Java Development Kit (JDK) 1.3 is
imminent, and the proposed changes for JDK 1.4 have been publicized.
Although these version numbers are still in the “ones,” the standard way
to refer to any version of the language that is JDK 1.2 or greater is to call it
“Java 2.” This indicates the very significant changes between “old Java”—
which had many warts that I complained about in the first edition of this
book—and this more modern and improved version of the language,
which has far fewer warts and many additions and nice designs.
This book is written for Java 2. I have the great luxury of getting rid of all
the old stuff and writing to only the new, improved language because the
old information still exists in the electronic 1st edition on the Web and on
the CD ROM (which is where you can go if you’re stuck using a pre-Java-2
version of the language). Also, because anyone can freely download the
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JDK from java.sun.com, it means that by writing to Java 2 I’m not
imposing a financial hardship on someone by forcing them to upgrade.
There is a bit of a catch, however. JDK 1.3 has some improvements that
I’d really like to use, but the version of Java that is currently being
released for Linux is JDK 1.2.2. Linux (see www.Linux.org) is a very
important development in conjunction with Java, because it is fast
becoming the most important server platform out there—fast, reliable,
robust, secure, well-maintained, and free, a true revolution in the history
of computing (I don’t think we’ve ever seen all of those features in any
tool before). And Java has found a very important niche in server-side
programming in the form of Servlets, a technology that is a huge
improvement over the traditional CGI programming (this is covered in
the “Distributed Programming” chapter).
So although I would like to only use the very newest features, it’s critical
that everything compiles under Linux, and so when you unpack the source
code and compile it under that OS (with the latest JDK) you’ll discover
that everything will compile. However, you will find that I’ve put notes
about features in JDK 1.3 here and there.
The CD ROM
Another bonus with this edition is the CD ROM that is packaged in the
back of the book. I’ve resisted putting CD ROMs in the back of my books
in the past because I felt the extra charge for a few Kbytes of source code
on this enormous CD was not justified, preferring instead to allow people
to download such things from my Web site. However, you’ll soon see that
this CD ROM is different.
The CD does contain the source code from the book, but it also contains
the book in its entirety, in several electronic formats. My favorite of these
is the HTML format, because it is fast and fully indexed—you just click on
an entry in the index or table of contents and you’re immediately at that
portion of the book.
The bulk of the 300+ Megabytes of the CD, however, is a full multimedia
course called Thinking in C: Foundations for C++ & Java. I originally
commissioned Chuck Allison to create this seminar-on-CD ROM as a
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stand-alone product, but decided to include it with the second editions of
both Thinking in C++ and Thinking in Java because of the consistent
experience of having people come to seminars without an adequate
background in C. The thinking apparently goes “I’m a smart programmer
and I don’t want to learn C, but rather C++ or Java, so I’ll just skip C and
go directly to C++/Java.” After arriving at the seminar, it slowly dawns on
folks that the prerequisite of understanding C syntax is there for a very
good reason. By including the CD ROM with the book, we can ensure that
everyone attends a seminar with adequate preparation.
The CD also allows the book to appeal to a wider audience. Even though
Chapter 3 (Controlling program flow) does cover the fundamentals of the
parts of Java that come from C, the CD is a gentler introduction, and
assumes even less about the student’s programming background than
does the book. It is my hope that by including the CD more people will be
able to be brought into the fold of Java programming.
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Introduction
Like any human language, Java provides a way to express
concepts. If successful, this medium of expression will be
significantly easier and more flexible than the alternatives
as problems grow larger and more complex.
You can’t look at Java as just a collection of features—some of the features
make no sense in isolation. You can use the sum of the parts only if you
are thinking about design, not simply coding. And to understand Java in
this way, you must understand the problems with it and with
programming in general. This book discusses programming problems,
why they are problems, and the approach Java has taken to solve them.
Thus, the set of features I explain in each chapter are based on the way I
see a particular type of problem being solved with the language. In this
way I hope to move you, a little at a time, to the point where the Java
mindset becomes your native tongue.
Throughout, I’ll be taking the attitude that you want to build a model in
your head that allows you to develop a deep understanding of the
language; if you encounter a puzzle you’ll be able to feed it to your model
and deduce the answer.
Prerequisites
This book assumes that you have some programming familiarity: you
understand that a program is a collection of statements, the idea of a
subroutine/function/macro, control statements such as “if” and looping
constructs such as “while,” etc. However, you might have learned this in
many places, such as programming with a macro language or working
with a tool like Perl. As long as you’ve programmed to the point where you
feel comfortable with the basic ideas of programming, you’ll be able to
work through this book. Of course, the book will be easier for the C
programmers and more so for the C++ programmers, but don’t count
yourself out if you’re not experienced with those languages (but come
9
willing to work hard; also, the multimedia CD that accompanies this book
will bring you up to speed on the basic C syntax necessary to learn Java).
I’ll be introducing the concepts of object-oriented programming (OOP)
and Java’s basic control mechanisms, so you’ll be exposed to those, and
the first exercises will involve the basic control-flow statements.
Although references will often be made to C and C++ language features,
these are not intended to be insider comments, but instead to help all
programmers put Java in perspective with those languages, from which,
after all, Java is descended. I will attempt to make these references simple
and to explain anything that I think a non- C/C++ programmer would not
be familiar with.
Learning Java
At about the same time that my first book Using C++ (Osborne/McGraw-
Hill, 1989) came out, I began teaching that language. Teaching
programming languages has become my profession; I’ve seen nodding
heads, blank faces, and puzzled expressions in audiences all over the
world since 1989. As I began giving in-house training with smaller groups
of people, I discovered something during the exercises. Even those people
who were smiling and nodding were confused about many issues. I found
out, by chairing the C++ track at the Software Development Conference
for a number of years (and later the Java track), that I and other speakers
tended to give the typical audience too many topics too fast. So eventually,
through both variety in the audience level and the way that I presented
the material, I would end up losing some portion of the audience. Maybe
it’s asking too much, but because I am one of those people resistant to
traditional lecturing (and for most people, I believe, such resistance
results from boredom), I wanted to try to keep everyone up to speed.
For a time, I was creating a number of different presentations in fairly
short order. Thus, I ended up learning by experiment and iteration (a
technique that also works well in Java program design). Eventually I
developed a course using everything I had learned from my teaching
experience—one that I would be happy giving for a long time. It tackles
the learning problem in discrete, easy-to-digest steps, and in a hands-on
seminar (the ideal learning situation) there are exercises following each of
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the short lessons. I now give this course in public Java seminars, which
you can find out about at www.BruceEckel.com. (The introductory
seminar is also available as a CD ROM. Information is available at the
same Web site.)
The feedback that I get from each seminar helps me change and refocus
the material until I think it works well as a teaching medium. But this
book isn’t just seminar notes—I tried to pack as much information as I
could within these pages, and structured it to draw you through onto the
next subject. More than anything, the book is designed to serve the
solitary reader who is struggling with a new programming language.
Goals
Like my previous book Thinking in C++, this book has come to be
structured around the process of teaching the language. In particular, my
motivation is to create something that provides me with a way to teach the
language in my own seminars. When I think of a chapter in the book, I
think in terms of what makes a good lesson during a seminar. My goal is
to get bite-sized pieces that can be taught in a reasonable amount of time,
followed by exercises that are feasible to accomplish in a classroom
situation.
My goals in this book are to:
1.
Present the material one simple step at a time so that you can easily
digest each concept before moving on.
2.
Use examples that are as simple and short as possible. This
sometimes prevents me from tackling “real world” problems, but
I’ve found that beginners are usually happier when they can
understand every detail of an example rather than being impressed
by the scope of the problem it solves. Also, there’s a severe limit to
the amount of code that can be absorbed in a classroom situation.
For this I will no doubt receive criticism for using “toy examples,”
but I’m willing to accept that in favor of producing something
pedagogically useful.
Introduction
11
3.
Carefully sequence the presentation of features so that you aren’t
seeing something that you haven’t been exposed to. Of course, this
isn’t always possible; in those situations, a brief introductory
description is given.
4.
Give you what I think is important for you to understand about the
language, rather than everything I know. I believe there is an
information importance hierarchy, and that there are some facts
that 95 percent of programmers will never need to know and that
just confuse people and adds to their perception of the complexity
of the language. To take an example from C, if you memorize the
operator precedence table (I never did), you can write clever code.
But if you need to think about it, it will also confuse the
reader/maintainer of that code. So forget about precedence, and
use parentheses when things aren’t clear.
5.
Keep each section focused enough so that the lecture time—and the
time between exercise periods—is small. Not only does this keep
the audience’s minds more active and involved during a hands-on
seminar, but it gives the reader a greater sense of accomplishment.
6.
Provide you with a solid foundation so that you can understand the
issues well enough to move on to more difficult coursework and
books.
Online documentation
The Java language and libraries from Sun Microsystems (a free download)
come with documentation in electronic form, readable using a Web
browser, and virtually every third party implementation of Java has this
or an equivalent documentation system. Almost all the books published
on Java have duplicated this documentation. So you either already have it
or you can download it, and unless necessary, this book will not repeat
that documentation because it’s usually much faster if you find the class
descriptions with your Web browser than if you look them up in a book
(and the on-line documentation is probably more up-to-date). This book
will provide extra descriptions of the classes only when it’s necessary to
supplement the documentation so you can understand a particular
example.
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Chapters
This book was designed with one thing in mind: the way people learn the
Java language. Seminar audience feedback helped me understand the
difficult parts that needed illumination. In the areas where I got ambitious
and included too many features all at once, I came to know—through the
process of presenting the material—that if you include a lot of new
features, you need to explain them all, and this easily compounds the
student’s confusion. As a result, I’ve taken a great deal of trouble to
introduce the features as few at a time as possible.
The goal, then, is for each chapter to teach a single feature, or a small
group of associated features, without relying on additional features. That
way you can digest each piece in the context of your current knowledge
before moving on.
Here is a brief description of the chapters contained in the book, which
correspond to lectures and exercise periods in my hands-on seminars.
Chapter 1:
Introduction to Objects
This chapter is an overview of what object-oriented
programming is all about, including the answer to the basic
question “What’s an object?”, interface vs. implementation,
abstraction and encapsulation, messages and functions,
inheritance and composition, and the all-important
polymorphism. You’ll also get an overview of issues of object
creation such as constructors, where the objects live, where to
put them once they’re created, and the magical garbage
collector that cleans up the objects that are no longer needed.
Other issues will be introduced, including error handling with
exceptions, multithreading for responsive user interfaces, and
networking and the Internet. You’ll learn what makes Java
special, why it’s been so successful, and about object-oriented
analysis and design.
Chapter 2:
Everything is an Object
This chapter moves you to the point where you can write your
first Java program, so it must give an overview of the
essentials, including the concept of a reference to an object;
Introduction
13
how to create an object; an introduction to primitive types
and arrays; scoping and the way objects are destroyed by the
garbage collector; how everything in Java is a new data type
(class) and how to create your own classes; functions,
arguments, and return values; name visibility and using
components from other libraries; the static keyword; and
comments and embedded documentation.
Chapter 3:
Controlling Program Flow
This chapter begins with all of the operators that come to Java
from C and C++. In addition, you’ll discover common
operator pitfalls, casting, promotion, and precedence. This is
followed by the basic control-flow and selection operations
that you get with virtually any programming language: choice
with if-else; looping with for and while; quitting a loop with
break and continue as well as Java’s labeled break and labeled
continue (which account for the “missing goto” in Java); and
selection using switch. Although much of this material has
common threads with C and C++ code, there are some
differences. In addition, all the examples will be full Java
examples so you’ll get more comfortable with what Java looks
like.
Chapter 4:
Initialization & Cleanup
This chapter begins by introducing the constructor, which
guarantees proper initialization. The definition of the
constructor leads into the concept of function overloading
(since you might want several constructors). This is followed
by a discussion of the process of cleanup, which is not always
as simple as it seems. Normally, you just drop an object when
you’re done with it and the garbage collector eventually comes
along and releases the memory. This portion explores the
garbage collector and some of its idiosyncrasies. The chapter
concludes with a closer look at how things are initialized:
automatic member initialization, specifying member
initialization, the order of initialization, static initialization
and array initialization.
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Chapter 5:
Hiding the Implementation
This chapter covers the way that code is packaged together,
and why some parts of a library are exposed while other parts
are hidden. It begins by looking at the package and import
keywords, which perform file-level packaging and allow you
to build libraries of classes. It then examines subject of
directory paths and file names. The remainder of the chapter
looks at the public, private, and protected keywords, the
concept of “friendly” access, and what the different levels of
access control mean when used in various contexts.
Chapter 6:
Reusing Classes
The concept of inheritance is standard in virtually all OOP
languages. It’s a way to take an existing class and add to its
functionality (as well as change it, the subject of Chapter 7).
Inheritance is often a way to reuse code by leaving the “base
class” the same, and just patching things here and there to
produce what you want. However, inheritance isn’t the only
way to make new classes from existing ones. You can also
embed an object inside your new class with composition. In
this chapter you’ll learn about these two ways to reuse code in
Java, and how to apply them.
Chapter 7:
Polymorphism
On your own, you might take nine months to discover and
understand polymorphism, a cornerstone of OOP. Through
small, simple examples you’ll see how to create a family of
types with inheritance and manipulate objects in that family
through their common base class. Java’s polymorphism
allows you to treat all objects in this family generically, which
means the bulk of your code doesn’t rely on specific type
information. This makes your programs extensible, so
building programs and code maintenance is easier and
cheaper.
Chapter 8:
Interfaces & Inner Classes
Java provides a third way to set up a reuse relationship,
through the interface, which is a pure abstraction of the
interface of an object. The interface is more than just an
Introduction
15
abstract class taken to the extreme, since it allows you to
perform a variation on C++’s “multiple inheritance,” by
creating a class that can be upcast to more than one base type.
At first, inner classes look like a simple code hiding
mechanism: you place classes inside other classes. You’ll
learn, however, that the inner class does more than that—it
knows about and can communicate with the surrounding
class—and that the kind of code you can write with inner
classes is more elegant and clear, although it is a new concept
to most and takes some time to become comfortable with
design using inner classes.
Chapter 9:
Holding your Objects
It’s a fairly simple program that has only a fixed quantity of
objects with known lifetimes. In general, your programs will
always be creating new objects at a variety of times that will
be known only while the program is running. In addition, you
won’t know until run-time the quantity or even the exact type
of the objects you need. To solve the general programming
problem, you need to create any number of objects, anytime,
anywhere. This chapter explores in depth the container
library that Java 2 supplies to hold objects while you’re
working with them: the simple arrays and more sophisticated
containers (data structures) such as ArrayList and
HashMap.
Chapter 10: Error Handling with Exceptions
The basic philosophy of Java is that badly-formed code will
not be run. As much as possible, the compiler catches
problems, but sometimes the problems—either programmer
error or a natural error condition that occurs as part of the
normal execution of the program—can be detected and dealt
with only at run-time. Java has exception handling to deal
with any problems that arise while the program is running.
This chapter examines how the keywords try, catch, throw,
throws, and finally work in Java; when you should throw
exceptions and what to do when you catch them. In addition,
you’ll see Java’s standard exceptions, how to create your own,
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what happens with exceptions in constructors, and how
exception handlers are located.
Chapter 11: The Java I/O System
Theoretically, you can divide any program into three parts:
input, process, and output. This implies that I/O
(input/output) is an important part of the equation. In this
chapter you’ll learn about the different classes that Java
provides for reading and writing files, blocks of memory, and
the console. The distinction between “old” I/O and “new”
Java I/O will be shown. In addition, this chapter examines the
process of taking an object, “streaming” it (so that it can be
placed on disk or sent across a network) and reconstructing it,
which is handled for you with Java’s object serialization. Also,
Java’s compression libraries, which are used in the Java
ARchive file format (JAR), are examined.
Chapter 12: Run-Time Type Identification
Java run-time type identification (RTTI) lets you find the
exact type of an object when you have a reference to only the
base type. Normally, you’ll want to intentionally ignore the
exact type of an object and let Java’s dynamic binding
mechanism (polymorphism) implement the correct behavior
for that type. But occasionally it is very helpful to know the
exact type of an object for which you have only a base
reference. Often this information allows you to perform a
special-case operation more efficiently. This chapter explains
what RTTI is for, how to use it, and how to get rid of it when it
doesn’t belong there. In addition, this chapter introduces the
Java reflection mechanism.
Chapter 13: Creating Windows and Applets
Java comes with the “Swing” GUI library, which is a set of
classes that handle windowing in a portable fashion. These
windowed programs can either be applets or stand-alone
applications. This chapter is an introduction to Swing and the
creation of World Wide Web applets. The important
“JavaBeans” technology is introduced. This is fundamental
Introduction
17
for the creation of Rapid-Application Development (RAD)
program-building tools.
Chapter 14: Multiple Threads
Java provides a built-in facility to support multiple
concurrent subtasks, called threads, running within a single
program. (Unless you have multiple processors on your
machine, this is only the appearance of multiple subtasks.)
Although these can be used anywhere, threads are most
apparent when trying to create a responsive user interface so,
for example, a user isn’t prevented from pressing a button or
entering data while some processing is going on. This chapter
looks at the syntax and semantics of multithreading in Java.
Chapter 15: Distributed Computing
All the Java features and libraries seem to really come
together when you start writing programs to work across
networks. This chapter explores communication across
networks and the Internet, and the classes that Java provides
to make this easier. It introduces the very important concepts
of Servlets and JSPs (for server-side programming), along
with Java DataBase Connectivity (JDBC), and Remote
Method Invocation (RMI). Finally, there’s an introduction to
the new technologies of JINI, JavaSpaces, and Enterprise
JavaBeans (EJBs).
Appendix A: Passing & Returning Objects
Since the only way you talk to objects in Java is through
references, the concepts of passing an object into a function
and returning an object from a function have some interesting
consequences. This appendix explains what you need to know
to manage objects when you’re moving in and out of
functions, and also shows the String class, which uses a
different approach to the problem.
Appendix B: The Java Native Interface (JNI)
A totally portable Java program has serious drawbacks: speed
and the inability to access platform-specific services. When
you know the platform that you’re running on, it’s possible to
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dramatically speed up certain operations by making them
native methods, which are functions that are written in
another programming language (currently, only C/C++ is
supported). This appendix gives you enough of an
introduction to this feature that you should be able to create
simple examples that interface with non-Java code.
Appendix C: Java Programming Guidelines
This appendix contains suggestions to help guide you while
performing low-level program design and writing code.
Appendix D: Recommended Reading
A list of some of the Java books I’ve found particularly useful.
Exercises
I’ve discovered that simple exercises are exceptionally useful to complete
a student’s understanding during a seminar, so you’ll find a set at the end
of each chapter.
Most exercises are designed to be easy enough that they can be finished in
a reasonable amount of time in a classroom situation while the instructor
observes, making sure that all the students are absorbing the material.
Some exercises are more advanced to prevent boredom for experienced
students. The majority are designed to be solved in a short time and test
and polish your knowledge. Some are more challenging, but none present
major challenges. (Presumably, you’ll find those on your own—or more
likely they’ll find you).
Solutions to selected exercises can be found in the electronic document
The Thinking in Java Annotated Solution Guide, available for a small fee
from www.BruceEckel.com.
Multimedia CD ROM
There are two multimedia CDs associated with this book. The first is
bound into the book itself: Thinking in C, described at the end of the
preface, which prepares you for the book by bringing you up to speed on
the necessary C syntax you need to be able to understand Java.
Introduction
19
A second Multimedia CD ROM is available, which is based on the contents
of the book. This CD ROM is a separate product and contains the entire
contents of the week-long “Hands-On Java” training seminar. This is
more than 15 hours of lectures that I have recorded, synchronized with
hundreds of slides of information. Because the seminar is based on this
book, it is an ideal accompaniment.
The CD ROM contains all the lectures (with the important exception of
personalized attention!) from the five-day full-immersion training
seminars. We believe that it sets a new standard for quality.
The Hands-On Java CD ROM is available only by ordering directly from
the Web site www.BruceEckel.com.
Source code
All the source code for this book is available as copyrighted freeware,
distributed as a single package, by visiting the Web site
www.BruceEckel.com. To make sure that you get the most current
version, this is the official site for distribution of the code and the
electronic version of the book. You can find mirrored versions of the
electronic book and the code on other sites (some of these sites are found
at www.BruceEckel.com), but you should check the official site to ensure
that the mirrored version is actually the most recent edition. You may
distribute the code in classroom and other educational situations.
The primary goal of the copyright is to ensure that the source of the code
is properly cited, and to prevent you from republishing the code in print
media without permission. (As long as the source is cited, using examples
from the book in most media is generally not a problem.)
In each source code file you will find a reference to the following copyright
notice:
//:! :CopyRight.txt
Copyright ©2000 Bruce Eckel
Source code file from the 2nd edition of the book
"Thinking in Java." All rights reserved EXCEPT as
allowed by the following statements:
You can freely use this file
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for your own work (personal or commercial),
including modifications and distribution in
executable form only. Permission is granted to use
this file in classroom situations, including its
use in presentation materials, as long as the book
"Thinking in Java" is cited as the source.
Except in classroom situations, you cannot copy
and distribute this code; instead, the sole
distribution point is http://www.BruceEckel.com
(and official mirror sites) where it is
freely available. You cannot remove this
copyright and notice. You cannot distribute
modified versions of the source code in this
package. You cannot use this file in printed
media without the express permission of the
author. Bruce Eckel makes no representation about
the suitability of this software for any purpose.
It is provided "as is" without express or implied
warranty of any kind, including any implied
warranty of merchantability, fitness for a
particular purpose or non-infringement. The entire
risk as to the quality and performance of the
software is with you. Bruce Eckel and the
publisher shall not be liable for any damages
suffered by you or any third party as a result of
using or distributing software. In no event will
Bruce Eckel or the publisher be liable for any
lost revenue, profit, or data, or for direct,
indirect, special, consequential, incidental, or
punitive damages, however caused and regardless of
the theory of liability, arising out of the use of
or inability to use software, even if Bruce Eckel
and the publisher have been advised of the
possibility of such damages. Should the software
prove defective, you assume the cost of all
necessary servicing, repair, or correction. If you
think you've found an error, please submit the
correction using the form you will find at
www.BruceEckel.com. (Please use the same
form for non-code errors found in the book.)
///:~
Introduction
21
You may use the code in your projects and in the classroom (including
your presentation materials) as long as the copyright notice that appears
in each source file is retained.
Coding standards
In the text of this book, identifiers (function, variable, and class names)
are set in bold. Most keywords are also set in bold, except for those
keywords that are used so much that the bolding can become tedious,
such as “class.”
I use a particular coding style for the examples in this book. This style
follows the style that Sun itself uses in virtually all of the code you will
find at its site (see java.sun.com/docs/codeconv/index.html), and seems
to be supported by most Java development environments. If you’ve read
my other works, you’ll also notice that Sun’s coding style coincides with
mine—this pleases me, although I had nothing to do with it. The subject of
formatting style is good for hours of hot debate, so I’ll just say I’m not
trying to dictate correct style via my examples; I have my own motivation
for using the style that I do. Because Java is a free-form programming
language, you can continue to use whatever style you’re comfortable with.
The programs in this book are files that are included by the word
processor in the text, directly from compiled files. Thus, the code files
printed in the book should all work without compiler errors. The errors
that should cause compile-time error messages are commented out with
the comment //! so they can be easily discovered and tested using
automatic means. Errors discovered and reported to the author will
appear first in the distributed source code and later in updates of the book
(which will also appear on the Web site www.BruceEckel.com).
Java versions
I generally rely on the Sun implementation of Java as a reference when
determining whether behavior is correct.
Over time, Sun has released three major versions of Java: 1.0, 1.1 and 2
(which is called version 2 even though the releases of the JDK from Sun
continue to use the numbering scheme of 1.2, 1.3, 1.4, etc.). Version 2
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seems to finally bring Java into the prime time, in particular where user
interface tools are concerned. This book focuses on and is tested with Java
2, although I do sometimes make concessions to earlier features of Java 2
so that the code will compile under Linux (via the Linux JDK that was
available at this writing).
If you need to learn about earlier releases of the language that are not
covered in this edition, the first edition of the book is freely downloadable
at www.BruceEckel.com and is also contained on the CD that is bound in
with this book.
One thing you’ll notice is that, when I do need to mention earlier versions
of the language, I don’t use the sub-revision numbers. In this book I will
refer to Java 1.0, Java 1.1, and Java 2 only, to guard against typographical
errors produced by further sub-revisioning of these products.
Seminars and mentoring
My company provides five-day, hands-on, public and in-house training
seminars based on the material in this book. Selected material from each
chapter represents a lesson, which is followed by a monitored exercise
period so each student receives personal attention. The audio lectures and
slides for the introductory seminar are also captured on CD ROM to
provide at least some of the experience of the seminar without the travel
and expense. For more information, go to www.BruceEckel.com.
My company also provides consulting, mentoring and walkthrough
services to help guide your project through its development cycle—
especially your company’s first Java project.
Errors
No matter how many tricks a writer uses to detect errors, some always
creep in and these often leap off the page for a fresh reader.
There is an error submission form linked from the beginning of each
chapter in the HTML version of this book (and on the CD ROM bound
into the back of this book, and downloadable from www.BruceEckel.com)
and also on the Web site itself, on the page for this book. If you discover
Introduction
23
anything you believe to be an error, please use this form to submit the
error along with your suggested correction. If necessary, include the
original source file and note any suggested modifications. Your help is
appreciated.
Note on the cover design
The cover of Thinking in Java is inspired by the American Arts & Crafts
Movement, which began near the turn of the century and reached its
zenith between 1900 and 1920. It began in England as a reaction to both
the machine production of the Industrial Revolution and the highly
ornamental style of the Victorian era. Arts & Crafts emphasized spare
design, the forms of nature as seen in the art nouveau movement, hand-
crafting, and the importance of the individual craftsperson, and yet it did
not eschew the use of modern tools. There are many echoes with the
situation we have today: the turn of the century, the evolution from the
raw beginnings of the computer revolution to something more refined and
meaningful to individual persons, and the emphasis on software
craftsmanship rather than just manufacturing code.
I see Java in this same way: as an attempt to elevate the programmer
away from an operating-system mechanic and toward being a “software
craftsman.”
Both the author and the book/cover designer (who have been friends
since childhood) find inspiration in this movement, and both own
furniture, lamps, and other pieces that are either original or inspired by
this period.
The other theme in this cover suggests a collection box that a naturalist
might use to display the insect specimens that he or she has preserved.
These insects are objects, which are placed within the box objects. The
box objects are themselves placed within the “cover object,” which
illustrates the fundamental concept of aggregation in object-oriented
programming. Of course, a programmer cannot help but make the
association with “bugs,” and here the bugs have been captured and
presumably killed in a specimen jar, and finally confined within a small
display box, as if to imply Java’s ability to find, display, and subdue bugs
(which is truly one of its most powerful attributes).
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Acknowledgements
First, thanks to associates who have worked with me to give seminars,
provide consulting, and develop teaching projects: Andrea Provaglio,
Dave Bartlett (who also contributed significantly to Chapter 15), Bill
Venners, and Larry O’Brien. I appreciate your patience as I continue to try
to develop the best model for independent folks like us to work together.
Thanks to Rolf André Klaedtke (Switzerland); Martin Vlcek, Martin Byer,
Vlada & Pavel Lahoda, Martin the Bear, and Hanka (Prague); and Marco
Cantu (Italy) for hosting me on my first self-organized European seminar
tour.
Thanks to the Doyle Street Cohousing Community for putting up with me
for the two years that it took me to write the first edition of this book (and
for putting up with me at all). Thanks very much to Kevin and Sonda
Donovan for subletting their great place in gorgeous Crested Butte,
Colorado for the summer while I worked on the first edition of the book.
Also thanks to the friendly residents of Crested Butte and the Rocky
Mountain Biological Laboratory who make me feel so welcome.
Thanks to Claudette Moore at Moore Literary Agency for her tremendous
patience and perseverance in getting me exactly what I wanted.
My first two books were published with Jeff Pepper as editor at
Osborne/McGraw-Hill. Jeff appeared at the right place and the right time
at Prentice-Hall and has cleared the path and made all the right things
happen to make this a very pleasant publishing experience. Thanks, Jeff—
it means a lot to me.
I’m especially indebted to Gen Kiyooka and his company Digigami, who
graciously provided my Web server for the first several years of my
presence on the Web. This was an invaluable learning aid.
Thanks to Cay Horstmann (co-author of Core Java, Prentice-Hall, 2000),
D’Arcy Smith (Symantec), and Paul Tyma (co-author of Java Primer Plus,
The Waite Group, 1996), for helping me clarify concepts in the language.
Introduction
25
Thanks to people who have spoken in my Java track at the Software
Development Conference, and students in my seminars, who ask the
questions I need to hear in order to make the material more clear.
Special thanks to Larry and Tina O’Brien, who helped turn my seminar
into the original Hands-On Java CD ROM. (You can find out more at
www.BruceEckel.com.)
Lots of people sent in corrections and I am indebted to them all, but
particular thanks go to (for the first edition): Kevin Raulerson (found tons
of great bugs), Bob Resendes (simply incredible), John Pinto, Joe Dante,
Joe Sharp (all three were fabulous), David Combs (many grammar and
clarification corrections), Dr. Robert Stephenson, John Cook, Franklin
Chen, Zev Griner, David Karr, Leander A. Stroschein, Steve Clark, Charles
A. Lee, Austin Maher, Dennis P. Roth, Roque Oliveira, Douglas Dunn,
Dejan Ristic, Neil Galarneau, David B. Malkovsky, Steve Wilkinson, and a
host of others. Prof. Ir. Marc Meurrens put in a great deal of effort to
publicize and make the electronic version of the first edition of the book
available in Europe.
There have been a spate of smart technical people in my life who have
become friends and have also been both influential and unusual in that
they do yoga and practice other forms of spiritual enhancement, which I
find quite inspirational and instructional. They are Kraig Brockschmidt,
Gen Kiyooka, and Andrea Provaglio, (who helps in the understanding of
Java and programming in general in Italy, and now in the United States as
an associate of the MindView team).
It’s not that much of a surprise to me that understanding Delphi helped
me understand Java, since there are many concepts and language design
decisions in common. My Delphi friends provided assistance by helping
me gain insight into that marvelous programming environment. They are
Marco Cantu (another Italian—perhaps being steeped in Latin gives one
aptitude for programming languages?), Neil Rubenking (who used to do
the yoga/vegetarian/Zen thing until he discovered computers), and of
course Zack Urlocker, a long-time pal whom I’ve traveled the world with.
My friend Richard Hale Shaw’s insights and support have been very
helpful (and Kim’s, too). Richard and I spent many months giving
seminars together and trying to work out the perfect learning experience
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for the attendees. Thanks also to KoAnn Vikoren, Eric Faurot, Marco
Pardi, and the rest of the cast and crew at MFI. Thanks especially to Tara
Arrowood, who re-inspired me about the possibilities of conferences.
The book design, cover design, and cover photo were created by my friend
Daniel Will-Harris, noted author and designer (www.Will-Harris.com),
who used to play with rub-on letters in junior high school while he
awaited the invention of computers and desktop publishing, and
complained of me mumbling over my algebra problems. However, I
produced the camera-ready pages myself, so the typesetting errors are
mine. Microsoft® Word 97 for Windows was used to write the book and to
create camera-ready pages in Adobe Acrobat; the book was created
directly from the Acrobat PDF files. (As a tribute to the electronic age, I
happened to be overseas both times the final version of the book was
produced—the first edition was sent from Capetown, South Africa and the
second edition was posted from Prague). The body typeface is Georgia
and the headlines are in Verdana. The cover typeface is ITC Rennie
Mackintosh.
Thanks to the vendors who created the compilers: Borland, the
Blackdown group (for Linux), and of course, Sun.
A special thanks to all my teachers and all my students (who are my
teachers as well). The most fun writing teacher was Gabrielle Rico (author
of Writing the Natural Way, Putnam, 1983). I’ll always treasure the
terrific week at Esalen.
The supporting cast of friends includes, but is not limited to: Andrew
Binstock, Steve Sinofsky, JD Hildebrandt, Tom Keffer, Brian McElhinney,
Brinkley Barr, Bill Gates at Midnight Engineering Magazine, Larry
Constantine and Lucy Lockwood, Greg Perry, Dan Putterman, Christi
Westphal, Gene Wang, Dave Mayer, David Intersimone, Andrea
Rosenfield, Claire Sawyers, more Italians (Laura Fallai, Corrado, Ilsa, and
Cristina Giustozzi), Chris and Laura Strand, the Almquists, Brad Jerbic,
Marilyn Cvitanic, the Mabrys, the Haflingers, the Pollocks, Peter Vinci,
the Robbins Families, the Moelter Families (and the McMillans), Michael
Wilk, Dave Stoner, Laurie Adams, the Cranstons, Larry Fogg, Mike and
Karen Sequeira, Gary Entsminger and Allison Brody, Kevin Donovan and
Sonda Eastlack, Chester and Shannon Andersen, Joe Lordi, Dave and
Introduction
27
Brenda Bartlett, David Lee, the Rentschlers, the Sudeks, Dick, Patty, and
Lee Eckel, Lynn and Todd, and their families. And of course, Mom and
Dad.
Internet contributors
Thanks to those who helped me rewrite the examples to use the Swing
library, and for other assistance: Jon Shvarts, Thomas Kirsch, Rahim
Adatia, Rajesh Jain, Ravi Manthena, Banu Rajamani, Jens Brandt, Nitin
Shivaram, Malcolm Davis, and everyone who expressed support. This
really helped me jump-start the project.
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1: Introduction
to Objects
The genesis of the computer revolution was in a machine.
The genesis of our programming languages thus tends to
look like that machine.
But computers are not so much machines as they are mind amplification
tools (“bicycles for the mind,” as Steve Jobs is fond of saying) and a
different kind of expressive medium. As a result, the tools are beginning
to look less like machines and more like parts of our minds, and also like
other forms of expression such as writing, painting, sculpture, animation,
and filmmaking. Object-oriented programming (OOP) is part of this
movement toward using the computer as an expressive medium.
This chapter will introduce you to the basic concepts of OOP, including an
overview of development methods. This chapter, and this book, assume
that you have had experience in a procedural programming language,
although not necessarily C. If you think you need more preparation in
programming and the syntax of C before tackling this book, you should
work through the Thinking in C: Foundations for C++ and Java training
CD ROM, bound in with this book and also available at
www.BruceEckel.com.
This chapter is background and supplementary material. Many people do
not feel comfortable wading into object-oriented programming without
understanding the big picture first. Thus, there are many concepts that
are introduced here to give you a solid overview of OOP. However, many
other people don’t get the big picture concepts until they’ve seen some of
the mechanics first; these people may become bogged down and lost
without some code to get their hands on. If you’re part of this latter group
and are eager to get to the specifics of the language, feel free to jump past
this chapter—skipping it at this point will not prevent you from writing
programs or learning the language. However, you will want to come back
29
here eventually to fill in your knowledge so you can understand why
objects are important and how to design with them.
The progress of
abstraction
All programming languages provide abstractions. It can be argued that
the complexity of the problems you’re able to solve is directly related to
the kind and quality of abstraction. By “kind” I mean, “What is it that you
are abstracting?” Assembly language is a small abstraction of the
underlying machine. Many so-called “imperative” languages that followed
(such as Fortran, BASIC, and C) were abstractions of assembly language.
These languages are big improvements over assembly language, but their
primary abstraction still requires you to think in terms of the structure of
the computer rather than the structure of the problem you are trying to
solve. The programmer must establish the association between the
machine model (in the “solution space,” which is the place where you’re
modeling that problem, such as a computer) and the model of the
problem that is actually being solved (in the “problem space,” which is the
place where the problem exists). The effort required to perform this
mapping, and the fact that it is extrinsic to the programming language,
produces programs that are difficult to write and expensive to maintain,
and as a side effect created the entire “programming methods” industry.
The alternative to modeling the machine is to model the problem you’re
trying to solve. Early languages such as LISP and APL chose particular
views of the world (“All problems are ultimately lists” or “All problems are
algorithmic,” respectively). PROLOG casts all problems into chains of
decisions. Languages have been created for constraint-based
programming and for programming exclusively by manipulating graphical
symbols. (The latter proved to be too restrictive.) Each of these
approaches is a good solution to the particular class of problem they’re
designed to solve, but when you step outside of that domain they become
awkward.
The object-oriented approach goes a step further by providing tools for
the programmer to represent elements in the problem space. This
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representation is general enough that the programmer is not constrained
to any particular type of problem. We refer to the elements in the problem
space and their representations in the solution space as “objects.” (Of
course, you will also need other objects that don’t have problem-space
analogs.) The idea is that the program is allowed to adapt itself to the
lingo of the problem by adding new types of objects, so when you read the
code describing the solution, you’re reading words that also express the
problem. This is a more flexible and powerful language abstraction than
what we’ve had before. Thus, OOP allows you to describe the problem in
terms of the problem, rather than in terms of the computer where the
solution will run. There’s still a connection back to the computer, though.
Each object looks quite a bit like a little computer; it has a state, and it has
operations that you can ask it to perform. However, this doesn’t seem like
such a bad analogy to objects in the real world—they all have
characteristics and behaviors.
Some language designers have decided that object-oriented programming
by itself is not adequate to easily solve all programming problems, and
advocate the combination of various approaches into multiparadigm
programming languages.1
Alan Kay summarized five basic characteristics of Smalltalk, the first
successful object-oriented language and one of the languages upon which
Java is based. These characteristics represent a pure approach to object-
oriented programming:
1.
Everything is an object. Think of an object as a fancy
variable; it stores data, but you can “make requests” to that object,
asking it to perform operations on itself. In theory, you can take
any conceptual component in the problem you’re trying to solve
(dogs, buildings, services, etc.) and represent it as an object in your
program.
2.
A program is a bunch of objects telling each other
what to do by sending messages. To make a request of an
object, you “send a message” to that object. More concretely, you
1 See Multiparadigm Programming in Leda by Timothy Budd (Addison-Wesley 1995).
Chapter 1: Introduction to Objects
31
can think of a message as a request to call a function that belongs
to a particular object.
3.
Each object has its own memory made up of other
objects. Put another way, you create a new kind of object by
making a package containing existing objects. Thus, you can build
complexity in a program while hiding it behind the simplicity of
objects.
4.
Every object has a type. Using the parlance, each object is an
instance of a class, in which “class” is synonymous with “type.” The
most important distinguishing characteristic of a class is “What
messages can you send to it?”
5.
All objects of a particular type can receive the same
messages. This is actually a loaded statement, as you will see
later. Because an object of type “circle” is also an object of type
“shape,” a circle is guaranteed to accept shape messages. This
means you can write code that talks to shapes and automatically
handle anything that fits the description of a shape. This
substitutability is one of the most powerful concepts in OOP.
An object has an interface
Aristotle was probably the first to begin a careful study of the concept of
type; he spoke of “the class of fishes and the class of birds.” The idea that
all objects, while being unique, are also part of a class of objects that have
characteristics and behaviors in common was used directly in the first
object-oriented language, Simula-67, with its fundamental keyword class
that introduces a new type into a program.
Simula, as its name implies, was created for developing simulations such
as the classic “bank teller problem.” In this, you have a bunch of tellers,
customers, accounts, transactions, and units of money—a lot of “objects.”
Objects that are identical except for their state during a program’s
execution are grouped together into “classes of objects” and that’s where
the keyword class came from. Creating abstract data types (classes) is a
fundamental concept in object-oriented programming. Abstract data
types work almost exactly like built-in types: You can create variables of a
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type (called objects or instances in object-oriented parlance) and
manipulate those variables (called sending messages or requests; you
send a message and the object figures out what to do with it). The
members (elements) of each class share some commonality: every account
has a balance, every teller can accept a deposit, etc. At the same time, each
member has its own state, each account has a different balance, each
teller has a name. Thus, the tellers, customers, accounts, transactions,
etc., can each be represented with a unique entity in the computer
program. This entity is the object, and each object belongs to a particular
class that defines its characteristics and behaviors.
So, although what we really do in object-oriented programming is create
new data types, virtually all object-oriented programming languages use
the “class” keyword. When you see the word “type” think “class” and vice
versa2.
Since a class describes a set of objects that have identical characteristics
(data elements) and behaviors (functionality), a class is really a data type
because a floating point number, for example, also has a set of
characteristics and behaviors. The difference is that a programmer defines
a class to fit a problem rather than being forced to use an existing data
type that was designed to represent a unit of storage in a machine. You
extend the programming language by adding new data types specific to
your needs. The programming system welcomes the new classes and gives
them all the care and type-checking that it gives to built-in types.
The object-oriented approach is not limited to building simulations.
Whether or not you agree that any program is a simulation of the system
you’re designing, the use of OOP techniques can easily reduce a large set
of problems to a simple solution.
Once a class is established, you can make as many objects of that class as
you like, and then manipulate those objects as if they are the elements
that exist in the problem you are trying to solve. Indeed, one of the
challenges of object-oriented programming is to create a one-to-one
2 Some people make a distinction, stating that type determines the interface while class is
a particular implementation of that interface.
Chapter 1: Introduction to Objects
33
mapping between the elements in the problem space and objects in the
solution space.
But how do you get an object to do useful work for you? There must be a
way to make a request of the object so that it will do something, such as
complete a transaction, draw something on the screen, or turn on a
switch. And each object can satisfy only certain requests. The requests you
can make of an object are defined by its interface, and the type is what
determines the interface. A simple example might be a representation of a
light bulb:
Light
Type Name
on()
off()
Interface brighten()
dim()
Light lt = new Light();
lt.on();
The interface establishes what requests you can make for a particular
object. However, there must be code somewhere to satisfy that request.
This, along with the hidden data, comprises the implementation. From a
procedural programming standpoint, it’s not that complicated. A type has
a function associated with each possible request, and when you make a
particular request to an object, that function is called. This process is
usually summarized by saying that you “send a message” (make a request)
to an object, and the object figures out what to do with that message (it
executes code).
Here, the name of the type/class is Light, the name of this particular
Light object is lt, and the requests that you can make of a Light object
are to turn it on, turn it off, make it brighter, or make it dimmer. You
create a Light object by defining a “reference” (lt) for that object and
calling new to request a new object of that type. To send a message to the
object, you state the name of the object and connect it to the message
request with a period (dot). From the standpoint of the user of a
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predefined class, that’s pretty much all there is to programming with
objects.
The diagram shown above follows the format of the Unified Modeling
Language (UML). Each class is represented by a box, with the type name
in the top portion of the box, any data members that you care to describe
in the middle portion of the box, and the member functions (the functions
that belong to this object, which receive any messages you send to that
object) in the bottom portion of the box. Often, only the name of the class
and the public member functions are shown in UML design diagrams, and
so the middle portion is not shown. If you’re interested only in the class
name, then the bottom portion doesn’t need to be shown, either.
The hidden
implementation
It is helpful to break up the playing field into class creators (those who
create new data types) and client programmers3 (the class consumers
who use the data types in their applications). The goal of the client
programmer is to collect a toolbox full of classes to use for rapid
application development. The goal of the class creator is to build a class
that exposes only what’s necessary to the client programmer and keeps
everything else hidden. Why? Because if it’s hidden, the client
programmer can’t use it, which means that the class creator can change
the hidden portion at will without worrying about the impact to anyone
else. The hidden portion usually represents the tender insides of an object
that could easily be corrupted by a careless or uninformed client
programmer, so hiding the implementation reduces program bugs. The
concept of implementation hiding cannot be overemphasized.
In any relationship it’s important to have boundaries that are respected by
all parties involved. When you create a library, you establish a
relationship with the client programmer, who is also a programmer, but
3 I’m indebted to my friend Scott Meyers for this term.
Chapter 1: Introduction to Objects
35
one who is putting together an application by using your library, possibly
to build a bigger library.
If all the members of a class are available to everyone, then the client
programmer can do anything with that class and there’s no way to enforce
rules. Even though you might really prefer that the client programmer not
directly manipulate some of the members of your class, without access
control there’s no way to prevent it. Everything’s naked to the world.
So the first reason for access control is to keep client programmers’ hands
off portions they shouldn’t touch—parts that are necessary for the internal
machinations of the data type but not part of the interface that users need
in order to solve their particular problems. This is actually a service to
users because they can easily see what’s important to them and what they
can ignore.
The second reason for access control is to allow the library designer to
change the internal workings of the class without worrying about how it
will affect the client programmer. For example, you might implement a
particular class in a simple fashion to ease development, and then later
discover that you need to rewrite it in order to make it run faster. If the
interface and implementation are clearly separated and protected, you
can accomplish this easily.
Java uses three explicit keywords to set the boundaries in a class: public,
private, and protected. Their use and meaning are quite
straightforward. These access specifiers determine who can use the
definitions that follow. public means the following definitions are
available to everyone. The private keyword, on the other hand, means
that no one can access those definitions except you, the creator of the
type, inside member functions of that type. private is a brick wall
between you and the client programmer. If someone tries to access a
private member, they’ll get a compile-time error. protected acts like
private, with the exception that an inheriting class has access to
protected members, but not private members. Inheritance will be
introduced shortly.
Java also has a “default” access, which comes into play if you don’t use
one of the aforementioned specifiers. This is sometimes called “friendly”
access because classes can access the friendly members of other classes in
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the same package, but outside of the package those same friendly
members appear to be private.
Reusing the
implementation
Once a class has been created and tested, it should (ideally) represent a
useful unit of code. It turns out that this reusability is not nearly so easy to
achieve as many would hope; it takes experience and insight to produce a
good design. But once you have such a design, it begs to be reused. Code
reuse is one of the greatest advantages that object-oriented programming
languages provide.
The simplest way to reuse a class is to just use an object of that class
directly, but you can also place an object of that class inside a new class.
We call this “creating a member object.” Your new class can be made up of
any number and type of other objects, in any combination that you need
to achieve the functionality desired in your new class. Because you are
composing a new class from existing classes, this concept is called
composition (or more generally, aggregation). Composition is often
referred to as a “has-a” relationship, as in “a car has an engine.”
Car
Engine
(The above UML diagram indicates composition with the filled diamond,
which states there is one car. I will typically use a simpler form: just a line,
without the diamond, to indicate an association.4)
Composition comes with a great deal of flexibility. The member objects of
your new class are usually private, making them inaccessible to the client
programmers who are using the class. This allows you to change those
4 This is usually enough detail for most diagrams, and you don’t need to get specific about
whether you’re using aggregation or composition.
Chapter 1: Introduction to Objects
37
members without disturbing existing client code. You can also change the
member objects at run-time, to dynamically change the behavior of your
program. Inheritance, which is described next, does not have this
flexibility since the compiler must place compile-time restrictions on
classes created with inheritance.
Because inheritance is so important in object-oriented programming it is
often highly emphasized, and the new programmer can get the idea that
inheritance should be used everywhere. This can result in awkward and
overly complicated designs. Instead, you should first look to composition
when creating new classes, since it is simpler and more flexible. If you
take this approach, your designs will be cleaner. Once you’ve had some
experience, it will be reasonably obvious when you need inheritance.
Inheritance:
reusing the interface
By itself, the idea of an object is a convenient tool. It allows you to
package data and functionality together by concept, so you can represent
an appropriate problem-space idea rather than being forced to use the
idioms of the underlying machine. These concepts are expressed as
fundamental units in the programming language by using the class
keyword.
It seems a pity, however, to go to all the trouble to create a class and then
be forced to create a brand new one that might have similar functionality.
It’s nicer if we can take the existing class, clone it, and then make
additions and modifications to the clone. This is effectively what you get
with inheritance, with the exception that if the original class (called the
base or super or parent class) is changed, the modified “clone” (called the
derived or inherited or sub or child class) also reflects those changes.
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Base
Derived
(The arrow in the above UML diagram points from the derived class to the
base class. As you will see, there can be more than one derived class.)
A type does more than describe the constraints on a set of objects; it also
has a relationship with other types. Two types can have characteristics
and behaviors in common, but one type may contain more characteristics
than another and may also handle more messages (or handle them
differently). Inheritance expresses this similarity between types using the
concept of base types and derived types. A base type contains all of the
characteristics and behaviors that are shared among the types derived
from it. You create a base type to represent the core of your ideas about
some objects in your system. From the base type, you derive other types to
express the different ways that this core can be realized.
For example, a trash-recycling machine sorts pieces of trash. The base
type is “trash,” and each piece of trash has a weight, a value, and so on,
and can be shredded, melted, or decomposed. From this, more specific
types of trash are derived that may have additional characteristics (a
bottle has a color) or behaviors (an aluminum can may be crushed, a steel
can is magnetic). In addition, some behaviors may be different (the value
of paper depends on its type and condition). Using inheritance, you can
build a type hierarchy that expresses the problem you’re trying to solve in
terms of its types.
A second example is the classic “shape” example, perhaps used in a
computer-aided design system or game simulation. The base type is
“shape,” and each shape has a size, a color, a position, and so on. Each
shape can be drawn, erased, moved, colored, etc. From this, specific types
of shapes are derived (inherited): circle, square, triangle, and so on, each
of which may have additional characteristics and behaviors. Certain
Chapter 1: Introduction to Objects
39
shapes can be flipped, for example. Some behaviors may be different, such
as when you want to calculate the area of a shape. The type hierarchy
embodies both the similarities and differences between the shapes.
Shape
draw()
erase()
move()
getColor()
setColor()
Circle
Square
Triangle
Casting the solution in the same terms as the problem is tremendously
beneficial because you don’t need a lot of intermediate models to get from
a description of the problem to a description of the solution. With objects,
the type hierarchy is the primary model, so you go directly from the
description of the system in the real world to the description of the system
in code. Indeed, one of the difficulties people have with object-oriented
design is that it’s too simple to get from the beginning to the end. A mind
trained to look for complex solutions is often stumped by this simplicity at
first.
When you inherit from an existing type, you create a new type. This new
type contains not only all the members of the existing type (although the
private ones are hidden away and inaccessible), but more important, it
duplicates the interface of the base class. That is, all the messages you can
send to objects of the base class you can also send to objects of the derived
class. Since we know the type of a class by the messages we can send to it,
this means that the derived class is the same type as the base class. In the
previous example, “a circle is a shape.” This type equivalence via
inheritance is one of the fundamental gateways in understanding the
meaning of object-oriented programming.
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Since both the base class and derived class have the same interface, there
must be some implementation to go along with that interface. That is,
there must be some code to execute when an object receives a particular
message. If you simply inherit a class and don’t do anything else, the
methods from the base-class interface come right along into the derived
class. That means objects of the derived class have not only the same type,
they also have the same behavior, which isn’t particularly interesting.
You have two ways to differentiate your new derived class from the
original base class. The first is quite straightforward: You simply add
brand new functions to the derived class. These new functions are not
part of the base class interface. This means that the base class simply
didn’t do as much as you wanted it to, so you added more functions. This
simple and primitive use for inheritance is, at times, the perfect solution
to your problem. However, you should look closely for the possibility that
your base class might also need these additional functions. This process of
discovery and iteration of your design happens regularly in object-
oriented programming.
Shape
draw()
erase()
move()
getColor()
setColor()
Circle
Square
Triangle
FlipVertical()
FlipHorizontal()
Although inheritance may sometimes imply (especially in Java, where the
keyword that indicates inheritance is extends) that you are going to add
new functions to the interface, that’s not necessarily true. The second and
Chapter 1: Introduction to Objects
41
more important way to differentiate your new class is to change the
behavior of an existing base-class function. This is referred to as
overriding that function.
Shape
draw()
erase()
move()
getColor()
setColor()
Circle
Square
Triangle
draw()
draw()
draw()
erase()
erase()
erase()
To override a function, you simply create a new definition for the function
in the derived class. You’re saying, “I’m using the same interface function
here, but I want it to do something different for my new type.”
Is-a vs. is-like-a relationships
There’s a certain debate that can occur about inheritance: Should
inheritance override only base-class functions (and not add new member
functions that aren’t in the base class)? This would mean that the derived
type is exactly the same type as the base class since it has exactly the same
interface. As a result, you can exactly substitute an object of the derived
class for an object of the base class. This can be thought of as pure
substitution, and it’s often referred to as the substitution principle. In a
sense, this is the ideal way to treat inheritance. We often refer to the
relationship between the base class and derived classes in this case as an
is-a relationship, because you can say “a circle is a shape.” A test for
inheritance is to determine whether you can state the is-a relationship
about the classes and have it make sense.
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There are times when you must add new interface elements to a derived
type, thus extending the interface and creating a new type. The new type
can still be substituted for the base type, but the substitution isn’t perfect
because your new functions are not accessible from the base type. This
can be described as an is-like-a5 relationship; the new type has the
interface of the old type but it also contains other functions, so you can’t
really say it’s exactly the same. For example, consider an air conditioner.
Suppose your house is wired with all the controls for cooling; that is, it has
an interface that allows you to control cooling. Imagine that the air
conditioner breaks down and you replace it with a heat pump, which can
both heat and cool. The heat pump is-like-an air conditioner, but it can do
more. Because the control system of your house is designed only to
control cooling, it is restricted to communication with the cooling part of
the new object. The interface of the new object has been extended, and the
existing system doesn’t know about anything except the original interface.
Thermostat
Controls
Cooling System
lowerTemperature()
cool()
Air Conditioner
Heat Pump
cool()
cool()
heat()
Of course, once you see this design it becomes clear that the base class
“cooling system” is not general enough, and should be renamed to
“temperature control system” so that it can also include heating—at which
point the substitution principle will work. However, the diagram above is
an example of what can happen in design and in the real world.
5 My term.
Chapter 1: Introduction to Objects
43
When you see the substitution principle it’s easy to feel like this approach
(pure substitution) is the only way to do things, and in fact it is nice if
your design works out that way. But you’ll find that there are times when
it’s equally clear that you must add new functions to the interface of a
derived class. With inspection both cases should be reasonably obvious.
Interchangeable objects
with polymorphism
When dealing with type hierarchies, you often want to treat an object not
as the specific type that it is, but instead as its base type. This allows you
to write code that doesn’t depend on specific types. In the shape example,
functions manipulate generic shapes without respect to whether they’re
circles, squares, triangles, or some shape that hasn’t even been defined
yet. All shapes can be drawn, erased, and moved, so these functions
simply send a message to a shape object; they don’t worry about how the
object copes with the message.
Such code is unaffected by the addition of new types, and adding new
types is the most common way to extend an object-oriented program to
handle new situations. For example, you can derive a new subtype of
shape called pentagon without modifying the functions that deal only with
generic shapes. This ability to extend a program easily by deriving new
subtypes is important because it greatly improves designs while reducing
the cost of software maintenance.
There’s a problem, however, with attempting to treat derived-type objects
as their generic base types (circles as shapes, bicycles as vehicles,
cormorants as birds, etc.). If a function is going to tell a generic shape to
draw itself, or a generic vehicle to steer, or a generic bird to move, the
compiler cannot know at compile-time precisely what piece of code will be
executed. That’s the whole point—when the message is sent, the
programmer doesn’t want to know what piece of code will be executed;
the draw function can be applied equally to a circle, a square, or a triangle,
and the object will execute the proper code depending on its specific type.
If you don’t have to know what piece of code will be executed, then when
you add a new subtype, the code it executes can be different without
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requiring changes to the function call. Therefore, the compiler cannot
know precisely what piece of code is executed, so what does it do? For
example, in the following diagram the BirdController object just works
with generic Bird objects, and does not know what exact type they are.
This is convenient from BirdController’s perspective because it doesn’t
have to write special code to determine the exact type of Bird it’s working
with, or that Bird’s behavior. So how does it happen that, when move( )
is called while ignoring the specific type of Bird, the right behavior will
occur (a Goose runs, flies, or swims, and a Penguin runs or swims)?
BirdController
Bird
What happens
reLocate()
when move() is
move()
called?
Goose
Penguin
move()
move()
The answer is the primary twist in object-oriented programming: the
compiler cannot make a function call in the traditional sense. The
function call generated by a non-OOP compiler causes what is called early
binding, a term you may not have heard before because you’ve never
thought about it any other way. It means the compiler generates a call to a
specific function name, and the linker resolves this call to the absolute
address of the code to be executed. In OOP, the program cannot
determine the address of the code until run-time, so some other scheme is
necessary when a message is sent to a generic object.
To solve the problem, object-oriented languages use the concept of late
binding. When you send a message to an object, the code being called isn’t
determined until run-time. The compiler does ensure that the function
exists and performs type checking on the arguments and return value (a
language in which this isn’t true is called weakly typed), but it doesn’t
know the exact code to execute.
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To perform late binding, Java uses a special bit of code in lieu of the
absolute call. This code calculates the address of the function body, using
information stored in the object (this process is covered in great detail in
Chapter 7). Thus, each object can behave differently according to the
contents of that special bit of code. When you send a message to an object,
the object actually does figure out what to do with that message.
In some languages (C++, in particular) you must explicitly state that you
want a function to have the flexibility of late-binding properties. In these
languages, by default, member functions are not dynamically bound. This
caused problems, so in Java dynamic binding is the default and you don’t
need to remember to add any extra keywords in order to get
polymorphism.
Consider the shape example. The family of classes (all based on the same
uniform interface) was diagrammed earlier in this chapter. To
demonstrate polymorphism, we want to write a single piece of code that
ignores the specific details of type and talks only to the base class. That
code is decoupled from type-specific information, and thus is simpler to
write and easier to understand. And, if a new type—a Hexagon, for
example—is added through inheritance, the code you write will work just
as well for the new type of Shape as it did on the existing types. Thus, the
program is extensible.
If you write a method in Java (as you will soon learn how to do):
void doStuff(Shape s) {
s.erase();
// ...
s.draw();
}
This function speaks to any Shape, so it is independent of the specific
type of object that it’s drawing and erasing. If in some other part of the
program we use the doStuff( ) function:
Circle c = new Circle();
Triangle t = new Triangle();
Line l = new Line();
doStuff(c);
doStuff(t);
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doStuff(l);
The calls to doStuff( ) automatically work correctly, regardless of the
exact type of the object.
This is actually a pretty amazing trick. Consider the line:
doStuff(c);
What’s happening here is that a Circle is being passed into a function
that’s expecting a Shape. Since a Circle is a Shape it can be treated as
one by doStuff( ). That is, any message that doStuff( ) can send to a
Shape, a Circle can accept. So it is a completely safe and logical thing to
do.
We call this process of treating a derived type as though it were its base
type upcasting. The name cast is used in the sense of casting into a mold
and the up comes from the way the inheritance diagram is typically
arranged, with the base type at the top and the derived classes fanning out
downward. Thus, casting to a base type is moving up the inheritance
diagram: “upcasting.”
Shape
"Upcasting"
Circle
Square
Triangle
An object-oriented program contains some upcasting somewhere, because
that’s how you decouple yourself from knowing about the exact type
you’re working with. Look at the code in doStuff( ):
s.erase();
// ...
s.draw();
Notice that it doesn’t say “If you’re a Circle, do this, if you’re a Square,
do that, etc.” If you write that kind of code, which checks for all the
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possible types that a Shape can actually be, it’s messy and you need to
change it every time you add a new kind of Shape. Here, you just say
“You’re a shape, I know you can erase( ) and draw( ) yourself, do it, and
take care of the details correctly.”
What’s impressive about the code in doStuff( ) is that, somehow, the
right thing happens. Calling draw( ) for Circle causes different code to
be executed than when calling draw( ) for a Square or a Line, but when
the draw( ) message is sent to an anonymous Shape, the correct
behavior occurs based on the actual type of the Shape. This is amazing
because, as mentioned earlier, when the Java compiler is compiling the
code for doStuff( ), it cannot know exactly what types it is dealing with.
So ordinarily, you’d expect it to end up calling the version of erase( ) and
draw( ) for the base class Shape, and not for the specific Circle,
Square, or Line. And yet the right thing happens because of
polymorphism. The compiler and run-time system handle the details; all
you need to know is that it happens, and more important how to design
with it. When you send a message to an object, the object will do the right
thing, even when upcasting is involved.
Abstract base classes and
interfaces
Often in a design, you want the base class to present only an interface for
its derived classes. That is, you don’t want anyone to actually create an
object of the base class, only to upcast to it so that its interface can be
used. This is accomplished by making that class abstract using the
abstract keyword. If anyone tries to make an object of an abstract class,
the compiler prevents them. This is a tool to enforce a particular design.
You can also use the abstract keyword to describe a method that hasn’t
been implemented yet—as a stub indicating “here is an interface function
for all types inherited from this class, but at this point I don’t have any
implementation for it.” An abstract method may be created only inside
an abstract class. When the class is inherited, that method must be
implemented, or the inheriting class becomes abstract as well. Creating
an abstract method allows you to put a method in an interface without
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being forced to provide a possibly meaningless body of code for that
method.
The interface keyword takes the concept of an abstract class one step
further by preventing any function definitions at all. The interface is a
very handy and commonly used tool, as it provides the perfect separation
of interface and implementation. In addition, you can combine many
interfaces together, if you wish, whereas inheriting from multiple regular
classes or abstract classes is not possible.
Object landscapes and
lifetimes
Technically, OOP is just about abstract data typing, inheritance, and
polymorphism, but other issues can be at least as important. The
remainder of this section will cover these issues.
One of the most important factors is the way objects are created and
destroyed. Where is the data for an object and how is the lifetime of the
object controlled? There are different philosophies at work here. C++
takes the approach that control of efficiency is the most important issue,
so it gives the programmer a choice. For maximum run-time speed, the
storage and lifetime can be determined while the program is being
written, by placing the objects on the stack (these are sometimes called
automatic or scoped variables) or in the static storage area. This places a
priority on the speed of storage allocation and release, and control of
these can be very valuable in some situations. However, you sacrifice
flexibility because you must know the exact quantity, lifetime, and type of
objects while you're writing the program. If you are trying to solve a more
general problem such as computer-aided design, warehouse management,
or air-traffic control, this is too restrictive.
The second approach is to create objects dynamically in a pool of memory
called the heap. In this approach, you don't know until run-time how
many objects you need, what their lifetime is, or what their exact type is.
Those are determined at the spur of the moment while the program is
running. If you need a new object, you simply make it on the heap at the
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point that you need it. Because the storage is managed dynamically, at
run-time, the amount of time required to allocate storage on the heap is
significantly longer than the time to create storage on the stack. (Creating
storage on the stack is often a single assembly instruction to move the
stack pointer down, and another to move it back up.) The dynamic
approach makes the generally logical assumption that objects tend to be
complicated, so the extra overhead of finding storage and releasing that
storage will not have an important impact on the creation of an object. In
addition, the greater flexibility is essential to solve the general
programming problem.
Java uses the second approach, exclusively6. Every time you want to
create an object, you use the new keyword to build a dynamic instance of
that object.
There's another issue, however, and that's the lifetime of an object. With
languages that allow objects to be created on the stack, the compiler
determines how long the object lasts and can automatically destroy it.
However, if you create it on the heap the compiler has no knowledge of its
lifetime. In a language like C++, you must determine programmatically
when to destroy the object, which can lead to memory leaks if you don’t
do it correctly (and this is a common problem in C++ programs). Java
provides a feature called a garbage collector that automatically discovers
when an object is no longer in use and destroys it. A garbage collector is
much more convenient because it reduces the number of issues that you
must track and the code you must write. More important, the garbage
collector provides a much higher level of insurance against the insidious
problem of memory leaks (which has brought many a C++ project to its
knees).
The rest of this section looks at additional factors concerning object
lifetimes and landscapes.
6 Primitive types, which you’ll learn about later, are a special case.
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Collections and iterators
If you don’t know how many objects you’re going to need to solve a
particular problem, or how long they will last, you also don’t know how to
store those objects. How can you know how much space to create for
those objects? You can’t, since that information isn’t known until run-
time.
The solution to most problems in object-oriented design seems flippant:
you create another type of object. The new type of object that solves this
particular problem holds references to other objects. Of course, you can
do the same thing with an array, which is available in most languages. But
there’s more. This new object, generally called a container (also called a
collection, but the Java library uses that term in a different sense so this
book will use “container”), will expand itself whenever necessary to
accommodate everything you place inside it. So you don’t need to know
how many objects you’re going to hold in a container. Just create a
container object and let it take care of the details.
Fortunately, a good OOP language comes with a set of containers as part
of the package. In C++, it’s part of the Standard C++ Library and is
sometimes called the Standard Template Library (STL). Object Pascal has
containers in its Visual Component Library (VCL). Smalltalk has a very
complete set of containers. Java also has containers in its standard
library. In some libraries, a generic container is considered good enough
for all needs, and in others (Java, for example) the library has different
types of containers for different needs: a vector (called an ArrayList in
Java) for consistent access to all elements, and a linked list for consistent
insertion at all elements, for example, so you can choose the particular
type that fits your needs. Container libraries may also include sets,
queues, hash tables, trees, stacks, etc.
All containers have some way to put things in and get things out; there are
usually functions to add elements to a container, and others to fetch those
elements back out. But fetching elements can be more problematic,
because a single-selection function is restrictive. What if you want to
manipulate or compare a set of elements in the container instead of just
one?
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The solution is an iterator, which is an object whose job is to select the
elements within a container and present them to the user of the iterator.
As a class, it also provides a level of abstraction. This abstraction can be
used to separate the details of the container from the code that’s accessing
that container. The container, via the iterator, is abstracted to be simply a
sequence. The iterator allows you to traverse that sequence without
worrying about the underlying structure—that is, whether it’s an
ArrayList, a LinkedList, a Stack, or something else. This gives you the
flexibility to easily change the underlying data structure without
disturbing the code in your program. Java began (in version 1.0 and 1.1)
with a standard iterator, called Enumeration, for all of its container
classes. Java 2 has added a much more complete container library that
contains an iterator called Iterator that does more than the older
Enumeration.
From a design standpoint, all you really want is a sequence that can be
manipulated to solve your problem. If a single type of sequence satisfied
all of your needs, there’d be no reason to have different kinds. There are
two reasons that you need a choice of containers. First, containers provide
different types of interfaces and external behavior. A stack has a different
interface and behavior than that of a queue, which is different from that of
a set or a list. One of these might provide a more flexible solution to your
problem than the other. Second, different containers have different
efficiencies for certain operations. The best example is an ArrayList and
a LinkedList. Both are simple sequences that can have identical
interfaces and external behaviors. But certain operations can have
radically different costs. Randomly accessing elements in an ArrayList is
a constant-time operation; it takes the same amount of time regardless of
the element you select. However, in a LinkedList it is expensive to move
through the list to randomly select an element, and it takes longer to find
an element that is further down the list. On the other hand, if you want to
insert an element in the middle of a sequence, it’s much cheaper in a
LinkedList than in an ArrayList. These and other operations have
different efficiencies depending on the underlying structure of the
sequence. In the design phase, you might start with a LinkedList and,
when tuning for performance, change to an ArrayList. Because of the
abstraction via iterators, you can change from one to the other with
minimal impact on your code.
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In the end, remember that a container is only a storage cabinet to put
objects in. If that cabinet solves all of your needs, it doesn’t really matter
how it is implemented (a basic concept with most types of objects). If
you’re working in a programming environment that has built-in overhead
due to other factors, then the cost difference between an ArrayList and a
LinkedList might not matter. You might need only one type of sequence.
You can even imagine the “perfect” container abstraction, which can
automatically change its underlying implementation according to the way
it is used.
The singly rooted hierarchy
One of the issues in OOP that has become especially prominent since the
introduction of C++ is whether all classes should ultimately be inherited
from a single base class. In Java (as with virtually all other OOP
languages) the answer is “yes” and the name of this ultimate base class is
simply Object. It turns out that the benefits of the singly rooted hierarchy
are many.
All objects in a singly rooted hierarchy have an interface in common, so
they are all ultimately the same type. The alternative (provided by C++) is
that you don’t know that everything is the same fundamental type. From a
backward-compatibility standpoint this fits the model of C better and can
be thought of as less restrictive, but when you want to do full-on object-
oriented programming you must then build your own hierarchy to provide
the same convenience that’s built into other OOP languages. And in any
new class library you acquire, some other incompatible interface will be
used. It requires effort (and possibly multiple inheritance) to work the
new interface into your design. Is the extra “flexibility” of C++ worth it? If
you need it—if you have a large investment in C—it’s quite valuable. If
you’re starting from scratch, other alternatives such as Java can often be
more productive.
All objects in a singly rooted hierarchy (such as Java provides) can be
guaranteed to have certain functionality. You know you can perform
certain basic operations on every object in your system. A singly rooted
hierarchy, along with creating all objects on the heap, greatly simplifies
argument passing (one of the more complex topics in C++).
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A singly rooted hierarchy makes it much easier to implement a garbage
collector (which is conveniently built into Java). The necessary support
can be installed in the base class, and the garbage collector can thus send
the appropriate messages to every object in the system. Without a singly
rooted hierarchy and a system to manipulate an object via a reference, it is
difficult to implement a garbage collector.
Since run-time type information is guaranteed to be in all objects, you’ll
never end up with an object whose type you cannot determine. This is
especially important with system level operations, such as exception
handling, and to allow greater flexibility in programming.
Collection libraries and support for
easy collection use
Because a container is a tool that you’ll use frequently, it makes sense to
have a library of containers that are built in a reusable fashion, so you can
take one off the shelf and plug it into your program. Java provides such a
library, which should satisfy most needs.
Downcasting vs. templates/generics
To make these containers reusable, they hold the one universal type in
Java that was previously mentioned: Object. The singly rooted hierarchy
means that everything is an Object, so a container that holds Objects
can hold anything. This makes containers easy to reuse.
To use such a container, you simply add object references to it, and later
ask for them back. But, since the container holds only Objects, when you
add your object reference into the container it is upcast to Object, thus
losing its identity. When you fetch it back, you get an Object reference,
and not a reference to the type that you put in. So how do you turn it back
into something that has the useful interface of the object that you put into
the container?
Here, the cast is used again, but this time you’re not casting up the
inheritance hierarchy to a more general type, you cast down the hierarchy
to a more specific type. This manner of casting is called downcasting.
With upcasting, you know, for example, that a Circle is a type of Shape
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so it’s safe to upcast, but you don’t know that an Object is necessarily a
Circle or a Shape so it’s hardly safe to downcast unless you know that’s
what you’re dealing with.
It’s not completely dangerous, however, because if you downcast to the
wrong thing you’ll get a run-time error called an exception, which will be
described shortly. When you fetch object references from a container,
though, you must have some way to remember exactly what they are so
you can perform a proper downcast.
Downcasting and the run-time checks require extra time for the running
program, and extra effort from the programmer. Wouldn’t it make sense
to somehow create the container so that it knows the types that it holds,
eliminating the need for the downcast and a possible mistake? The
solution is parameterized types, which are classes that the compiler can
automatically customize to work with particular types. For example, with
a parameterized container, the compiler could customize that container so
that it would accept only Shapes and fetch only Shapes.
Parameterized types are an important part of C++, partly because C++
has no singly rooted hierarchy. In C++, the keyword that implements
parameterized types is “template.” Java currently has no parameterized
types since it is possible for it to get by—however awkwardly—using the
singly rooted hierarchy. However, a current proposal for parameterized
types uses a syntax that is strikingly similar to C++ templates.
The housekeeping dilemma: who
should clean up?
Each object requires resources in order to exist, most notably memory.
When an object is no longer needed it must be cleaned up so that these
resources are released for reuse. In simple programming situations the
question of how an object is cleaned up doesn’t seem too challenging: you
create the object, use it for as long as it’s needed, and then it should be
destroyed. It’s not hard, however, to encounter situations in which the
situation is more complex.
Suppose, for example, you are designing a system to manage air traffic for
an airport. (The same model might also work for managing crates in a
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warehouse, or a video rental system, or a kennel for boarding pets.) At
first it seems simple: make a container to hold airplanes, then create a
new airplane and place it in the container for each airplane that enters the
air-traffic-control zone. For cleanup, simply delete the appropriate
airplane object when a plane leaves the zone.
But perhaps you have some other system to record data about the planes;
perhaps data that doesn’t require such immediate attention as the main
controller function. Maybe it’s a record of the flight plans of all the small
planes that leave the airport. So you have a second container of small
planes, and whenever you create a plane object you also put it in this
second container if it’s a small plane. Then some background process
performs operations on the objects in this container during idle moments.
Now the problem is more difficult: how can you possibly know when to
destroy the objects? When you’re done with the object, some other part of
the system might not be. This same problem can arise in a number of
other situations, and in programming systems (such as C++) in which you
must explicitly delete an object when you’re done with it this can become
quite complex.
With Java, the garbage collector is designed to take care of the problem of
releasing the memory (although this doesn’t include other aspects of
cleaning up an object). The garbage collector “knows” when an object is
no longer in use, and it then automatically releases the memory for that
object. This (combined with the fact that all objects are inherited from the
single root class Object and that you can create objects only one way, on
the heap) makes the process of programming in Java much simpler than
programming in C++. You have far fewer decisions to make and hurdles
to overcome.
Garbage collectors vs. efficiency and
flexibility
If all this is such a good idea, why didn’t they do the same thing in C++?
Well of course there’s a price you pay for all this programming
convenience, and that price is run-time overhead. As mentioned before, in
C++ you can create objects on the stack, and in this case they’re
automatically cleaned up (but you don’t have the flexibility of creating as
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many as you want at run-time). Creating objects on the stack is the most
efficient way to allocate storage for objects and to free that storage.
Creating objects on the heap can be much more expensive. Always
inheriting from a base class and making all function calls polymorphic
also exacts a small toll. But the garbage collector is a particular problem
because you never quite know when it’s going to start up or how long it
will take. This means that there’s an inconsistency in the rate of execution
of a Java program, so you can’t use it in certain situations, such as when
the rate of execution of a program is uniformly critical. (These are
generally called real time programs, although not all real time
programming problems are this stringent.)
The designers of the C++ language, trying to woo C programmers (and
most successfully, at that), did not want to add any features to the
language that would impact the speed or the use of C++ in any situation
where programmers might otherwise choose C. This goal was realized, but
at the price of greater complexity when programming in C++. Java is
simpler than C++, but the trade-off is in efficiency and sometimes
applicability. For a significant portion of programming problems,
however, Java is the superior choice.
Exception handling:
dealing with errors
Ever since the beginning of programming languages, error handling has
been one of the most difficult issues. Because it’s so hard to design a good
error handling scheme, many languages simply ignore the issue, passing
the problem on to library designers who come up with halfway measures
that can work in many situations but can easily be circumvented,
generally by just ignoring them. A major problem with most error
handling schemes is that they rely on programmer vigilance in following
an agreed-upon convention that is not enforced by the language. If the
programmer is not vigilant—often the case if they are in a hurry—these
schemes can easily be forgotten.
Exception handling wires error handling directly into the programming
language and sometimes even the operating system. An exception is an
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object that is “thrown” from the site of the error and can be “caught” by an
appropriate exception handler designed to handle that particular type of
error. It’s as if exception handling is a different, parallel path of execution
that can be taken when things go wrong. And because it uses a separate
execution path, it doesn’t need to interfere with your normally executing
code. This makes that code simpler to write since you aren’t constantly
forced to check for errors. In addition, a thrown exception is unlike an
error value that’s returned from a function or a flag that’s set by a function
in order to indicate an error condition—these can be ignored. An
exception cannot be ignored, so it’s guaranteed to be dealt with at some
point. Finally, exceptions provide a way to reliably recover from a bad
situation. Instead of just exiting you are often able to set things right and
restore the execution of a program, which produces much more robust
programs.
Java’s exception handling stands out among programming languages,
because in Java, exception handling was wired in from the beginning and
you’re forced to use it. If you don’t write your code to properly handle
exceptions, you’ll get a compile-time error message. This guaranteed
consistency makes error handling much easier.
It’s worth noting that exception handling isn’t an object-oriented feature,
although in object-oriented languages the exception is normally
represented with an object. Exception handling existed before object-
oriented languages.
Multithreading
A fundamental concept in computer programming is the idea of handling
more than one task at a time. Many programming problems require that
the program be able to stop what it’s doing, deal with some other
problem, and then return to the main process. The solution has been
approached in many ways. Initially, programmers with low-level
knowledge of the machine wrote interrupt service routines and the
suspension of the main process was initiated through a hardware
interrupt. Although this worked well, it was difficult and nonportable, so
it made moving a program to a new type of machine slow and expensive.
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Sometimes interrupts are necessary for handling time-critical tasks, but
there’s a large class of problems in which you’re simply trying to partition
the problem into separately running pieces so that the whole program can
be more responsive. Within a program, these separately running pieces
are called threads, and the general concept is called multithreading. A
common example of multithreading is the user interface. By using
threads, a user can press a button and get a quick response rather than
being forced to wait until the program finishes its current task.
Ordinarily, threads are just a way to allocate the time of a single
processor. But if the operating system supports multiple processors, each
thread can be assigned to a different processor and they can truly run in
parallel. One of the convenient features of multithreading at the language
level is that the programmer doesn’t need to worry about whether there
are many processors or just one. The program is logically divided into
threads and if the machine has more than one processor then the program
runs faster, without any special adjustments.
All this makes threading sound pretty simple. There is a catch: shared
resources. If you have more than one thread running that’s expecting to
access the same resource you have a problem. For example, two processes
can’t simultaneously send information to a printer. To solve the problem,
resources that can be shared, such as the printer, must be locked while
they are being used. So a thread locks a resource, completes its task, and
then releases the lock so that someone else can use the resource.
Java’s threading is built into the language, which makes a complicated
subject much simpler. The threading is supported on an object level, so
one thread of execution is represented by one object. Java also provides
limited resource locking. It can lock the memory of any object (which is,
after all, one kind of shared resource) so that only one thread can use it at
a time. This is accomplished with the synchronized keyword. Other
types of resources must be locked explicitly by the programmer, typically
by creating an object to represent the lock that all threads must check
before accessing that resource.
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Persistence
When you create an object, it exists for as long as you need it, but under
no circumstances does it exist when the program terminates. While this
makes sense at first, there are situations in which it would be incredibly
useful if an object could exist and hold its information even while the
program wasn’t running. Then the next time you started the program, the
object would be there and it would have the same information it had the
previous time the program was running. Of course, you can get a similar
effect by writing the information to a file or to a database, but in the spirit
of making everything an object it would be quite convenient to be able to
declare an object persistent and have all the details taken care of for you.
Java provides support for “lightweight persistence,” which means that you
can easily store objects on disk and later retrieve them. The reason it’s
“lightweight” is that you’re still forced to make explicit calls to do the
storage and retrieval. In addition, JavaSpaces (described in Chapter 15)
provide for a kind of persistent storage of objects. In some future release
more complete support for persistence might appear.
Java and the Internet
If Java is, in fact, yet another computer programming language, you may
question why it is so important and why it is being promoted as a
revolutionary step in computer programming. The answer isn’t
immediately obvious if you’re coming from a traditional programming
perspective. Although Java is very useful for solving traditional stand-
alone programming problems, it is also important because it will solve
programming problems on the World Wide Web.
What is the Web?
The Web can seem a bit of a mystery at first, with all this talk of “surfing,”
“presence,” and “home pages.” There has even been a growing reaction
against “Internet-mania,” questioning the economic value and outcome of
such a sweeping movement. It’s helpful to step back and see what it really
is, but to do this you must understand client/server systems, another
aspect of computing that’s full of confusing issues.
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Client/Server computing
The primary idea of a client/server system is that you have a central
repository of information—some kind of data, often in a database—that
you want to distribute on demand to some set of people or machines. A
key to the client/server concept is that the repository of information is
centrally located so that it can be changed and so that those changes will
propagate out to the information consumers. Taken together, the
information repository, the software that distributes the information, and
the machine(s) where the information and software reside is called the
server. The software that resides on the remote machine, communicates
with the server, fetches the information, processes it, and then displays it
on the remote machine is called the client.
The basic concept of client/server computing, then, is not so complicated.
The problems arise because you have a single server trying to serve many
clients at once. Generally, a database management system is involved so
the designer “balances” the layout of data into tables for optimal use. In
addition, systems often allow a client to insert new information into a
server. This means you must ensure that one client’s new data doesn’t
walk over another client’s new data, or that data isn’t lost in the process of
adding it to the database. (This is called transaction processing.) As client
software changes, it must be built, debugged, and installed on the client
machines, which turns out to be more complicated and expensive than
you might think. It’s especially problematic to support multiple types of
computers and operating systems. Finally, there’s the all-important
performance issue: you might have hundreds of clients making requests
of your server at any one time, and so any small delay is crucial. To
minimize latency, programmers work hard to offload processing tasks,
often to the client machine, but sometimes to other machines at the server
site, using so-called middleware. (Middleware is also used to improve
maintainability.)
The simple idea of distributing information to people has so many layers
of complexity in implementing it that the whole problem can seem
hopelessly enigmatic. And yet it’s crucial: client/server computing
accounts for roughly half of all programming activities. It’s responsible for
everything from taking orders and credit-card transactions to the
distribution of any kind of data—stock market, scientific, government, you
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name it. What we’ve come up with in the past is individual solutions to
individual problems, inventing a new solution each time. These were hard
to create and hard to use, and the user had to learn a new interface for
each one. The entire client/server problem needs to be solved in a big
way.
The Web as a giant server
The Web is actually one giant client/server system. It’s a bit worse than
that, since you have all the servers and clients coexisting on a single
network at once. You don’t need to know that, since all you care about is
connecting to and interacting with one server at a time (even though you
might be hopping around the world in your search for the correct server).
Initially it was a simple one-way process. You made a request of a server
and it handed you a file, which your machine’s browser software (i.e., the
client) would interpret by formatting onto your local machine. But in
short order people began wanting to do more than just deliver pages from
a server. They wanted full client/server capability so that the client could
feed information back to the server, for example, to do database lookups
on the server, to add new information to the server, or to place an order
(which required more security than the original systems offered). These
are the changes we’ve been seeing in the development of the Web.
The Web browser was a big step forward: the concept that one piece of
information could be displayed on any type of computer without change.
However, browsers were still rather primitive and rapidly bogged down by
the demands placed on them. They weren’t particularly interactive, and
tended to clog up both the server and the Internet because any time you
needed to do something that required programming you had to send
information back to the server to be processed. It could take many
seconds or minutes to find out you had misspelled something in your
request. Since the browser was just a viewer it couldn’t perform even the
simplest computing tasks. (On the other hand, it was safe, since it couldn’t
execute any programs on your local machine that might contain bugs or
viruses.)
To solve this problem, different approaches have been taken. To begin
with, graphics standards have been enhanced to allow better animation
and video within browsers. The remainder of the problem can be solved
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only by incorporating the ability to run programs on the client end, under
the browser. This is called client-side programming.
Client-side programming
The Web’s initial server-browser design provided for interactive content,
but the interactivity was completely provided by the server. The server
produced static pages for the client browser, which would simply interpret
and display them. Basic HTML contains simple mechanisms for data
gathering: text-entry boxes, check boxes, radio boxes, lists and drop-down
lists, as well as a button that can only be programmed to reset the data on
the form or “submit” the data on the form back to the server. This
submission passes through the Common Gateway Interface (CGI)
provided on all Web servers. The text within the submission tells CGI
what to do with it. The most common action is to run a program located
on the server in a directory that’s typically called “cgi-bin.” (If you watch
the address window at the top of your browser when you push a button on
a Web page, you can sometimes see “cgi-bin” within all the gobbledygook
there.) These programs can be written in most languages. Perl is a
common choice because it is designed for text manipulation and is
interpreted, so it can be installed on any server regardless of processor or
operating system.
Many powerful Web sites today are built strictly on CGI, and you can in
fact do nearly anything with it. However, Web sites built on CGI programs
can rapidly become overly complicated to maintain, and there is also the
problem of response time. The response of a CGI program depends on
how much data must be sent, as well as the load on both the server and
the Internet. (On top of this, starting a CGI program tends to be slow.)
The initial designers of the Web did not foresee how rapidly this
bandwidth would be exhausted for the kinds of applications people
developed. For example, any sort of dynamic graphing is nearly
impossible to perform with consistency because a GIF file must be created
and moved from the server to the client for each version of the graph. And
you’ve no doubt had direct experience with something as simple as
validating the data on an input form. You press the submit button on a
page; the data is shipped back to the server; the server starts a CGI
program that discovers an error, formats an HTML page informing you of
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the error, and then sends the page back to you; you must then back up a
page and try again. Not only is this slow, it’s inelegant.
The solution is client-side programming. Most machines that run Web
browsers are powerful engines capable of doing vast work, and with the
original static HTML approach they are sitting there, just idly waiting for
the server to dish up the next page. Client-side programming means that
the Web browser is harnessed to do whatever work it can, and the result
for the user is a much speedier and more interactive experience at your
Web site.
The problem with discussions of client-side programming is that they
aren’t very different from discussions of programming in general. The
parameters are almost the same, but the platform is different: a Web
browser is like a limited operating system. In the end, you must still
program, and this accounts for the dizzying array of problems and
solutions produced by client-side programming. The rest of this section
provides an overview of the issues and approaches in client-side
programming.
Plug-ins
One of the most significant steps forward in client-side programming is
the development of the plug-in. This is a way for a programmer to add
new functionality to the browser by downloading a piece of code that
plugs itself into the appropriate spot in the browser. It tells the browser
“from now on you can perform this new activity.” (You need to download
the plug-in only once.) Some fast and powerful behavior is added to
browsers via plug-ins, but writing a plug-in is not a trivial task, and isn’t
something you’d want to do as part of the process of building a particular
site. The value of the plug-in for client-side programming is that it allows
an expert programmer to develop a new language and add that language
to a browser without the permission of the browser manufacturer. Thus,
plug-ins provide a “back door” that allows the creation of new client-side
programming languages (although not all languages are implemented as
plug-ins).
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Scripting languages
Plug-ins resulted in an explosion of scripting languages. With a scripting
language you embed the source code for your client-side program directly
into the HTML page, and the plug-in that interprets that language is
automatically activated while the HTML page is being displayed. Scripting
languages tend to be reasonably easy to understand and, because they are
simply text that is part of an HTML page, they load very quickly as part of
the single server hit required to procure that page. The trade-off is that
your code is exposed for everyone to see (and steal). Generally, however,
you aren’t doing amazingly sophisticated things with scripting languages
so this is not too much of a hardship.
This points out that the scripting languages used inside Web browsers are
really intended to solve specific types of problems, primarily the creation
of richer and more interactive graphical user interfaces (GUIs). However,
a scripting language might solve 80 percent of the problems encountered
in client-side programming. Your problems might very well fit completely
within that 80 percent, and since scripting languages can allow easier and
faster development, you should probably consider a scripting language
before looking at a more involved solution such as Java or ActiveX
programming.
The most commonly discussed browser scripting languages are JavaScript
(which has nothing to do with Java; it’s named that way just to grab some
of Java’s marketing momentum), VBScript (which looks like Visual
Basic), and Tcl/Tk, which comes from the popular cross-platform GUI-
building language. There are others out there, and no doubt more in
development.
JavaScript is probably the most commonly supported. It comes built into
both Netscape Navigator and the Microsoft Internet Explorer (IE). In
addition, there are probably more JavaScript books available than there
are for the other browser languages, and some tools automatically create
pages using JavaScript. However, if you’re already fluent in Visual Basic
or Tcl/Tk, you’ll be more productive using those scripting languages
rather than learning a new one. (You’ll have your hands full dealing with
the Web issues already.)
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Java
If a scripting language can solve 80 percent of the client-side
programming problems, what about the other 20 percent—the “really
hard stuff?” The most popular solution today is Java. Not only is it a
powerful programming language built to be secure, cross-platform, and
international, but Java is being continually extended to provide language
features and libraries that elegantly handle problems that are difficult in
traditional programming languages, such as multithreading, database
access, network programming, and distributed computing. Java allows
client-side programming via the applet.
An applet is a mini-program that will run only under a Web browser. The
applet is downloaded automatically as part of a Web page (just as, for
example, a graphic is automatically downloaded). When the applet is
activated it executes a program. This is part of its beauty—it provides you
with a way to automatically distribute the client software from the server
at the time the user needs the client software, and no sooner. The user
gets the latest version of the client software without fail and without
difficult reinstallation. Because of the way Java is designed, the
programmer needs to create only a single program, and that program
automatically works with all computers that have browsers with built-in
Java interpreters. (This safely includes the vast majority of machines.)
Since Java is a full-fledged programming language, you can do as much
work as possible on the client before and after making requests of the
server. For example, you won’t need to send a request form across the
Internet to discover that you’ve gotten a date or some other parameter
wrong, and your client computer can quickly do the work of plotting data
instead of waiting for the server to make a plot and ship a graphic image
back to you. Not only do you get the immediate win of speed and
responsiveness, but the general network traffic and load on servers can be
reduced, preventing the entire Internet from slowing down.
One advantage a Java applet has over a scripted program is that it’s in
compiled form, so the source code isn’t available to the client. On the
other hand, a Java applet can be decompiled without too much trouble,
but hiding your code is often not an important issue. Two other factors
can be important. As you will see later in this book, a compiled Java
applet can comprise many modules and take multiple server “hits”
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(accesses) to download. (In Java 1.1 and higher this is minimized by Java
archives, called JAR files, that allow all the required modules to be
packaged together and compressed for a single download.) A scripted
program will just be integrated into the Web page as part of its text (and
will generally be smaller and reduce server hits). This could be important
to the responsiveness of your Web site. Another factor is the all-important
learning curve. Regardless of what you’ve heard, Java is not a trivial
language to learn. If you’re a Visual Basic programmer, moving to
VBScript will be your fastest solution, and since it will probably solve
most typical client/server problems you might be hard pressed to justify
learning Java. If you’re experienced with a scripting language you will
certainly benefit from looking at JavaScript or VBScript before
committing to Java, since they might fit your needs handily and you’ll be
more productive sooner.
ActiveX
To some degree, the competitor to Java is Microsoft’s ActiveX, although it
takes a completely different approach. ActiveX was originally a Windows-
only solution, although it is now being developed via an independent
consortium to become cross-platform. Effectively, ActiveX says “if your
program connects to its environment just so, it can be dropped into a Web
page and run under a browser that supports ActiveX.” (IE directly
supports ActiveX and Netscape does so using a plug-in.) Thus, ActiveX
does not constrain you to a particular language. If, for example, you’re
already an experienced Windows programmer using a language such as
C++, Visual Basic, or Borland’s Delphi, you can create ActiveX
components with almost no changes to your programming knowledge.
ActiveX also provides a path for the use of legacy code in your Web pages.
Security
Automatically downloading and running programs across the Internet can
sound like a virus-builder’s dream. ActiveX especially brings up the
thorny issue of security in client-side programming. If you click on a Web
site, you might automatically download any number of things along with
the HTML page: GIF files, script code, compiled Java code, and ActiveX
components. Some of these are benign; GIF files can’t do any harm, and
scripting languages are generally limited in what they can do. Java was
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also designed to run its applets within a “sandbox” of safety, which
prevents it from writing to disk or accessing memory outside the sandbox.
ActiveX is at the opposite end of the spectrum. Programming with
ActiveX is like programming Windows—you can do anything you want. So
if you click on a page that downloads an ActiveX component, that
component might cause damage to the files on your disk. Of course,
programs that you load onto your computer that are not restricted to
running inside a Web browser can do the same thing. Viruses downloaded
from Bulletin-Board Systems (BBSs) have long been a problem, but the
speed of the Internet amplifies the difficulty.
The solution seems to be “digital signatures,” whereby code is verified to
show who the author is. This is based on the idea that a virus works
because its creator can be anonymous, so if you remove the anonymity
individuals will be forced to be responsible for their actions. This seems
like a good plan because it allows programs to be much more functional,
and I suspect it will eliminate malicious mischief. If, however, a program
has an unintentional destructive bug it will still cause problems.
The Java approach is to prevent these problems from occurring, via the
sandbox. The Java interpreter that lives on your local Web browser
examines the applet for any untoward instructions as the applet is being
loaded. In particular, the applet cannot write files to disk or erase files
(one of the mainstays of viruses). Applets are generally considered to be
safe, and since this is essential for reliable client/server systems, any bugs
in the Java language that allow viruses are rapidly repaired. (It’s worth
noting that the browser software actually enforces these security
restrictions, and some browsers allow you to select different security
levels to provide varying degrees of access to your system.)
You might be skeptical of this rather draconian restriction against writing
files to your local disk. For example, you may want to build a local
database or save data for later use offline. The initial vision seemed to be
that eventually everyone would get online to do anything important, but
that was soon seen to be impractical (although low-cost “Internet
appliances” might someday satisfy the needs of a significant segment of
users). The solution is the “signed applet” that uses public-key encryption
to verify that an applet does indeed come from where it claims it does. A
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signed applet can still trash your disk, but the theory is that since you can
now hold the applet creator accountable they won’t do vicious things. Java
provides a framework for digital signatures so that you will eventually be
able to allow an applet to step outside the sandbox if necessary.
Digital signatures have missed an important issue, which is the speed that
people move around on the Internet. If you download a buggy program
and it does something untoward, how long will it be before you discover
the damage? It could be days or even weeks. By then, how will you track
down the program that’s done it? And what good will it do you at that
point?
Internet vs. intranet
The Web is the most general solution to the client/server problem, so it
makes sense that you can use the same technology to solve a subset of the
problem, in particular the classic client/server problem within a
company. With traditional client/server approaches you have the problem
of multiple types of client computers, as well as the difficulty of installing
new client software, both of which are handily solved with Web browsers
and client-side programming. When Web technology is used for an
information network that is restricted to a particular company, it is
referred to as an intranet. Intranets provide much greater security than
the Internet, since you can physically control access to the servers within
your company. In terms of training, it seems that once people understand
the general concept of a browser it’s much easier for them to deal with
differences in the way pages and applets look, so the learning curve for
new kinds of systems seems to be reduced.
The security problem brings us to one of the divisions that seems to be
automatically forming in the world of client-side programming. If your
program is running on the Internet, you don’t know what platform it will
be working under, and you want to be extra careful that you don’t
disseminate buggy code. You need something cross-platform and secure,
like a scripting language or Java.
If you’re running on an intranet, you might have a different set of
constraints. It’s not uncommon that your machines could all be
Intel/Windows platforms. On an intranet, you’re responsible for the
quality of your own code and can repair bugs when they’re discovered. In
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addition, you might already have a body of legacy code that you’ve been
using in a more traditional client/server approach, whereby you must
physically install client programs every time you do an upgrade. The time
wasted in installing upgrades is the most compelling reason to move to
browsers, because upgrades are invisible and automatic. If you are
involved in such an intranet, the most sensible approach to take is the
shortest path that allows you to use your existing code base, rather than
trying to recode your programs in a new language.
When faced with this bewildering array of solutions to the client-side
programming problem, the best plan of attack is a cost-benefit analysis.
Consider the constraints of your problem and what would be the shortest
path to your solution. Since client-side programming is still
programming, it’s always a good idea to take the fastest development
approach for your particular situation. This is an aggressive stance to
prepare for inevitable encounters with the problems of program
development.
Server-side programming
This whole discussion has ignored the issue of server-side programming.
What happens when you make a request of a server? Most of the time the
request is simply “send me this file.” Your browser then interprets the file
in some appropriate fashion: as an HTML page, a graphic image, a Java
applet, a script program, etc. A more complicated request to a server
generally involves a database transaction. A common scenario involves a
request for a complex database search, which the server then formats into
an HTML page and sends to you as the result. (Of course, if the client has
more intelligence via Java or a scripting language, the raw data can be
sent and formatted at the client end, which will be faster and less load on
the server.) Or you might want to register your name in a database when
you join a group or place an order, which will involve changes to that
database. These database requests must be processed via some code on
the server side, which is generally referred to as server-side programming.
Traditionally, server-side programming has been performed using Perl
and CGI scripts, but more sophisticated systems have been appearing.
These include Java-based Web servers that allow you to perform all your
server-side programming in Java by writing what are called servlets.
Servlets and their offspring, JSPs, are two of the most compelling reasons
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that companies who develop Web sites are moving to Java, especially
because they eliminate the problems of dealing with differently abled
browsers.
A separate arena: applications
Much of the brouhaha over Java has been over applets. Java is actually a
general-purpose programming language that can solve any type of
problem—at least in theory. And as pointed out previously, there might be
more effective ways to solve most client/server problems. When you move
out of the applet arena (and simultaneously release the restrictions, such
as the one against writing to disk) you enter the world of general-purpose
applications that run standalone, without a Web browser, just like any
ordinary program does. Here, Java’s strength is not only in its portability,
but also its programmability. As you’ll see throughout this book, Java has
many features that allow you to create robust programs in a shorter
period than with previous programming languages.
Be aware that this is a mixed blessing. You pay for the improvements
through slower execution speed (although there is significant work going
on in this area—JDK 1.3, in particular, introduces the so-called “hotspot”
performance improvements). Like any language, Java has built-in
limitations that might make it inappropriate to solve certain types of
programming problems. Java is a rapidly evolving language, however, and
as each new release comes out it becomes more and more attractive for
solving larger sets of problems.
Analysis and design
The object-oriented paradigm is a new and different way of thinking
about programming. Many folks have trouble at first knowing how to
approach an OOP project. Once you know that everything is supposed to
be an object, and as you learn to think more in an object-oriented style,
you can begin to create “good” designs that take advantage of all the
benefits that OOP has to offer.
A method (often called a methodology) is a set of processes and heuristics
used to break down the complexity of a programming problem. Many
OOP methods have been formulated since the dawn of object-oriented
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programming. This section will give you a feel for what you’re trying to
accomplish when using a method.
Especially in OOP, methodology is a field of many experiments, so it is
important to understand what problem the method is trying to solve
before you consider adopting one. This is particularly true with Java, in
which the programming language is intended to reduce the complexity
(compared to C) involved in expressing a program. This may in fact
alleviate the need for ever-more-complex methodologies. Instead, simple
methodologies may suffice in Java for a much larger class of problems
than you could handle using simple methodologies with procedural
languages.
It’s also important to realize that the term “methodology” is often too
grand and promises too much. Whatever you do now when you design
and write a program is a method. It may be your own method, and you
may not be conscious of doing it, but it is a process you go through as you
create. If it is an effective process, it may need only a small tune-up to
work with Java. If you are not satisfied with your productivity and the way
your programs turn out, you may want to consider adopting a formal
method, or choosing pieces from among the many formal methods.
While you’re going through the development process, the most important
issue is this: Don’t get lost. It’s easy to do. Most of the analysis and design
methods are intended to solve the largest of problems. Remember that
most projects don’t fit into that category, so you can usually have
successful analysis and design with a relatively small subset of what a
method recommends7. But some sort of process, no matter how limited,
will generally get you on your way in a much better fashion than simply
beginning to code.
It’s also easy to get stuck, to fall into “analysis paralysis,” where you feel
like you can’t move forward because you haven’t nailed down every little
detail at the current stage. Remember, no matter how much analysis you
do, there are some things about a system that won’t reveal themselves
7 An excellent example of this is UML Distilled, 2nd edition, by Martin Fowler (Addison-
Wesley 2000), which reduces the sometimes-overwhelming UML process to a manageable
subset.
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until design time, and more things that won’t reveal themselves until
you’re coding, or not even until a program is up and running. Because of
this, it’s crucial to move fairly quickly through analysis and design, and to
implement a test of the proposed system.
This point is worth emphasizing. Because of the history we’ve had with
procedural languages, it is commendable that a team will want to proceed
carefully and understand every minute detail before moving to design and
implementation. Certainly, when creating a DBMS, it pays to understand
a customer’s needs thoroughly. But a DBMS is in a class of problems that
is very well-posed and well-understood; in many such programs, the
database structure is the problem to be tackled. The class of programming
problem discussed in this chapter is of the “wild-card” (my term) variety,
in which the solution isn’t simply re-forming a well-known solution, but
instead involves one or more “wild-card factors”—elements for which
there is no well-understood previous solution, and for which research is
necessary8. Attempting to thoroughly analyze a wild-card problem before
moving into design and implementation results in analysis paralysis
because you don’t have enough information to solve this kind of problem
during the analysis phase. Solving such a problem requires iteration
through the whole cycle, and that requires risk-taking behavior (which
makes sense, because you’re trying to do something new and the potential
rewards are higher). It may seem like the risk is compounded by “rushing”
into a preliminary implementation, but it can instead reduce the risk in a
wild-card project because you’re finding out early whether a particular
approach to the problem is viable. Product development is risk
management.
It’s often proposed that you “build one to throw away.” With OOP, you
may still throw part of it away, but because code is encapsulated into
classes, during the first pass you will inevitably produce some useful class
designs and develop some worthwhile ideas about the system design that
do not need to be thrown away. Thus, the first rapid pass at a problem not
8 My rule of thumb for estimating such projects: If there’s more than one wild card, don’t
even try to plan how long it’s going to take or how much it will cost until you’ve created a
working prototype. There are too many degrees of freedom.
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only produces critical information for the next analysis, design, and
implementation pass, it also creates a code foundation.
That said, if you’re looking at a methodology that contains tremendous
detail and suggests many steps and documents, it’s still difficult to know
when to stop. Keep in mind what you’re trying to discover:
1.
What are the objects? (How do you partition your project into its
component parts?)
2.
What are their interfaces? (What messages do you need to send to
each object?)
If you come up with nothing more than the objects and their interfaces,
then you can write a program. For various reasons you might need more
descriptions and documents than this, but you can’t get away with any
less.
The process can be undertaken in five phases, and a Phase 0 that is just
the initial commitment to using some kind of structure.
Phase 0: Make a plan
You must first decide what steps you’re going to have in your process. It
sounds simple (in fact, all of this sounds simple), and yet people often
don’t make this decision before they start coding. If your plan is “let’s
jump in and start coding,” fine. (Sometimes that’s appropriate when you
have a well-understood problem.) At least agree that this is the plan.
You might also decide at this phase that some additional process structure
is necessary, but not the whole nine yards. Understandably, some
programmers like to work in “vacation mode,” in which no structure is
imposed on the process of developing their work; “It will be done when
it’s done.” This can be appealing for a while, but I’ve found that having a
few milestones along the way helps to focus and galvanize your efforts
around those milestones instead of being stuck with the single goal of
“finish the project.” In addition, it divides the project into more bite-sized
pieces and makes it seem less threatening (plus the milestones offer more
opportunities for celebration).
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When I began to study story structure (so that I will someday write a
novel) I was initially resistant to the idea of structure, feeling that I wrote
best when I simply let it flow onto the page. But I later realized that when
I write about computers the structure is clear enough to me that I don’t
have to think about it very much. But I still structure my work, albeit only
semi-consciously in my head. Even if you think that your plan is to just
start coding, you still somehow go through the subsequent phases while
asking and answering certain questions.
The mission statement
Any system you build, no matter how complicated, has a fundamental
purpose; the business that it’s in, the basic need that it satisfies. If you can
look past the user interface, the hardware- or system-specific details, the
coding algorithms and the efficiency problems, you will eventually find
the core of its being—simple and straightforward. Like the so-called high
concept from a Hollywood movie, you can describe it in one or two
sentences. This pure description is the starting point.
The high concept is quite important because it sets the tone for your
project; it’s a mission statement. You won’t necessarily get it right the first
time (you may be in a later phase of the project before it becomes
completely clear), but keep trying until it feels right. For example, in an
air-traffic control system you may start out with a high concept focused on
the system that you’re building: “The tower program keeps track of the
aircraft.” But consider what happens when you shrink the system to a very
small airfield; perhaps there’s only a human controller, or none at all. A
more useful model won’t concern the solution you’re creating as much as
it describes the problem: “Aircraft arrive, unload, service and reload, then
depart.”
Phase 1: What are we making?
In the previous generation of program design (called procedural design),
this is called “creating the requirements analysis and system
specification.” These, of course, were places to get lost; intimidatingly
named documents that could become big projects in their own right. Their
intention was good, however. The requirements analysis says “Make a list
of the guidelines we will use to know when the job is done and the
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customer is satisfied.” The system specification says “Here’s a description
of what the program will do (not how) to satisfy the requirements.” The
requirements analysis is really a contract between you and the customer
(even if the customer works within your company, or is some other object
or system). The system specification is a top-level exploration into the
problem and in some sense a discovery of whether it can be done and how
long it will take. Since both of these will require consensus among people
(and because they will usually change over time), I think it’s best to keep
them as bare as possible—ideally, to lists and basic diagrams—to save
time. You might have other constraints that require you to expand them
into bigger documents, but by keeping the initial document small and
concise, it can be created in a few sessions of group brainstorming with a
leader who dynamically creates the description. This not only solicits
input from everyone, it also fosters initial buy-in and agreement by
everyone on the team. Perhaps most importantly, it can kick off a project
with a lot of enthusiasm.
It’s necessary to stay focused on the heart of what you’re trying to
accomplish in this phase: determine what the system is supposed to do.
The most valuable tool for this is a collection of what are called “use
cases.” Use cases identify key features in the system that will reveal some
of the fundamental classes you’ll be using. These are essentially
descriptive answers to questions like9:
• “Who will use this system?”
• “What can those actors do with the system?”
• “How does this actor do that with this system?”
• “How else might this work if someone else were doing this, or if
the same actor had a different objective?” (to reveal variations)
• “What problems might happen while doing this with the system?”
(to reveal exceptions)
If you are designing an auto-teller, for example, the use case for a
particular aspect of the functionality of the system is able to describe what
the auto-teller does in every possible situation. Each of these “situations”
9 Thanks for help from James H Jarrett.
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is referred to as a scenario, and a use case can be considered a collection
of scenarios. You can think of a scenario as a question that starts with:
“What does the system do if…?” For example, “What does the auto-teller
do if a customer has just deposited a check within the last 24 hours, and
there’s not enough in the account without the check having cleared to
provide a desired withdrawal?”
Use case diagrams are intentionally simple to prevent you from getting
bogged down in system implementation details prematurely:
Bank
Make
Deposit
Uses
Make
Teller
Withdrawal
Customer
Get Account
Balance
Transfer
Between
Accounts
ATM
Each stick person represents an “actor,” which is typically a human or
some other kind of free agent. (These can even be other computer
systems, as is the case with “ATM.”) The box represents the boundary of
your system. The ellipses represent the use cases, which are descriptions
of valuable work that can be performed with the system. The lines
between the actors and the use cases represent the interactions.
It doesn’t matter how the system is actually implemented, as long as it
looks like this to the user.
A use case does not need to be terribly complex, even if the underlying
system is complex. It is only intended to show the system as it appears to
the user. For example:
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Greenhouse
Maintain
Growing
Temperature
Gardener
The use cases produce the requirements specifications by determining all
the interactions that the user may have with the system. You try to
discover a full set of use cases for your system, and once you’ve done that
you have the core of what the system is supposed to do. The nice thing
about focusing on use cases is that they always bring you back to the
essentials and keep you from drifting off into issues that aren’t critical for
getting the job done. That is, if you have a full set of use cases, you can
describe your system and move onto the next phase. You probably won’t
get it all figured out perfectly on the first try, but that’s OK. Everything
will reveal itself in time, and if you demand a perfect system specification
at this point you’ll get stuck.
If you do get stuck, you can kick-start this phase by using a rough
approximation tool: describe the system in a few paragraphs and then
look for nouns and verbs. The nouns can suggest actors, context of the use
case (e.g., “lobby”), or artifacts manipulated in the use case. Verbs can
suggest interactions between actors and use cases, and specify steps
within the use case. You’ll also discover that nouns and verbs produce
objects and messages during the design phase (and note that use cases
describe interactions between subsystems, so the “noun and verb”
technique can be used only as a brainstorming tool as it does not generate
use cases) 10.
The boundary between a use case and an actor can point out the existence
of a user interface, but it does not define such a user interface. For a
process of defining and creating user interfaces, see Software for Use by
10 More information on use cases can be found in Applying Use Cases by Schneider &
Winters (Addison-Wesley 1998) and Use Case Driven Object Modeling with UML by
Rosenberg (Addison-Wesley 1999).
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Larry Constantine and Lucy Lockwood, (Addison-Wesley Longman, 1999)
or go to www.ForUse.com.
Although it’s a black art, at this point some kind of basic scheduling is
important. You now have an overview of what you’re building, so you’ll
probably be able to get some idea of how long it will take. A lot of factors
come into play here. If you estimate a long schedule then the company
might decide not to build it (and thus use their resources on something
more reasonable—that’s a good thing). Or a manager might have already
decided how long the project should take and will try to influence your
estimate. But it’s best to have an honest schedule from the beginning and
deal with the tough decisions early. There have been a lot of attempts to
come up with accurate scheduling techniques (much like techniques to
predict the stock market), but probably the best approach is to rely on
your experience and intuition. Get a gut feeling for how long it will really
take, then double that and add 10 percent. Your gut feeling is probably
correct; you can get something working in that time. The “doubling” will
turn that into something decent, and the 10 percent will deal with the
final polishing and details11. However you want to explain it, and
regardless of the moans and manipulations that happen when you reveal
such a schedule, it just seems to work out that way.
Phase 2: How will we build it?
In this phase you must come up with a design that describes what the
classes look like and how they will interact. An excellent technique in
determining classes and interactions is the Class-Responsibility-
Collaboration (CRC) card. Part of the value of this tool is that it’s so low-
tech: you start out with a set of blank 3 x 5 cards, and you write on them.
Each card represents a single class, and on the card you write:
1.
The name of the class. It’s important that this name capture the
essence of what the class does, so that it makes sense at a glance.
11 My personal take on this has changed lately. Doubling and adding 10 percent will give
you a reasonably accurate estimate (assuming there are not too many wild-card factors),
but you still have to work quite diligently to finish in that time. If you want time to really
make it elegant and to enjoy yourself in the process, the correct multiplier is more like
three or four times, I believe.
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2.
The “responsibilities” of the class: what it should do. This can
typically be summarized by just stating the names of the member
functions (since those names should be descriptive in a good
design), but it does not preclude other notes. If you need to seed
the process, look at the problem from a lazy programmer’s
standpoint: What objects would you like to magically appear to
solve your problem?
3.
The “collaborations” of the class: what other classes does it interact
with? “Interact” is an intentionally broad term; it could mean
aggregation or simply that some other object exists that will
perform services for an object of the class. Collaborations should
also consider the audience for this class. For example, if you create
a class Firecracker, who is going to observe it, a Chemist or a
Spectator? The former will want to know what chemicals go into
the construction, and the latter will respond to the colors and
shapes released when it explodes.
You may feel like the cards should be bigger because of all the information
you’d like to get on them, but they are intentionally small, not only to keep
your classes small but also to keep you from getting into too much detail
too early. If you can’t fit all you need to know about a class on a small
card, the class is too complex (either you’re getting too detailed, or you
should create more than one class). The ideal class should be understood
at a glance. The idea of CRC cards is to assist you in coming up with a first
cut of the design so that you can get the big picture and then refine your
design.
One of the great benefits of CRC cards is in communication. It’s best done
real time, in a group, without computers. Each person takes responsibility
for several classes (which at first have no names or other information).
You run a live simulation by solving one scenario at a time, deciding
which messages are sent to the various objects to satisfy each scenario. As
you go through this process, you discover the classes that you need along
with their responsibilities and collaborations, and you fill out the cards as
you do this. When you’ve moved through all the use cases, you should
have a fairly complete first cut of your design.
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Before I began using CRC cards, the most successful consulting
experiences I had when coming up with an initial design involved
standing in front of a team—who hadn’t built an OOP project before—and
drawing objects on a whiteboard. We talked about how the objects should
communicate with each other, and erased some of them and replaced
them with other objects. Effectively, I was managing all the “CRC cards”
on the whiteboard. The team (who knew what the project was supposed to
do) actually created the design; they “owned” the design rather than
having it given to them. All I was doing was guiding the process by asking
the right questions, trying out the assumptions, and taking the feedback
from the team to modify those assumptions. The true beauty of the
process was that the team learned how to do object-oriented design not by
reviewing abstract examples, but by working on the one design that was
most interesting to them at that moment: theirs.
Once you’ve come up with a set of CRC cards, you may want to create a
more formal description of your design using UML12. You don’t need to
use UML, but it can be helpful, especially if you want to put up a diagram
on the wall for everyone to ponder, which is a good idea. An alternative to
UML is a textual description of the objects and their interfaces, or,
depending on your programming language, the code itself13.
UML also provides an additional diagramming notation for describing the
dynamic model of your system. This is helpful in situations in which the
state transitions of a system or subsystem are dominant enough that they
need their own diagrams (such as in a control system). You may also need
to describe the data structures, for systems or subsystems in which data is
a dominant factor (such as a database).
You’ll know you’re done with Phase 2 when you have described the objects
and their interfaces. Well, most of them—there are usually a few that slip
through the cracks and don’t make themselves known until Phase 3. But
that’s OK. All you are concerned with is that you eventually discover all of
your objects. It’s nice to discover them early in the process, but OOP
12 For starters, I recommend the aforementioned UML Distilled, 2nd edition.
13 Python (www.Python.org) is often used as “executable pseudocode.”
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provides enough structure so that it’s not so bad if you discover them
later. In fact, the design of an object tends to happen in five stages,
throughout the process of program development.
Five stages of object design
The design life of an object is not limited to the time when you’re writing
the program. Instead, the design of an object appears over a sequence of
stages. It’s helpful to have this perspective because you stop expecting
perfection right away; instead, you realize that the understanding of what
an object does and what it should look like happens over time. This view
also applies to the design of various types of programs; the pattern for a
particular type of program emerges through struggling again and again
with that problem (This is chronicled in the book Thinking in Patterns
with Java, downloadable at www.BruceEckel.com). Objects, too, have
their patterns that emerge through understanding, use, and reuse.
1. Object discovery. This stage occurs during the initial analysis of a
program. Objects may be discovered by looking for external factors and
boundaries, duplication of elements in the system, and the smallest
conceptual units. Some objects are obvious if you already have a set of
class libraries. Commonality between classes suggesting base classes and
inheritance may appear right away, or later in the design process.
2. Object assembly. As you’re building an object you’ll discover the
need for new members that didn’t appear during discovery. The internal
needs of the object may require other classes to support it.
3. System construction. Once again, more requirements for an
object may appear at this later stage. As you learn, you evolve your
objects. The need for communication and interconnection with other
objects in the system may change the needs of your classes or require new
classes. For example, you may discover the need for facilitator or helper
classes, such as a linked list, that contain little or no state information and
simply help other classes function.
4. System extension. As you add new features to a system you may
discover that your previous design doesn’t support easy system extension.
With this new information, you can restructure parts of the system,
possibly adding new classes or class hierarchies.
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5. Object reuse. This is the real stress test for a class. If someone tries
to reuse it in an entirely new situation, they’ll probably discover some
shortcomings. As you change a class to adapt to more new programs, the
general principles of the class will become clearer, until you have a truly
reusable type. However, don’t expect most objects from a system design to
be reusable—it is perfectly acceptable for the bulk of your objects to be
system-specific. Reusable types tend to be less common, and they must
solve more general problems in order to be reusable.
Guidelines for object development
These stages suggest some guidelines when thinking about developing
your classes:
1.
Let a specific problem generate a class, then let the class grow and
mature during the solution of other problems.
2.
Remember, discovering the classes you need (and their interfaces)
is the majority of the system design. If you already had those
classes, this would be an easy project.
3.
Don’t force yourself to know everything at the beginning; learn as
you go. This will happen anyway.
4.
Start programming; get something working so you can prove or
disprove your design. Don’t fear that you’ll end up with procedural-
style spaghetti code—classes partition the problem and help control
anarchy and entropy. Bad classes do not break good classes.
5.
Always keep it simple. Little clean objects with obvious utility are
better than big complicated interfaces. When decision points come
up, use an Occam’s Razor approach: Consider the choices and
select the one that is simplest, because simple classes are almost
always best. Start small and simple, and you can expand the class
interface when you understand it better. As time goes on, it’s
difficult to remove elements from a class.
Phase 3: Build the core
This is the initial conversion from the rough design into a compiling and
executing body of code that can be tested, and especially that will prove or
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disprove your architecture. This is not a one-pass process, but rather the
beginning of a series of steps that will iteratively build the system, as
you’ll see in Phase 4.
Your goal is to find the core of your system architecture that needs to be
implemented in order to generate a running system, no matter how
incomplete that system is in this initial pass. You’re creating a framework
that you can build on with further iterations. You’re also performing the
first of many system integrations and tests, and giving the stakeholders
feedback about what their system will look like and how it is progressing.
Ideally, you are also exposing some of the critical risks. You’ll probably
also discover changes and improvements that can be made to your
original architecture—things you would not have learned without
implementing the system.
Part of building the system is the reality check that you get from testing
against your requirements analysis and system specification (in whatever
form they exist). Make sure that your tests verify the requirements and
use cases. When the core of the system is stable, you’re ready to move on
and add more functionality.
Phase 4: Iterate the use cases
Once the core framework is running, each feature set you add is a small
project in itself. You add a feature set during an iteration, a reasonably
short period of development.
How big is an iteration? Ideally, each iteration lasts one to three weeks
(this can vary based on the implementation language). At the end of that
period, you have an integrated, tested system with more functionality
than it had before. But what’s particularly interesting is the basis for the
iteration: a single use case. Each use case is a package of related
functionality that you build into the system all at once, during one
iteration. Not only does this give you a better idea of what the scope of a
use case should be, but it also gives more validation to the idea of a use
case, since the concept isn’t discarded after analysis and design, but
instead it is a fundamental unit of development throughout the software-
building process.
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You stop iterating when you achieve target functionality or an external
deadline arrives and the customer can be satisfied with the current
version. (Remember, software is a subscription business.) Because the
process is iterative, you have many opportunities to ship a product rather
than a single endpoint; open-source projects work exclusively in an
iterative, high-feedback environment, which is precisely what makes them
successful.
An iterative development process is valuable for many reasons. You can
reveal and resolve critical risks early, the customers have ample
opportunity to change their minds, programmer satisfaction is higher,
and the project can be steered with more precision. But an additional
important benefit is the feedback to the stakeholders, who can see by the
current state of the product exactly where everything lies. This may
reduce or eliminate the need for mind-numbing status meetings and
increase the confidence and support from the stakeholders.
Phase 5: Evolution
This is the point in the development cycle that has traditionally been
called “maintenance,” a catch-all term that can mean everything from
“getting it to work the way it was really supposed to in the first place” to
“adding features that the customer forgot to mention” to the more
traditional “fixing the bugs that show up” and “adding new features as the
need arises.” So many misconceptions have been applied to the term
“maintenance” that it has taken on a slightly deceiving quality, partly
because it suggests that you’ve actually built a pristine program and all
you need to do is change parts, oil it, and keep it from rusting. Perhaps
there’s a better term to describe what’s going on.
I’ll use the term evolution14. That is, “You won’t get it right the first time,
so give yourself the latitude to learn and to go back and make changes.”
You might need to make a lot of changes as you learn and understand the
problem more deeply. The elegance you’ll produce if you evolve until you
get it right will pay off, both in the short and the long term. Evolution is
14 At least one aspect of evolution is covered in Martin Fowler’s book Refactoring:
improving the design of existing code (Addison-Wesley 1999), which uses Java examples
exclusively.
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where your program goes from good to great, and where those issues that
you didn’t really understand in the first pass become clear. It’s also where
your classes can evolve from single-project usage to reusable resources.
What it means to “get it right” isn’t just that the program works according
to the requirements and the use cases. It also means that the internal
structure of the code makes sense to you, and feels like it fits together
well, with no awkward syntax, oversized objects, or ungainly exposed bits
of code. In addition, you must have some sense that the program
structure will survive the changes that it will inevitably go through during
its lifetime, and that those changes can be made easily and cleanly. This is
no small feat. You must not only understand what you’re building, but
also how the program will evolve (what I call the vector of change).
Fortunately, object-oriented programming languages are particularly
adept at supporting this kind of continuing modification—the boundaries
created by the objects are what tend to keep the structure from breaking
down. They also allow you to make changes—ones that would seem
drastic in a procedural program—without causing earthquakes
throughout your code. In fact, support for evolution might be the most
important benefit of OOP.
With evolution, you create something that at least approximates what you
think you’re building, and then you kick the tires, compare it to your
requirements, and see where it falls short. Then you can go back and fix it
by redesigning and reimplementing the portions of the program that
didn’t work right15. You might actually need to solve the problem, or an
aspect of the problem, several times before you hit on the right solution.
(A study of Design Patterns is usually helpful here. You can find
information in Thinking in Patterns with Java, downloadable at
www.BruceEckel.com.)
15 This is something like “rapid prototyping,” where you were supposed to build a quick-
and-dirty version so that you could learn about the system, and then throw away your
prototype and build it right. The trouble with rapid prototyping is that people didn’t throw
away the prototype, but instead built upon it. Combined with the lack of structure in
procedural programming, this often leads to messy systems that are expensive to
maintain.
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Evolution also occurs when you build a system, see that it matches your
requirements, and then discover it wasn’t actually what you wanted.
When you see the system in operation, you find that you really wanted to
solve a different problem. If you think this kind of evolution is going to
happen, then you owe it to yourself to build your first version as quickly as
possible so you can find out if it is indeed what you want.
Perhaps the most important thing to remember is that by default—by
definition, really—if you modify a class, its super- and subclasses will still
function. You need not fear modification (especially if you have a built-in
set of unit tests to verify the correctness of your modifications).
Modification won’t necessarily break the program, and any change in the
outcome will be limited to subclasses and/or specific collaborators of the
class you change.
Plans pay off
Of course you wouldn’t build a house without a lot of carefully drawn
plans. If you build a deck or a dog house your plans won’t be so elaborate,
but you’ll probably still start with some kind of sketches to guide you on
your way. Software development has gone to extremes. For a long time,
people didn’t have much structure in their development, but then big
projects began failing. In reaction, we ended up with methodologies that
had an intimidating amount of structure and detail, primarily intended
for those big projects. These methodologies were too scary to use—it
looked like you’d spend all your time writing documents and no time
programming. (This was often the case.) I hope that what I’ve shown you
here suggests a middle path—a sliding scale. Use an approach that fits
your needs (and your personality). No matter how minimal you choose to
make it, some kind of plan will make a big improvement in your project as
opposed to no plan at all. Remember that, by most estimates, over 50
percent of projects fail (some estimates go up to 70 percent!).
By following a plan—preferably one that is simple and brief—and coming
up with design structure before coding, you’ll discover that things fall
together far more easily than if you dive in and start hacking. You’ll also
realize a great deal of satisfaction. It’s my experience that coming up with
an elegant solution is deeply satisfying at an entirely different level; it
feels closer to art than technology. And elegance always pays off; it’s not a
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frivolous pursuit. Not only does it give you a program that’s easier to build
and debug, but it’s also easier to understand and maintain, and that’s
where the financial value lies.
Extreme programming
I have studied analysis and design techniques, on and off, since I was in
graduate school. The concept of Extreme Programming (XP) is the most
radical, and delightful, that I’ve seen. You can find it chronicled in
Extreme Programming Explained by Kent Beck (Addison-Wesley, 2000)
and on the Web at www.xprogramming.com.
XP is both a philosophy about programming work and a set of guidelines
to do it. Some of these guidelines are reflected in other recent
methodologies, but the two most important and distinct contributions, in
my opinion, are “write tests first” and “pair programming.” Although he
argues strongly for the whole process, Beck points out that if you adopt
only these two practices you’ll greatly improve your productivity and
reliability.
Write tests first
Testing has traditionally been relegated to the last part of a project, after
you’ve “gotten everything working, but just to be sure.” It’s implicitly had
a low priority, and people who specialize in it have not been given a lot of
status and have often even been cordoned off in a basement, away from
the “real programmers.” Test teams have responded in kind, going so far
as to wear black clothing and cackling with glee whenever they break
something (to be honest, I’ve had this feeling myself when breaking
compilers).
XP completely revolutionizes the concept of testing by giving it equal (or
even greater) priority than the code. In fact, you write the tests before you
write the code that will be tested, and the tests stay with the code forever.
The tests must be executed successfully every time you do an integration
of the project (which is often, sometimes more than once a day).
Writing tests first has two extremely important effects.
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First, it forces a clear definition of the interface of a class. I’ve often
suggested that people “imagine the perfect class to solve a particular
problem” as a tool when trying to design the system. The XP testing
strategy goes further than that—it specifies exactly what the class must
look like, to the consumer of that class, and exactly how the class must
behave. In no uncertain terms. You can write all the prose, or create all
the diagrams you want, describing how a class should behave and what it
looks like, but nothing is as real as a set of tests. The former is a wish list,
but the tests are a contract that is enforced by the compiler and the
running program. It’s hard to imagine a more concrete description of a
class than the tests.
While creating the tests, you are forced to completely think out the class
and will often discover needed functionality that might be missed during
the thought experiments of UML diagrams, CRC cards, use cases, etc.
The second important effect of writing the tests first comes from running
the tests every time you do a build of your software. This activity gives you
the other half of the testing that’s performed by the compiler. If you look
at the evolution of programming languages from this perspective, you’ll
see that the real improvements in the technology have actually revolved
around testing. Assembly language checked only for syntax, but C
imposed some semantic restrictions, and these prevented you from
making certain types of mistakes. OOP languages impose even more
semantic restrictions, which if you think about it are actually forms of
testing. “Is this data type being used properly?” and “Is this function being
called properly?” are the kinds of tests that are being performed by the
compiler or run-time system. We’ve seen the results of having these tests
built into the language: people have been able to write more complex
systems, and get them to work, with much less time and effort. I’ve
puzzled over why this is, but now I realize it’s the tests: you do something
wrong, and the safety net of the built-in tests tells you there’s a problem
and points you to where it is.
But the built-in testing afforded by the design of the language can only go
so far. At some point, you must step in and add the rest of the tests that
produce a full suite (in cooperation with the compiler and run-time
system) that verifies all of your program. And, just like having a compiler
watching over your shoulder, wouldn’t you want these tests helping you
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right from the beginning? That’s why you write them first, and run them
automatically with every build of your system. Your tests become an
extension of the safety net provided by the language.
One of the things that I’ve discovered about the use of more and more
powerful programming languages is that I am emboldened to try more
brazen experiments, because I know that the language will keep me from
wasting my time chasing bugs. The XP test scheme does the same thing
for your entire project. Because you know your tests will always catch any
problems that you introduce (and you regularly add any new tests as you
think of them), you can make big changes when you need to without
worrying that you’ll throw the whole project into complete disarray. This
is incredibly powerful.
Pair programming
Pair programming goes against the rugged individualism that we’ve been
indoctrinated into from the beginning, through school (where we succeed
or fail on our own, and working with our neighbors is considered
“cheating”), and media, especially Hollywood movies in which the hero is
usually fighting against mindless conformity16. Programmers, too, are
considered paragons of individuality—“cowboy coders” as Larry
Constantine likes to say. And yet XP, which is itself battling against
conventional thinking, says that code should be written with two people
per workstation. And that this should be done in an area with a group of
workstations, without the barriers that the facilities-design people are so
fond of. In fact, Beck says that the first task of converting to XP is to arrive
with screwdrivers and Allen wrenches and take apart everything that gets
in the way.17 (This will require a manager who can deflect the ire of the
facilities department.)
16 Although this may be a more American perspective, the stories of Hollywood reach
everywhere.
17 Including (especially) the PA system. I once worked in a company that insisted on
broadcasting every phone call that arrived for every executive, and it constantly
interrupted our productivity (but the managers couldn’t begin to conceive of stifling such
an important service as the PA). Finally, when no one was looking I started snipping
speaker wires.
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The value of pair programming is that one person is actually doing the
coding while the other is thinking about it. The thinker keeps the big
picture in mind—not only the picture of the problem at hand, but the
guidelines of XP. If two people are working, it’s less likely that one of
them will get away with saying, “I don’t want to write the tests first,” for
example. And if the coder gets stuck, they can swap places. If both of them
get stuck, their musings may be overheard by someone else in the work
area who can contribute. Working in pairs keeps things flowing and on
track. Probably more important, it makes programming a lot more social
and fun.
I’ve begun using pair programming during the exercise periods in some of
my seminars and it seems to significantly improve everyone’s experience.
Why Java succeeds
The reason Java has been so successful is that the goal was to solve many
of the problems facing developers today. The goal of Java is improved
productivity. This productivity comes in many ways, but the language is
designed to aid you as much as possible, while hindering you as little as
possible with arbitrary rules or any requirement that you use a particular
set of features. Java is designed to be practical; Java language design
decisions were based on providing the maximum benefits to the
programmer.
Systems are easier
to express and understand
Classes designed to fit the problem tend to express it better. This means
that when you write the code, you’re describing your solution in the terms
of the problem space (“Put the grommet in the bin”) rather than the terms
of the computer, which is the solution space (“Set the bit in the chip that
means that the relay will close”). You deal with higher-level concepts and
can do much more with a single line of code.
The other benefit of this ease of expression is maintenance, which (if
reports can be believed) takes a huge portion of the cost over a program’s
lifetime. If a program is easier to understand, then it’s easier to maintain.
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This can also reduce the cost of creating and maintaining the
documentation.
Maximal leverage with libraries
The fastest way to create a program is to use code that’s already written: a
library. A major goal in Java is to make library use easier. This is
accomplished by casting libraries into new data types (classes), so that
bringing in a library means adding new types to the language. Because the
Java compiler takes care of how the library is used—guaranteeing proper
initialization and cleanup, and ensuring that functions are called
properly—you can focus on what you want the library to do, not how you
have to do it.
Error handling
Error handling in C is a notorious problem, and one that is often
ignored—finger-crossing is usually involved. If you’re building a large,
complex program, there’s nothing worse than having an error buried
somewhere with no clue as to where it came from. Java exception
handling is a way to guarantee that an error is noticed, and that
something happens as a result.
Programming in the large
Many traditional languages have built-in limitations to program size and
complexity. BASIC, for example, can be great for pulling together quick
solutions for certain classes of problems, but if the program gets more
than a few pages long, or ventures out of the normal problem domain of
that language, it’s like trying to swim through an ever-more viscous fluid.
There’s no clear line that tells you when your language is failing you, and
even if there were, you’d ignore it. You don’t say, “My BASIC program just
got too big; I’ll have to rewrite it in C!” Instead, you try to shoehorn a few
more lines in to add that one new feature. So the extra costs come
creeping up on you.
Java is designed to aid programming in the large—that is, to erase those
creeping-complexity boundaries between a small program and a large
one. You certainly don’t need to use OOP when you’re writing a “hello
world” style utility program, but the features are there when you need
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them. And the compiler is aggressive about ferreting out bug-producing
errors for small and large programs alike.
Strategies for transition
If you buy into OOP, your next question is probably, “How can I get my
manager/colleagues/department/peers to start using objects?” Think
about how you—one independent programmer—would go about learning
to use a new language and a new programming paradigm. You’ve done it
before. First comes education and examples; then comes a trial project to
give you a feel for the basics without doing anything too confusing. Then
comes a “real world” project that actually does something useful.
Throughout your first projects you continue your education by reading,
asking questions of experts, and trading hints with friends. This is the
approach many experienced programmers suggest for the switch to Java.
Switching an entire company will of course introduce certain group
dynamics, but it will help at each step to remember how one person would
do it.
Guidelines
Here are some guidelines to consider when making the transition to OOP
and Java:
1. Training
The first step is some form of education. Remember the company’s
investment in code, and try not to throw everything into disarray for six to
nine months while everyone puzzles over how interfaces work. Pick a
small group for indoctrination, preferably one composed of people who
are curious, work well together, and can function as their own support
network while they’re learning Java.
An alternative approach that is sometimes suggested is the education of
all company levels at once, including overview courses for strategic
managers as well as design and programming courses for project builders.
This is especially good for smaller companies making fundamental shifts
in the way they do things, or at the division level of larger companies.
Because the cost is higher, however, some may choose to start with
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project-level training, do a pilot project (possibly with an outside mentor),
and let the project team become the teachers for the rest of the company.
2. Low-risk project
Try a low-risk project first and allow for mistakes. Once you’ve gained
some experience, you can either seed other projects from members of this
first team or use the team members as an OOP technical support staff.
This first project may not work right the first time, so it should not be
mission-critical for the company. It should be simple, self-contained, and
instructive; this means that it should involve creating classes that will be
meaningful to the other programmers in the company when they get their
turn to learn Java.
3. Model from success
Seek out examples of good object-oriented design before starting from
scratch. There’s a good probability that someone has solved your problem
already, and if they haven’t solved it exactly you can probably apply what
you’ve learned about abstraction to modify an existing design to fit your
needs. This is the general concept of design patterns, covered in Thinking
in Patterns with Java, downloadable at www.BruceEckel.com.
4. Use existing class libraries
The primary economic motivation for switching to OOP is the easy use of
existing code in the form of class libraries (in particular, the Standard
Java libraries, which are covered throughout this book). The shortest
application development cycle will result when you can create and use
objects from off-the-shelf libraries. However, some new programmers
don’t understand this, are unaware of existing class libraries, or, through
fascination with the language, desire to write classes that may already
exist. Your success with OOP and Java will be optimized if you make an
effort to seek out and reuse other people’s code early in the transition
process.
5. Don’t rewrite existing code in Java
It is not usually the best use of your time to take existing, functional code
and rewrite it in Java. (If you must turn it into objects, you can interface
to the C or C++ code using the Java Native Interface, described in
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Appendix B.) There are incremental benefits, especially if the code is
slated for reuse. But chances are you aren’t going to see the dramatic
increases in productivity that you hope for in your first few projects unless
that project is a new one. Java and OOP shine best when taking a project
from concept to reality.
Management obstacles
If you’re a manager, your job is to acquire resources for your team, to
overcome barriers to your team’s success, and in general to try to provide
the most productive and enjoyable environment so your team is most
likely to perform those miracles that are always being asked of you.
Moving to Java falls in all three of these categories, and it would be
wonderful if it didn’t cost you anything as well. Although moving to Java
may be cheaper—depending on your constraints—than the OOP
alternatives for a team of C programmers (and probably for programmers
in other procedural languages), it isn’t free, and there are obstacles you
should be aware of before trying to sell the move to Java within your
company and embarking on the move itself.
Startup costs
The cost of moving to Java is more than just the acquisition of Java
compilers (the Sun Java compiler is free, so this is hardly an obstacle).
Your medium- and long-term costs will be minimized if you invest in
training (and possibly mentoring for your first project) and also if you
identify and purchase class libraries that solve your problem rather than
trying to build those libraries yourself. These are hard-money costs that
must be factored into a realistic proposal. In addition, there are the
hidden costs in loss of productivity while learning a new language and
possibly a new programming environment. Training and mentoring can
certainly minimize these, but team members must overcome their own
struggles to understand the new technology. During this process they will
make more mistakes (this is a feature, because acknowledged mistakes
are the fastest path to learning) and be less productive. Even then, with
some types of programming problems, the right classes, and the right
development environment, it’s possible to be more productive while
you’re learning Java (even considering that you’re making more mistakes
and writing fewer lines of code per day) than if you’d stayed with C.
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Performance issues
A common question is, “Doesn’t OOP automatically make my programs a
lot bigger and slower?” The answer is, “It depends.” The extra safety
features in Java have traditionally extracted a performance penalty over a
language like C++. Technologies such as “hotspot” and compilation
technologies have improved the speed significantly in most cases, and
efforts continue toward higher performance.
When your focus is on rapid prototyping, you can throw together
components as fast as possible while ignoring efficiency issues. If you’re
using any third-party libraries, these are usually already optimized by
their vendors; in any case it’s not an issue while you’re in rapid-
development mode. When you have a system that you like, if it’s small and
fast enough, then you’re done. If not, you begin tuning with a profiling
tool, looking first for speedups that can be done by rewriting small
portions of code. If that doesn’t help, you look for modifications that can
be made in the underlying implementation so no code that uses a
particular class needs to be changed. Only if nothing else solves the
problem do you need to change the design. The fact that performance is so
critical in that portion of the design is an indicator that it must be part of
the primary design criteria. You have the benefit of finding this out early
using rapid development.
If you find a function that is a particular bottleneck, you can rewrite it in
C/C++ using Java’s native methods, the subject of Appendix B.
Common design errors
When starting your team into OOP and Java, programmers will typically
go through a series of common design errors. This often happens due to
insufficient feedback from experts during the design and implementation
of early projects, because no experts have been developed within the
company, and because there may be resistance to retaining consultants.
It’s easy to feel that you understand OOP too early in the cycle and go off
on a bad tangent. Something that’s obvious to someone experienced with
the language may be a subject of great internal debate for a novice. Much
of this trauma can be skipped by using an experienced outside expert for
training and mentoring.
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Java vs. C++?
Java looks a lot like C++, and so naturally it would seem that C++ will be
replaced by Java. But I’m starting to question this logic. For one thing,
C++ still has some features that Java doesn’t, and although there have
been a lot of promises about Java someday being as fast or faster than
C++, we’ve seen steady improvements but no dramatic breakthroughs.
Also, there seems to be a continuing interest in C++, so I don’t think that
language is going away any time soon. (Languages seem to hang around.
Speaking at one of my “Intermediate/Advanced Java Seminars,” Allen
Holub asserted that the two most commonly used languages are Rexx and
COBOL, in that order.)
I’m beginning to think that the strength of Java lies in a slightly different
arena than that of C++. C++ is a language that doesn’t try to fit a mold.
Certainly it has been adapted in a number of ways to solve particular
problems. Some C++ tools combine libraries, component models, and
code-generation tools to solve the problem of developing windowed end-
user applications (for Microsoft Windows). And yet, what do the vast
majority of Windows developers use? Microsoft’s Visual Basic (VB). This
despite the fact that VB produces the kind of code that becomes
unmanageable when the program is only a few pages long (and syntax
that can be positively mystifying). As successful and popular as VB is, it’s
not a very good example of language design. It would be nice to have the
ease and power of VB without the resulting unmanageable code. And
that’s where I think Java will shine: as the “next VB.” You may or may not
shudder to hear this, but think about it: so much of Java is intended to
make it easy for the programmer to solve application-level problems like
networking and cross-platform UI, and yet it has a language design that
allows the creation of very large and flexible bodies of code. Add to this
the fact that Java has the most robust type checking and error handling
systems I’ve ever seen in a language and you have the makings of a
significant leap forward in programming productivity.
Should you use Java instead of C++ for your project? Other than Web
applets, there are two issues to consider. First, if you want to use a lot of
existing C++ libraries (and you’ll certainly get a lot of productivity gains
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there), or if you have an existing C or C++ code base, Java might slow
your development down rather than speeding it up.
If you’re developing all your code primarily from scratch, then the
simplicity of Java over C++ will significantly shorten your development
time—the anecdotal evidence (stories from C++ teams that I’ve talked to
who have switched to Java) suggests a doubling of development speed
over C++. If Java performance doesn’t matter or you can somehow
compensate for it, sheer time-to-market issues make it difficult to choose
C++ over Java.
The biggest issue is performance. Interpreted Java has been slow, even 20
to 50 times slower than C in the original Java interpreters. This has
improved greatly over time, but it will still remain an important number.
Computers are about speed; if it wasn’t significantly faster to do
something on a computer then you’d do it by hand. (I’ve even heard it
suggested that you start with Java, to gain the short development time,
then use a tool and support libraries to translate your code to C++, if you
need faster execution speed.)
The key to making Java feasible for most development projects is the
appearance of speed improvements like so-called “just-in time” (JIT)
compilers, Sun’s own “hotspot” technology, and even native code
compilers. Of course, native code compilers will eliminate the touted
cross-platform execution of the compiled programs, but they will also
bring the speed of the executable closer to that of C and C++. And cross-
compiling a program in Java should be a lot easier than doing so in C or
C++. (In theory, you just recompile, but that promise has been made
before for other languages.)
You can find comparisons of Java and C++ and observations about Java
realities in the appendices of the first edition of this book (Available on
this book’s accompanying CD ROM, as well as at www.BruceEckel.com).
Summary
This chapter attempts to give you a feel for the broad issues of object-
oriented programming and Java, including why OOP is different, and why
Java in particular is different, concepts of OOP methodologies, and finally
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the kinds of issues you will encounter when moving your own company to
OOP and Java.
OOP and Java may not be for everyone. It’s important to evaluate your
own needs and decide whether Java will optimally satisfy those needs, or
if you might be better off with another programming system (including
the one you’re currently using). If you know that your needs will be very
specialized for the foreseeable future and if you have specific constraints
that may not be satisfied by Java, then you owe it to yourself to investigate
the alternatives18. Even if you eventually choose Java as your language,
you’ll at least understand what the options were and have a clear vision of
why you took that direction.
You know what a procedural program looks like: data definitions and
function calls. To find the meaning of such a program you have to work a
little, looking through the function calls and low-level concepts to create a
model in your mind. This is the reason we need intermediate
representations when designing procedural programs—by themselves,
these programs tend to be confusing because the terms of expression are
oriented more toward the computer than to the problem you’re solving.
Because Java adds many new concepts on top of what you find in a
procedural language, your natural assumption may be that the main( ) in
a Java program will be far more complicated than for the equivalent C
program. Here, you’ll be pleasantly surprised: A well-written Java
program is generally far simpler and much easier to understand than the
equivalent C program. What you’ll see are the definitions of the objects
that represent concepts in your problem space (rather than the issues of
the computer representation) and messages sent to those objects to
represent the activities in that space. One of the delights of object-
oriented programming is that, with a well-designed program, it’s easy to
understand the code by reading it. Usually there’s a lot less code as well,
because many of your problems will be solved by reusing existing library
code.
18 In particular, I recommend looking at Python (http://www.Python.org).
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2: Everything
is an Object
Although it is based on C++, Java is more of a “pure”
object-oriented language.
Both C++ and Java are hybrid languages, but in Java the designers felt
that the hybridization was not as important as it was in C++. A hybrid
language allows multiple programming styles; the reason C++ is hybrid is
to support backward compatibility with the C language. Because C++ is a
superset of the C language, it includes many of that language’s
undesirable features, which can make some aspects of C++ overly
complicated.
The Java language assumes that you want to do only object-oriented
programming. This means that before you can begin you must shift your
mindset into an object-oriented world (unless it’s already there). The
benefit of this initial effort is the ability to program in a language that is
simpler to learn and to use than many other OOP languages. In this
chapter we’ll see the basic components of a Java program and we’ll learn
that everything in Java is an object, even a Java program.
You manipulate objects
with references
Each programming language has its own means of manipulating data.
Sometimes the programmer must be constantly aware of what type of
manipulation is going on. Are you manipulating the object directly, or are
you dealing with some kind of indirect representation (a pointer in C or
C++) that must be treated with a special syntax?
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All this is simplified in Java. You treat everything as an object, so there is
a single consistent syntax that you use everywhere. Although you treat
everything as an object, the identifier you manipulate is actually a
“reference” to an object1. You might imagine this scene as a television (the
object) with your remote control (the reference). As long as you’re holding
this reference, you have a connection to the television, but when someone
says “change the channel” or “lower the volume,” what you’re
manipulating is the reference, which in turn modifies the object. If you
want to move around the room and still control the television, you take
the remote/reference with you, not the television.
Also, the remote control can stand on its own, with no television. That is,
just because you have a reference doesn’t mean there’s necessarily an
object connected to it. So if you want to hold a word or sentence, you
create a String reference:
String s;
But here you’ve created only the reference, not an object. If you decided to
send a message to s at this point, you’ll get an error (at run-time) because
s isn’t actually attached to anything (there’s no television). A safer
practice, then, is always to initialize a reference when you create it:
String s = "asdf";
1 This can be a flashpoint. There are those who say “clearly, it’s a pointer,” but this
presumes an underlying implementation. Also, Java references are much more akin to
C++ references than pointers in their syntax. In the first edition of this book, I chose to
invent a new term, “handle,” because C++ references and Java references have some
important differences. I was coming out of C++ and did not want to confuse the C++
programmers whom I assumed would be the largest audience for Java. In the 2nd edition, I
decided that “reference” was the more commonly used term, and that anyone changing
from C++ would have a lot more to cope with than the terminology of references, so they
might as well jump in with both feet. However, there are people who disagree even with
the term “reference.” I read in one book where it was “completely wrong to say that Java
supports pass by reference,” because Java object identifiers (according to that author) are
actually “object references.” And (he goes on) everything is actually pass by value. So
you’re not passing by reference, you’re “passing an object reference by value.” One could
argue for the precision of such convoluted explanations, but I think my approach
simplifies the understanding of the concept without hurting anything (well, the language
lawyers may claim that I’m lying to you, but I’ll say that I’m providing an appropriate
abstraction.)
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However, this uses a special Java feature: strings can be initialized with
quoted text. Normally, you must use a more general type of initialization
for objects.
You must create
all the objects
When you create a reference, you want to connect it with a new object.
You do so, in general, with the new keyword. new says, “Make me a new
one of these objects.” So in the above example, you can say:
String s = new String("asdf");
Not only does this mean “Make me a new String,” but it also gives
information about how to make the String by supplying an initial
character string.
Of course, String is not the only type that exists. Java comes with a
plethora of ready-made types. What’s more important is that you can
create your own types. In fact, that’s the fundamental activity in Java
programming, and it’s what you’ll be learning about in the rest of this
book.
Where storage lives
It’s useful to visualize some aspects of how things are laid out while the
program is running, in particular how memory is arranged. There are six
different places to store data:
1.
Registers. This is the fastest storage because it exists in a place
different from that of other storage: inside the processor. However,
the number of registers is severely limited, so registers are
allocated by the compiler according to its needs. You don’t have
direct control, nor do you see any evidence in your programs that
registers even exist.
2.
The stack. This lives in the general RAM (random-access
memory) area, but has direct support from the processor via its
stack pointer. The stack pointer is moved down to create new
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memory and moved up to release that memory. This is an
extremely fast and efficient way to allocate storage, second only to
registers. The Java compiler must know, while it is creating the
program, the exact size and lifetime of all the data that is stored on
the stack, because it must generate the code to move the stack
pointer up and down. This constraint places limits on the flexibility
of your programs, so while some Java storage exists on the stack—
in particular, object references—Java objects themselves are not
placed on the stack.
3.
The heap. This is a general-purpose pool of memory (also in the
RAM area) where all Java objects live. The nice thing about the
heap is that, unlike the stack, the compiler doesn’t need to know
how much storage it needs to allocate from the heap or how long
that storage must stay on the heap. Thus, there’s a great deal of
flexibility in using storage on the heap. Whenever you need to
create an object, you simply write the code to create it using new,
and the storage is allocated on the heap when that code is executed.
Of course there’s a price you pay for this flexibility: it takes more
time to allocate heap storage than it does to allocate stack storage
(that is, if you even could create objects on the stack in Java, as you
can in C++).
4.
Static storage. “Static” is used here in the sense of “in a fixed
location” (although it’s also in RAM). Static storage contains data
that is available for the entire time a program is running. You can
use the static keyword to specify that a particular element of an
object is static, but Java objects themselves are never placed in
static storage.
5.
Constant storage. Constant values are often placed directly in
the program code, which is safe since they can never change.
Sometimes constants are cordoned off by themselves so that they
can be optionally placed in read-only memory (ROM).
6.
Non-RAM storage. If data lives completely outside a program it
can exist while the program is not running, outside the control of
the program. The two primary examples of this are streamed
objects, in which objects are turned into streams of bytes, generally
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to be sent to another machine, and persistent objects, in which the
objects are placed on disk so they will hold their state even when
the program is terminated. The trick with these types of storage is
turning the objects into something that can exist on the other
medium, and yet can be resurrected into a regular RAM-based
object when necessary. Java provides support for lightweight
persistence, and future versions of Java might provide more
complete solutions for persistence.
Special case: primitive types
There is a group of types that gets special treatment; you can think of
these as “primitive” types that you use quite often in your programming.
The reason for the special treatment is that to create an object with new—
especially a small, simple variable—isn’t very efficient because new places
objects on the heap. For these types Java falls back on the approach taken
by C and C++. That is, instead of creating the variable using new, an
“automatic” variable is created that is not a reference. The variable holds
the value, and it’s placed on the stack so it’s much more efficient.
Java determines the size of each primitive type. These sizes don’t change
from one machine architecture to another as they do in most languages.
This size invariance is one reason Java programs are so portable.
Primitive
Size Minimum
Maximum Wrapper
type
type
boolean
— —
—
Boolean
char
16-bit
Unicode 0
Unicode 216- 1
Character
byte
8-bit -128
+127
Byte
short
16-bit -215 +215—1
Short
int
32-bit -231 +231—1
Integer
long
64-bit -263 +263—1
Long
float
32-bit IEEE754
IEEE754
Float
double
64-bit IEEE754
IEEE754
Double
void
— —
—
Void
All numeric types are signed, so don’t go looking for unsigned types.
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The size of the boolean type is not explicitly defined; it is only specified
to be able to take the literal values true or false.
The primitive data types also have “wrapper” classes for them. That
means that if you want to make a nonprimitive object on the heap to
represent that primitive type, you use the associated wrapper. For
example:
char c = 'x';
Character C = new Character(c);
Or you could also use:
Character C = new Character('x');
The reasons for doing this will be shown in a later chapter.
High-precision numbers
Java includes two classes for performing high-precision arithmetic:
BigInteger and BigDecimal. Although these approximately fit into the
same category as the “wrapper” classes, neither one has a primitive
analogue.
Both classes have methods that provide analogues for the operations that
you perform on primitive types. That is, you can do anything with a
BigInteger or BigDecimal that you can with an int or float, it’s just
that you must use method calls instead of operators. Also, since there’s
more involved, the operations will be slower. You’re exchanging speed for
accuracy.
BigInteger supports arbitrary-precision integers. This means that you
can accurately represent integral values of any size without losing any
information during operations.
BigDecimal is for arbitrary-precision fixed-point numbers; you can use
these for accurate monetary calculations, for example.
Consult your online documentation for details about the constructors and
methods you can call for these two classes.
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Arrays in Java
Virtually all programming languages support arrays. Using arrays in C
and C++ is perilous because those arrays are only blocks of memory. If a
program accesses the array outside of its memory block or uses the
memory before initialization (common programming errors) there will be
unpredictable results.
One of the primary goals of Java is safety, so many of the problems that
plague programmers in C and C++ are not repeated in Java. A Java array
is guaranteed to be initialized and cannot be accessed outside of its range.
The range checking comes at the price of having a small amount of
memory overhead on each array as well as verifying the index at run-time,
but the assumption is that the safety and increased productivity is worth
the expense.
When you create an array of objects, you are really creating an array of
references, and each of those references is automatically initialized to a
special value with its own keyword: null. When Java sees null, it
recognizes that the reference in question isn’t pointing to an object. You
must assign an object to each reference before you use it, and if you try to
use a reference that’s still null, the problem will be reported at run-time.
Thus, typical array errors are prevented in Java.
You can also create an array of primitives. Again, the compiler guarantees
initialization because it zeroes the memory for that array.
Arrays will be covered in detail in later chapters.
You never need to
destroy an object
In most programming languages, the concept of the lifetime of a variable
occupies a significant portion of the programming effort. How long does
the variable last? If you are supposed to destroy it, when should you?
Confusion over variable lifetimes can lead to a lot of bugs, and this section
shows how Java greatly simplifies the issue by doing all the cleanup work
for you.
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Scoping
Most procedural languages have the concept of scope. This determines
both the visibility and lifetime of the names defined within that scope. In
C, C++, and Java, scope is determined by the placement of curly braces
{}. So for example:
{
int x = 12;
/* only x available */
{
int q = 96;
/* both x & q available */
}
/* only x available */
/* q “out of scope” */
}
A variable defined within a scope is available only to the end of that scope.
Indentation makes Java code easier to read. Since Java is a free-form
language, the extra spaces, tabs, and carriage returns do not affect the
resulting program.
Note that you cannot do the following, even though it is legal in C and
C++:
{
int x = 12;
{
int x = 96; /* illegal */
}
}
The compiler will announce that the variable x has already been defined.
Thus the C and C++ ability to “hide” a variable in a larger scope is not
allowed because the Java designers thought that it led to confusing
programs.
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Scope of objects
Java objects do not have the same lifetimes as primitives. When you
create a Java object using new, it hangs around past the end of the scope.
Thus if you use:
{
String s = new String("a string");
} /* end of scope */
the reference s vanishes at the end of the scope. However, the String
object that s was pointing to is still occupying memory. In this bit of code,
there is no way to access the object because the only reference to it is out
of scope. In later chapters you’ll see how the reference to the object can be
passed around and duplicated during the course of a program.
It turns out that because objects created with new stay around for as long
as you want them, a whole slew of C++ programming problems simply
vanish in Java. The hardest problems seem to occur in C++ because you
don’t get any help from the language in making sure that the objects are
available when they’re needed. And more important, in C++ you must
make sure that you destroy the objects when you’re done with them.
That brings up an interesting question. If Java leaves the objects lying
around, what keeps them from filling up memory and halting your
program? This is exactly the kind of problem that would occur in C++.
This is where a bit of magic happens. Java has a garbage collector, which
looks at all the objects that were created with new and figures out which
ones are not being referenced anymore. Then it releases the memory for
those objects, so the memory can be used for new objects. This means that
you never need to worry about reclaiming memory yourself. You simply
create objects, and when you no longer need them they will go away by
themselves. This eliminates a certain class of programming problem: the
so-called “memory leak,” in which a programmer forgets to release
memory.
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Creating new
data types: class
If everything is an object, what determines how a particular class of object
looks and behaves? Put another way, what establishes the type of an
object? You might expect there to be a keyword called “type,” and that
certainly would have made sense. Historically, however, most object-
oriented languages have used the keyword class to mean “I’m about to
tell you what a new type of object looks like.” The class keyword (which is
so common that it will not be emboldened throughout this book) is
followed by the name of the new type. For example:
class ATypeName { /* class body goes here */ }
This introduces a new type, so you can now create an object of this type
using new:
ATypeName a = new ATypeName();
In ATypeName, the class body consists only of a comment (the stars and
slashes and what is inside, which will be discussed later in this chapter),
so there is not too much that you can do with it. In fact, you cannot tell it
to do much of anything (that is, you cannot send it any interesting
messages) until you define some methods for it.
Fields and methods
When you define a class (and all you do in Java is define classes, make
objects of those classes, and send messages to those objects), you can put
two types of elements in your class: data members (sometimes called
fields), and member functions (typically called methods). A data member
is an object of any type that you can communicate with via its reference. It
can also be one of the primitive types (which isn’t a reference). If it is a
reference to an object, you must initialize that reference to connect it to an
actual object (using new, as seen earlier) in a special function called a
constructor (described fully in Chapter 4). If it is a primitive type you can
initialize it directly at the point of definition in the class. (As you’ll see
later, references can also be initialized at the point of definition.)
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Each object keeps its own storage for its data members; the data members
are not shared among objects. Here is an example of a class with some
data members:
class DataOnly {
int i;
float f;
boolean b;
}
This class doesn’t do anything, but you can create an object:
DataOnly d = new DataOnly();
You can assign values to the data members, but you must first know how
to refer to a member of an object. This is accomplished by stating the
name of the object reference, followed by a period (dot), followed by the
name of the member inside the object:
objectReference.member
For example:
d.i = 47;
d.f = 1.1f;
d.b = false;
It is also possible that your object might contain other objects that contain
data you’d like to modify. For this, you just keep “connecting the dots.”
For example:
myPlane.leftTank.capacity = 100;
The DataOnly class cannot do much of anything except hold data,
because it has no member functions (methods). To understand how those
work, you must first understand arguments and return values, which will
be described shortly.
Default values for primitive members
When a primitive data type is a member of a class, it is guaranteed to get a
default value if you do not initialize it:
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Primitive type
Default
boolean false
char ‘\u0000’
(null)
byte (byte)0
short (short)0
int 0
long 0L
float 0.0f
double 0.0d
Note carefully that the default values are what Java guarantees when the
variable is used as a member of a class. This ensures that member
variables of primitive types will always be initialized (something C++
doesn’t do), reducing a source of bugs. However, this initial value may not
be correct or even legal for the program you are writing. It’s best to always
explicitly initialize your variables.
This guarantee doesn’t apply to “local” variables—those that are not fields
of a class. Thus, if within a function definition you have:
int x;
Then x will get some arbitrary value (as in C and C++); it will not
automatically be initialized to zero. You are responsible for assigning an
appropriate value before you use x. If you forget, Java definitely improves
on C++: you get a compile-time error telling you the variable might not
have been initialized. (Many C++ compilers will warn you about
uninitialized variables, but in Java these are errors.)
Methods, arguments,
and return values
Up until now, the term function has been used to describe a named
subroutine. The term that is more commonly used in Java is method, as in
“a way to do something.” If you want, you can continue thinking in terms
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of functions. It’s really only a syntactic difference, but from now on
“method” will be used in this book rather than “function.”
Methods in Java determine the messages an object can receive. In this
section you will learn how simple it is to define a method.
The fundamental parts of a method are the name, the arguments, the
return type, and the body. Here is the basic form:
returnType methodName( /* argument list */ ) {
/* Method body */
}
The return type is the type of the value that pops out of the method after
you call it. The argument list gives the types and names for the
information you want to pass into the method. The method name and
argument list together uniquely identify the method.
Methods in Java can be created only as part of a class. A method can be
called only for an object,2 and that object must be able to perform that
method call. If you try to call the wrong method for an object, you’ll get an
error message at compile-time. You call a method for an object by naming
the object followed by a period (dot), followed by the name of the method
and its argument list, like this: objectName.methodName(arg1,
arg2, arg3). For example, suppose you have a method f( ) that takes no
arguments and returns a value of type int. Then, if you have an object
called a for which f( ) can be called, you can say this:
int x = a.f();
The type of the return value must be compatible with the type of x.
This act of calling a method is commonly referred to as sending a
message to an object. In the above example, the message is f( ) and the
object is a. Object-oriented programming is often summarized as simply
“sending messages to objects.”
2 static methods, which you’ll learn about soon, can be called for the class, without an
object.
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The argument list
The method argument list specifies what information you pass into the
method. As you might guess, this information—like everything else in
Java—takes the form of objects. So, what you must specify in the
argument list are the types of the objects to pass in and the name to use
for each one. As in any situation in Java where you seem to be handing
objects around, you are actually passing references3. The type of the
reference must be correct, however. If the argument is supposed to be a
String, what you pass in must be a string.
Consider a method that takes a String as its argument. Here is the
definition, which must be placed within a class definition for it to be
compiled:
int storage(String s) {
return s.length() * 2;
}
This method tells you how many bytes are required to hold the
information in a particular String. (Each char in a String is 16 bits, or
two bytes, long, to support Unicode characters.) The argument is of type
String and is called s. Once s is passed into the method, you can treat it
just like any other object. (You can send messages to it.) Here, the
length( ) method is called, which is one of the methods for Strings; it
returns the number of characters in a string.
You can also see the use of the return keyword, which does two things.
First, it means “leave the method, I’m done.” Second, if the method
produces a value, that value is placed right after the return statement. In
this case, the return value is produced by evaluating the expression
s.length( ) * 2.
You can return any type you want, but if you don’t want to return
anything at all, you do so by indicating that the method returns void.
Here are some examples:
3 With the usual exception of the aforementioned “special” data types boolean, char,
byte, short, int, long, float, and double. In general, though, you pass objects, which
really means you pass references to objects.
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boolean flag() { return true; }
float naturalLogBase() { return 2.718f; }
void nothing() { return; }
void nothing2() {}
When the return type is void, then the return keyword is used only to
exit the method, and is therefore unnecessary when you reach the end of
the method. You can return from a method at any point, but if you’ve
given a non-void return type then the compiler will force you (with error
messages) to return the appropriate type of value regardless of where you
return.
At this point, it can look like a program is just a bunch of objects with
methods that take other objects as arguments and send messages to those
other objects. That is indeed much of what goes on, but in the following
chapter you’ll learn how to do the detailed low-level work by making
decisions within a method. For this chapter, sending messages will
suffice.
Building a Java program
There are several other issues you must understand before seeing your
first Java program.
Name visibility
A problem in any programming language is the control of names. If you
use a name in one module of the program, and another programmer uses
the same name in another module, how do you distinguish one name
from another and prevent the two names from “clashing?” In C this is a
particular problem because a program is often an unmanageable sea of
names. C++ classes (on which Java classes are based) nest functions
within classes so they cannot clash with function names nested within
other classes. However, C++ still allowed global data and global functions,
so clashing was still possible. To solve this problem, C++ introduced
namespaces using additional keywords.
Java was able to avoid all of this by taking a fresh approach. To produce
an unambiguous name for a library, the specifier used is not unlike an
Internet domain name. In fact, the Java creators want you to use your
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Internet domain name in reverse since those are guaranteed to be unique.
Since my domain name is BruceEckel.com, my utility library of foibles
would be named com.bruceeckel.utility.foibles. After your reversed
domain name, the dots are intended to represent subdirectories.
In Java 1.0 and Java 1.1 the domain extensions com, edu, org, net, etc.,
were capitalized by convention, so the library would appear:
COM.bruceeckel.utility.foibles. Partway through the development of
Java 2, however, it was discovered that this caused problems, and so now
the entire package name is lowercase.
This mechanism means that all of your files automatically live in their
own namespaces, and each class within a file must have a unique
identifier. So you do not need to learn special language features to solve
this problem—the language takes care of it for you.
Using other components
Whenever you want to use a predefined class in your program, the
compiler must know how to locate it. Of course, the class might already
exist in the same source code file that it’s being called from. In that case,
you simply use the class—even if the class doesn’t get defined until later in
the file. Java eliminates the “forward referencing” problem so you don’t
need to think about it.
What about a class that exists in some other file? You might think that the
compiler should be smart enough to simply go and find it, but there is a
problem. Imagine that you want to use a class of a particular name, but
more than one definition for that class exists (presumably these are
different definitions). Or worse, imagine that you’re writing a program,
and as you’re building it you add a new class to your library that conflicts
with the name of an existing class.
To solve this problem, you must eliminate all potential ambiguities. This
is accomplished by telling the Java compiler exactly what classes you want
using the import keyword. import tells the compiler to bring in a
package, which is a library of classes. (In other languages, a library could
consist of functions and data as well as classes, but remember that all
code in Java must be written inside a class.)
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Most of the time you’ll be using components from the standard Java
libraries that come with your compiler. With these, you don’t need to
worry about long, reversed domain names; you just say, for example:
import java.util.ArrayList;
to tell the compiler that you want to use Java’s ArrayList class. However,
util contains a number of classes and you might want to use several of
them without declaring them all explicitly. This is easily accomplished by
using ‘*’ to indicate a wild card:
import java.util.*;
It is more common to import a collection of classes in this manner than to
import classes individually.
The static keyword
Ordinarily, when you create a class you are describing how objects of that
class look and how they will behave. You don’t actually get anything until
you create an object of that class with new, and at that point data storage
is created and methods become available.
But there are two situations in which this approach is not sufficient. One
is if you want to have only one piece of storage for a particular piece of
data, regardless of how many objects are created, or even if no objects are
created. The other is if you need a method that isn’t associated with any
particular object of this class. That is, you need a method that you can call
even if no objects are created. You can achieve both of these effects with
the static keyword. When you say something is static, it means that data
or method is not tied to any particular object instance of that class. So
even if you’ve never created an object of that class you can call a static
method or access a piece of static data. With ordinary, non-static data
and methods you must create an object and use that object to access the
data or method, since non-static data and methods must know the
particular object they are working with. Of course, since static methods
don’t need any objects to be created before they are used, they cannot
directly access non-static members or methods by simply calling those
other members without referring to a named object (since non-static
members and methods must be tied to a particular object).
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Some object-oriented languages use the terms class data and class
methods, meaning that the data and methods exist only for the class as a
whole, and not for any particular objects of the class. Sometimes the Java
literature uses these terms too.
To make a data member or method static, you simply place the keyword
before the definition. For example, the following produces a static data
member and initializes it:
class StaticTest {
static int i = 47;
}
Now even if you make two StaticTest objects, there will still be only one
piece of storage for StaticTest.i. Both objects will share the same i.
Consider:
StaticTest st1 = new StaticTest();
StaticTest st2 = new StaticTest();
At this point, both st1.i and st2.i have the same value of 47 since they
refer to the same piece of memory.
There are two ways to refer to a static variable. As indicated above, you
can name it via an object, by saying, for example, st2.i. You can also refer
to it directly through its class name, something you cannot do with a non-
static member. (This is the preferred way to refer to a static variable
since it emphasizes that variable’s static nature.)
StaticTest.i++;
The ++ operator increments the variable. At this point, both st1.i and
st2.i will have the value 48.
Similar logic applies to static methods. You can refer to a static method
either through an object as you can with any method, or with the special
additional syntax ClassName.method( ). You define a static method in
a similar way:
class StaticFun {
static void incr() { StaticTest.i++; }
}
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You can see that the StaticFun method incr( ) increments the static
data i. You can call incr( ) in the typical way, through an object:
StaticFun sf = new StaticFun();
sf.incr();
Or, because incr( ) is a static method, you can call it directly through its
class:
StaticFun.incr();
While static, when applied to a data member, definitely changes the way
the data is created (one for each class vs. the non-static one for each
object), when applied to a method it’s not so dramatic. An important use
of static for methods is to allow you to call that method without creating
an object. This is essential, as we will see, in defining the main( ) method
that is the entry point for running an application.
Like any method, a static method can create or use named objects of its
type, so a static method is often used as a “shepherd” for a flock of
instances of its own type.
Your first Java program
Finally, here’s the program.4 It starts by printing a string, and then the
date, using the Date class from the Java standard library. Note that an
additional style of comment is introduced here: the ‘//’, which is a
comment until the end of the line:
// HelloDate.java
4 Some programming environments will flash programs up on the screen and close them
before you've had a chance to see the results. You can put in the following bit of code at the
end of main( ) to pause the output:
try {
System.in.read();
} catch(Exception e) {}
This will pause the output until you press “Enter” (or any other key). This code involves
concepts that will not be introduced until much later in the book, so you won’t understand
it until then, but it will do the trick.
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import java.util.*;
public class HelloDate {
public static void main(String[] args) {
System.out.println("Hello, it's: ");
System.out.println(new Date());
}
}
At the beginning of each program file, you must place the import
statement to bring in any extra classes you’ll need for the code in that file.
Note that I say “extra;” that’s because there’s a certain library of classes
that are automatically brought into every Java file: java.lang. Start up
your Web browser and look at the documentation from Sun. (If you
haven’t downloaded it from java.sun.com or otherwise installed the Java
documentation, do so now). If you look at the list of the packages, you’ll
see all the different class libraries that come with Java. Select java.lang.
This will bring up a list of all the classes that are part of that library. Since
java.lang is implicitly included in every Java code file, these classes are
automatically available. There’s no Date class listed in java.lang, which
means you must import another library to use that. If you don’t know the
library where a particular class is, or if you want to see all of the classes,
you can select “Tree” in the Java documentation. Now you can find every
single class that comes with Java. Then you can use the browser’s “find”
function to find Date. When you do you’ll see it listed as java.util.Date,
which lets you know that it’s in the util library and that you must import
java.util.* in order to use Date.
If you go back to the beginning, select java.lang and then System, you’ll
see that the System class has several fields, and if you select out you’ll
discover that it’s a static PrintStream object. Since it’s static you don’t
need to create anything. The out object is always there and you can just
use it. What you can do with this out object is determined by the type it
is: a PrintStream. Conveniently, PrintStream is shown in the
description as a hyperlink, so if you click on that you’ll see a list of all the
methods you can call for PrintStream. There are quite a few and these
will be covered later in this book. For now all we’re interested in is
println( ), which in effect means “print what I’m giving you out to the
console and end with a new line.” Thus, in any Java program you write
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you can say System.out.println(“things”) whenever you want to print
something to the console.
The name of the class is the same as the name of the file. When you’re
creating a stand-alone program such as this one, one of the classes in the
file must have the same name as the file. (The compiler complains if you
don’t do this.) That class must contain a method called main( ) with the
signature shown:
public static void main(String[] args) {
The public keyword means that the method is available to the outside
world (described in detail in Chapter 5). The argument to main( ) is an
array of String objects. The args won’t be used in this program, but the
Java compiler insists that they be there because they hold the arguments
invoked on the command line.
The line that prints the date is quite interesting:
System.out.println(new Date());
Consider the argument: a Date object is being created just to send its
value to println( ). As soon as this statement is finished, that Date is
unnecessary, and the garbage collector can come along and get it anytime.
We don’t need to worry about cleaning it up.
Compiling and running
To compile and run this program, and all the other programs in this book,
you must first have a Java programming environment. There are a
number of third-party development environments, but in this book we
will assume that you are using the JDK from Sun, which is free. If you are
using another development system, you will need to look in the
documentation for that system to determine how to compile and run
programs.
Get on the Internet and go to java.sun.com. There you will find
information and links that will lead you through the process of
downloading and installing the JDK for your particular platform.
Once the JDK is installed, and you’ve set up your computer’s path
information so that it will find javac and java, download and unpack the
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source code for this book (you can find it on the CD ROM that’s bound in
with this book, or at www.BruceEckel.com). This will create a
subdirectory for each chapter in this book. Move to subdirectory c02 and
type:
javac HelloDate.java
This command should produce no response. If you get any kind of an
error message it means you haven’t installed the JDK properly and you
need to investigate those problems.
On the other hand, if you just get your command prompt back, you can
type:
java HelloDate
and you’ll get the message and the date as output.
This is the process you can use to compile and run each of the programs in
this book. However, you will see that the source code for this book also
has a file called makefile in each chapter, and this contains “make”
commands for automatically building the files for that chapter. See this
book’s Web page at www.BruceEckel.com for details on how to use the
makefiles.
Comments and embedded
documentation
There are two types of comments in Java. The first is the traditional C-
style comment that was inherited by C++. These comments begin with a
/* and continue, possibly across many lines, until a */. Note that many
programmers will begin each line of a continued comment with a *, so
you’ll often see:
/* This is a comment
* that continues
* across lines
*/
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Remember, however, that everything inside the /* and */ is ignored, so
there’s no difference in saying:
/* This is a comment that
continues across lines */
The second form of comment comes from C++. It is the single-line
comment, which starts at a // and continues until the end of the line. This
type of comment is convenient and commonly used because it’s easy. You
don’t need to hunt on the keyboard to find / and then * (instead, you just
press the same key twice), and you don’t need to close the comment. So
you will often see:
// this is a one-line comment
Comment documentation
One of the thoughtful parts of the Java language is that the designers
didn’t consider writing code to be the only important activity—they also
thought about documenting it. Possibly the biggest problem with
documenting code has been maintaining that documentation. If the
documentation and the code are separate, it becomes a hassle to change
the documentation every time you change the code. The solution seems
simple: link the code to the documentation. The easiest way to do this is
to put everything in the same file. To complete the picture, however, you
need a special comment syntax to mark special documentation, and a tool
to extract those comments and put them in a useful form. This is what
Java has done.
The tool to extract the comments is called javadoc. It uses some of the
technology from the Java compiler to look for special comment tags you
put in your programs. It not only extracts the information marked by
these tags, but it also pulls out the class name or method name that
adjoins the comment. This way you can get away with the minimal
amount of work to generate decent program documentation.
The output of javadoc is an HTML file that you can view with your Web
browser. This tool allows you to create and maintain a single source file
and automatically generate useful documentation. Because of javadoc we
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have a standard for creating documentation, and it’s easy enough that we
can expect or even demand documentation with all Java libraries.
Syntax
All of the javadoc commands occur only within /** comments. The
comments end with */ as usual. There are two primary ways to use
javadoc: embed HTML, or use “doc tags.” Doc tags are commands that
start with a ‘@’ and are placed at the beginning of a comment line. (A
leading ‘*’, however, is ignored.)
There are three “types” of comment documentation, which correspond to
the element the comment precedes: class, variable, or method. That is, a
class comment appears right before the definition of a class; a variable
comment appears right in front of the definition of a variable, and a
method comment appears right in front of the definition of a method. As a
simple example:
/** A class comment */
public class docTest {
/** A variable comment */
public int i;
/** A method comment */
public void f() {}
}
Note that javadoc will process comment documentation for only public
and protected members. Comments for private and “friendly”
members (see Chapter 5) are ignored and you’ll see no output. (However,
you can use the -private flag to include private members as well.) This
makes sense, since only public and protected members are available
outside the file, which is the client programmer’s perspective. However,
all class comments are included in the output.
The output for the above code is an HTML file that has the same standard
format as all the rest of the Java documentation, so users will be
comfortable with the format and can easily navigate your classes. It’s
worth entering the above code, sending it through javadoc and viewing
the resulting HTML file to see the results.
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Embedded HTML
Javadoc passes HTML commands through to the generated HTML
document. This allows you full use of HTML; however, the primary
motive is to let you format code, such as:
/**
* <pre>
* System.out.println(new Date());
* </pre>
*/
You can also use HTML just as you would in any other Web document to
format the regular text in your descriptions:
/**
* You can <em>even</em> insert a list:
* <ol>
* <li> Item one
* <li> Item two
* <li> Item three
* </ol>
*/
Note that within the documentation comment, asterisks at the beginning
of a line are thrown away by javadoc, along with leading spaces. Javadoc
reformats everything so that it conforms to the standard documentation
appearance. Don’t use headings such as <h1> or <hr> as embedded
HTML because javadoc inserts its own headings and yours will interfere
with them.
All types of comment documentation—class, variable, and method—can
support embedded HTML.
@see: referring to other classes
All three types of comment documentation (class, variable, and method)
can contain @see tags, which allow you to refer to the documentation in
other classes. Javadoc will generate HTML with the @see tags
hyperlinked to the other documentation. The forms are:
@see classname
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@see fully-qualified-classname
@see fully-qualified-classname#method-name
Each one adds a hyperlinked “See Also” entry to the generated
documentation. Javadoc will not check the hyperlinks you give it to make
sure they are valid.
Class documentation tags
Along with embedded HTML and @see references, class documentation
can include tags for version information and the author’s name. Class
documentation can also be used for interfaces (see Chapter 8).
@version
This is of the form:
@version version-information
in which version-information is any significant information you see fit
to include. When the -version flag is placed on the javadoc command
line, the version information will be called out specially in the generated
HTML documentation.
@author
This is of the form:
@author author-information
in which author-information is, presumably, your name, but it could
also include your email address or any other appropriate information.
When the -author flag is placed on the javadoc command line, the author
information will be called out specially in the generated HTML
documentation.
You can have multiple author tags for a list of authors, but they must be
placed consecutively. All the author information will be lumped together
into a single paragraph in the generated HTML.
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@since
This tag allows you to indicate the version of this code that began using a
particular feature. You’ll see it appearing in the HTML Java
documentation to indicate what version of the JDK is used.
Variable documentation tags
Variable documentation can include only embedded HTML and @see
references.
Method documentation tags
As well as embedded documentation and @see references, methods allow
documentation tags for parameters, return values, and exceptions.
@param
This is of the form:
@param parameter-name description
in which parameter-name is the identifier in the parameter list, and
description is text that can continue on subsequent lines. The
description is considered finished when a new documentation tag is
encountered. You can have any number of these, presumably one for each
parameter.
@return
This is of the form:
@return description
in which description gives you the meaning of the return value. It can
continue on subsequent lines.
@throws
Exceptions will be demonstrated in Chapter 10, but briefly they are
objects that can be “thrown” out of a method if that method fails.
Although only one exception object can emerge when you call a method, a
particular method might produce any number of different types of
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exceptions, all of which need descriptions. So the form for the exception
tag is:
@throws fully-qualified-class-name description
in which fully-qualified-class-name gives an unambiguous name of an
exception class that’s defined somewhere, and description (which can
continue on subsequent lines) tells you why this particular type of
exception can emerge from the method call.
@deprecated
This is used to tag features that were superseded by an improved feature.
The deprecated tag is a suggestion that you no longer use this particular
feature, since sometime in the future it is likely to be removed. A method
that is marked @deprecated causes the compiler to issue a warning if it
is used.
Documentation example
Here is the first Java program again, this time with documentation
comments added:
//: c02:HelloDate.java
import java.util.*;
/** The first Thinking in Java example program.
* Displays a string and today's date.
* @author Bruce Eckel
* @author www.BruceEckel.com
* @version 2.0
*/
public class HelloDate {
/** Sole entry point to class & application
* @param args array of string arguments
* @return No return value
* @exception exceptions No exceptions thrown
*/
public static void main(String[] args) {
System.out.println("Hello, it's: ");
System.out.println(new Date());
}
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} ///:~
The first line of the file uses my own technique of putting a ‘:’ as a special
marker for the comment line containing the source file name. That line
contains the path information to the file (in this case, c02 indicates
Chapter 2) followed by the file name5. The last line also finishes with a
comment, and this one indicates the end of the source code listing, which
allows it to be automatically extracted from the text of this book and
checked with a compiler.
Coding style
The unofficial standard in Java is to capitalize the first letter of a class
name. If the class name consists of several words, they are run together
(that is, you don’t use underscores to separate the names), and the first
letter of each embedded word is capitalized, such as:
class AllTheColorsOfTheRainbow { // ...
For almost everything else: methods, fields (member variables), and
object reference names, the accepted style is just as it is for classes except
that the first letter of the identifier is lowercase. For example:
class AllTheColorsOfTheRainbow {
int anIntegerRepresentingColors;
void changeTheHueOfTheColor(int newHue) {
// ...
}
// ...
}
Of course, you should remember that the user must also type all these
long names, and so be merciful.
The Java code you will see in the Sun libraries also follows the placement
of open-and-close curly braces that you see used in this book.
5 A tool that I created using Python (see www.Python.org) uses this information to extract
the code files, put them in appropriate subdirectories, and create makefiles.
Chapter 2: Everything is an Object
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Summary
In this chapter you have seen enough of Java programming to understand
how to write a simple program, and you have gotten an overview of the
language and some of its basic ideas. However, the examples so far have
all been of the form “do this, then do that, then do something else.” What
if you want the program to make choices, such as “if the result of doing
this is red, do that; if not, then do something else”? The support in Java
for this fundamental programming activity will be covered in the next
chapter.
Exercises
Solutions to selected exercises can be found in the electronic document The Thinking in Java
Annotated Solution Guide, available for a small fee from www.BruceEckel.com.
1. Following the HelloDate.java example in this chapter, create a
“hello, world” program that simply prints out that statement. You
need only a single method in your class (the “main” one that gets
executed when the program starts). Remember to make it static
and to include the argument list, even though you don’t use the
argument list. Compile the program with javac and run it using
java. If you are using a different development environment than
the JDK, learn how to compile and run programs in that
environment.
2. Find the code fragments involving ATypeName and turn them
into a program that compiles and runs.
3. Turn the DataOnly code fragments into a program that compiles
and runs.
4. Modify Exercise 3 so that the values of the data in DataOnly are
assigned to and printed in main( ).
5. Write a program that includes and calls the storage( ) method
defined as a code fragment in this chapter.
6. Turn the StaticFun code fragments into a working program.
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7. Write a program that prints three arguments taken from the
command line. To do this, you’ll need to index into the command-
line array of Strings.
8. Turn the AllTheColorsOfTheRainbow example into a program
that compiles and runs.
9. Find the code for the second version of HelloDate.java, which is
the simple comment documentation example. Execute javadoc
on the file and view the results with your Web browser.
10. Turn docTest into a file that compiles and then run it through
javadoc. Verify the resulting documentation with your Web
browser.
11. Add an HTML list of items to the documentation in Exercise 10.
12. Take the program in Exercise 1 and add comment documentation
to it. Extract this comment documentation into an HTML file
using javadoc and view it with your Web browser.
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3: Controlling
Program Flow
Like a sentient creature, a program must manipulate its
world and make choices during execution.
In Java you manipulate objects and data using operators, and you make
choices with execution control statements. Java was inherited from C++,
so most of these statements and operators will be familiar to C and C++
programmers. Java has also added some improvements and
simplifications.
If you find yourself floundering a bit in this chapter, make sure you go
through the multimedia CD ROM bound into this book: Thinking in C:
Foundations for Java and C++. It contains audio lectures, slides,
exercises, and solutions specifically designed to bring you up to speed
with the C syntax necessary to learn Java.
Using Java operators
An operator takes one or more arguments and produces a new value. The
arguments are in a different form than ordinary method calls, but the
effect is the same. You should be reasonably comfortable with the general
concept of operators from your previous programming experience.
Addition (+), subtraction and unary minus (-), multiplication (*), division
(/), and assignment (=) all work much the same in any programming
language.
All operators produce a value from their operands. In addition, an
operator can change the value of an operand. This is called a side effect.
The most common use for operators that modify their operands is to
generate the side effect, but you should keep in mind that the value
produced is available for your use just as in operators without side effects.
133
Almost all operators work only with primitives. The exceptions are ‘=’,
‘==’ and ‘!=’, which work with all objects (and are a point of confusion for
objects). In addition, the String class supports ‘+’ and ‘+=’.
Precedence
Operator precedence defines how an expression evaluates when several
operators are present. Java has specific rules that determine the order of
evaluation. The easiest one to remember is that multiplication and
division happen before addition and subtraction. Programmers often
forget the other precedence rules, so you should use parentheses to make
the order of evaluation explicit. For example:
A = X + Y - 2/2 + Z;
has a very different meaning from the same statement with a particular
grouping of parentheses:
A = X + (Y - 2)/(2 + Z);
Assignment
Assignment is performed with the operator =. It means “take the value of
the right-hand side (often called the rvalue) and copy it into the left-hand
side (often called the lvalue). An rvalue is any constant, variable or
expression that can produce a value, but an lvalue must be a distinct,
named variable. (That is, there must be a physical space to store a value.)
For instance, you can assign a constant value to a variable (A = 4;), but
you cannot assign anything to constant value—it cannot be an lvalue. (You
can’t say 4 = A;.)
Assignment of primitives is quite straightforward. Since the primitive
holds the actual value and not a reference to an object, when you assign
primitives you copy the contents from one place to another. For example,
if you say A = B for primitives, then the contents of B are copied into A. If
you then go on to modify A, B is naturally unaffected by this modification.
As a programmer, this is what you’ve come to expect for most situations.
When you assign objects, however, things change. Whenever you
manipulate an object, what you’re manipulating is the reference, so when
you assign “from one object to another” you’re actually copying a
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reference from one place to another. This means that if you say C = D for
objects, you end up with both C and D pointing to the object that,
originally, only D pointed to. The following example will demonstrate
this.
Here’s the example:
//: c03:Assignment.java
// Assignment with objects is a bit tricky.
class Number {
int i;
}
public class Assignment {
public static void main(String[] args) {
Number n1 = new Number();
Number n2 = new Number();
n1.i = 9;
n2.i = 47;
System.out.println("1: n1.i: " + n1.i +
", n2.i: " + n2.i);
n1 = n2;
System.out.println("2: n1.i: " + n1.i +
", n2.i: " + n2.i);
n1.i = 27;
System.out.println("3: n1.i: " + n1.i +
", n2.i: " + n2.i);
}
} ///:~
The Number class is simple, and two instances of it (n1 and n2) are
created within main( ). The i value within each Number is given a
different value, and then n2 is assigned to n1, and n1 is changed. In many
programming languages you would expect n1 and n2 to be independent
at all times, but because you’ve assigned a reference here’s the output
you’ll see:
1: n1.i: 9, n2.i: 47
2: n1.i: 47, n2.i: 47
3: n1.i: 27, n2.i: 27
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Changing the n1 object appears to change the n2 object as well! This is
because both n1 and n2 contain the same reference, which is pointing to
the same object. (The original reference that was in n1 that pointed to the
object holding a value of 9 was overwritten during the assignment and
effectively lost; its object will be cleaned up by the garbage collector.)
This phenomenon is often called aliasing and it’s a fundamental way that
Java works with objects. But what if you don’t want aliasing to occur in
this case? You could forego the assignment and say:
n1.i = n2.i;
This retains the two separate objects instead of tossing one and tying n1
and n2 to the same object, but you’ll soon realize that manipulating the
fields within objects is messy and goes against good object-oriented
design principles. This is a nontrivial topic, so it is left for Appendix A,
which is devoted to aliasing. In the meantime, you should keep in mind
that assignment for objects can add surprises.
Aliasing during method calls
Aliasing will also occur when you pass an object into a method:
//: c03:PassObject.java
// Passing objects to methods may not be what
// you're used to.
class Letter {
char c;
}
public class PassObject {
static void f(Letter y) {
y.c = 'z';
}
public static void main(String[] args) {
Letter x = new Letter();
x.c = 'a';
System.out.println("1: x.c: " + x.c);
f(x);
System.out.println("2: x.c: " + x.c);
}
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} ///:~
In many programming languages, the method f( ) would appear to be
making a copy of its argument Letter y inside the scope of the method.
But once again a reference is being passed so the line
y.c = 'z';
is actually changing the object outside of f( ). The output shows this:
1: x.c: a
2: x.c: z
Aliasing and its solution is a complex issue and, although you must wait
until Appendix A for all the answers, you should be aware of it at this
point so you can watch for pitfalls.
Mathematical operators
The basic mathematical operators are the same as the ones available in
most programming languages: addition (+), subtraction (-), division (/),
multiplication (*) and modulus (%, which produces the remainder from
integer division). Integer division truncates, rather than rounds, the
result.
Java also uses a shorthand notation to perform an operation and an
assignment at the same time. This is denoted by an operator followed by
an equal sign, and is consistent with all the operators in the language
(whenever it makes sense). For example, to add 4 to the variable x and
assign the result to x, use: x += 4.
This example shows the use of the mathematical operators:
//: c03:MathOps.java
// Demonstrates the mathematical operators.
import java.util.*;
public class MathOps {
// Create a shorthand to save typing:
static void prt(String s) {
System.out.println(s);
}
// shorthand to print a string and an int:
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static void pInt(String s, int i) {
prt(s + " = " + i);
}
// shorthand to print a string and a float:
static void pFlt(String s, float f) {
prt(s + " = " + f);
}
public static void main(String[] args) {
// Create a random number generator,
// seeds with current time by default:
Random rand = new Random();
int i, j, k;
// '%' limits maximum value to 99:
j = rand.nextInt() % 100;
k = rand.nextInt() % 100;
pInt("j",j); pInt("k",k);
i = j + k; pInt("j + k", i);
i = j - k; pInt("j - k", i);
i = k / j; pInt("k / j", i);
i = k * j; pInt("k * j", i);
i = k % j; pInt("k % j", i);
j %= k; pInt("j %= k", j);
// Floating-point number tests:
float u,v,w; // applies to doubles, too
v = rand.nextFloat();
w = rand.nextFloat();
pFlt("v", v); pFlt("w", w);
u = v + w; pFlt("v + w", u);
u = v - w; pFlt("v - w", u);
u = v * w; pFlt("v * w", u);
u = v / w; pFlt("v / w", u);
// the following also works for
// char, byte, short, int, long,
// and double:
u += v; pFlt("u += v", u);
u -= v; pFlt("u -= v", u);
u *= v; pFlt("u *= v", u);
u /= v; pFlt("u /= v", u);
}
} ///:~
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The first thing you will see are some shorthand methods for printing: the
prt( ) method prints a String, the pInt( ) prints a String followed by an
int and the pFlt( ) prints a String followed by a float. Of course, they all
ultimately end up using System.out.println( ).
To generate numbers, the program first creates a Random object.
Because no arguments are passed during creation, Java uses the current
time as a seed for the random number generator. The program generates
a number of different types of random numbers with the Random object
simply by calling different methods: nextInt( ), nextLong( ),
nextFloat( ) or nextDouble( ).
The modulus operator, when used with the result of the random number
generator, limits the result to an upper bound of the operand minus one
(99 in this case).
Unary minus and plus operators
The unary minus (-) and unary plus (+) are the same operators as binary
minus and plus. The compiler figures out which use is intended by the
way you write the expression. For instance, the statement
x = -a;
has an obvious meaning. The compiler is able to figure out:
x = a * -b;
but the reader might get confused, so it is clearer to say:
x = a * (-b);
The unary minus produces the negative of the value. Unary plus provides
symmetry with unary minus, although it doesn’t have any effect.
Auto increment and decrement
Java, like C, is full of shortcuts. Shortcuts can make code much easier to
type, and either easier or harder to read.
Two of the nicer shortcuts are the increment and decrement operators
(often referred to as the auto-increment and auto-decrement operators).
The decrement operator is -- and means “decrease by one unit.” The
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increment operator is ++ and means “increase by one unit.” If a is an int,
for example, the expression ++a is equivalent to (a = a + 1). Increment
and decrement operators produce the value of the variable as a result.
There are two versions of each type of operator, often called the prefix and
postfix versions. Pre-increment means the ++ operator appears before
the variable or expression, and post-increment means the ++ operator
appears after the variable or expression. Similarly, pre-decrement means
the -- operator appears before the variable or expression, and post-
decrement means the -- operator appears after the variable or expression.
For pre-increment and pre-decrement, (i.e., ++a or --a), the operation is
performed and the value is produced. For post-increment and post-
decrement (i.e. a++ or a--), the value is produced, then the operation is
performed. As an example:
//: c03:AutoInc.java
// Demonstrates the ++ and -- operators.
public class AutoInc {
public static void main(String[] args) {
int i = 1;
prt("i : " + i);
prt("++i : " + ++i); // Pre-increment
prt("i++ : " + i++); // Post-increment
prt("i : " + i);
prt("--i : " + --i); // Pre-decrement
prt("i-- : " + i--); // Post-decrement
prt("i : " + i);
}
static void prt(String s) {
System.out.println(s);
}
} ///:~
The output for this program is:
i : 1
++i : 2
i++ : 2
i : 3
--i : 2
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i-- : 2
i : 1
You can see that for the prefix form you get the value after the operation
has been performed, but with the postfix form you get the value before the
operation is performed. These are the only operators (other than those
involving assignment) that have side effects. (That is, they change the
operand rather than using just its value.)
The increment operator is one explanation for the name C++, implying
“one step beyond C.” In an early Java speech, Bill Joy (one of the
creators), said that “Java=C++--” (C plus plus minus minus), suggesting
that Java is C++ with the unnecessary hard parts removed and therefore a
much simpler language. As you progress in this book you’ll see that many
parts are simpler, and yet Java isn’t that much easier than C++.
Relational operators
Relational operators generate a boolean result. They evaluate the
relationship between the values of the operands. A relational expression
produces true if the relationship is true, and false if the relationship is
untrue. The relational operators are less than (<), greater than (>), less
than or equal to (<=), greater than or equal to (>=), equivalent (==) and
not equivalent (!=). Equivalence and nonequivalence works with all built-
in data types, but the other comparisons won’t work with type boolean.
Testing object equivalence
The relational operators == and != also work with all objects, but their
meaning often confuses the first-time Java programmer. Here’s an
example:
//: c03:Equivalence.java
public class Equivalence {
public static void main(String[] args) {
Integer n1 = new Integer(47);
Integer n2 = new Integer(47);
System.out.println(n1 == n2);
System.out.println(n1 != n2);
}
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} ///:~
The expression System.out.println(n1 == n2) will print the result of
the boolean comparison within it. Surely the output should be true and
then false, since both Integer objects are the same. But while the
contents of the objects are the same, the references are not the same and
the operators == and != compare object references. So the output is
actually false and then true. Naturally, this surprises people at first.
What if you want to compare the actual contents of an object for
equivalence? You must use the special method equals( ) that exists for
all objects (not primitives, which work fine with == and !=). Here’s how
it’s used:
//: c03:EqualsMethod.java
public class EqualsMethod {
public static void main(String[] args) {
Integer n1 = new Integer(47);
Integer n2 = new Integer(47);
System.out.println(n1.equals(n2));
}
} ///:~
The result will be true, as you would expect. Ah, but it’s not as simple as
that. If you create your own class, like this:
//: c03:EqualsMethod2.java
class Value {
int i;
}
public class EqualsMethod2 {
public static void main(String[] args) {
Value v1 = new Value();
Value v2 = new Value();
v1.i = v2.i = 100;
System.out.println(v1.equals(v2));
}
} ///:~
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you’re back to square one: the result is false. This is because the default
behavior of equals( ) is to compare references. So unless you override
equals( ) in your new class you won’t get the desired behavior.
Unfortunately, you won’t learn about overriding until Chapter 7, but being
aware of the way equals( ) behaves might save you some grief in the
meantime.
Most of the Java library classes implement equals( ) so that it compares
the contents of objects instead of their references.
Logical operators
The logical operators AND (&&), OR (||) and NOT (!) produce a boolean
value of true or false based on the logical relationship of its arguments.
This example uses the relational and logical operators:
//: c03:Bool.java
// Relational and logical operators.
import java.util.*;
public class Bool {
public static void main(String[] args) {
Random rand = new Random();
int i = rand.nextInt() % 100;
int j = rand.nextInt() % 100;
prt("i = " + i);
prt("j = " + j);
prt("i > j is " + (i > j));
prt("i < j is " + (i < j));
prt("i >= j is " + (i >= j));
prt("i <= j is " + (i <= j));
prt("i == j is " + (i == j));
prt("i != j is " + (i != j));
// Treating an int as a boolean is
// not legal Java
//! prt("i && j is " + (i && j));
//! prt("i || j is " + (i || j));
//! prt("!i is " + !i);
prt("(i < 10) && (j < 10) is "
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+ ((i < 10) && (j < 10)) );
prt("(i < 10) || (j < 10) is "
+ ((i < 10) || (j < 10)) );
}
static void prt(String s) {
System.out.println(s);
}
} ///:~
You can apply AND, OR, or NOT to boolean values only. You can’t use a
non-boolean as if it were a boolean in a logical expression as you can in
C and C++. You can see the failed attempts at doing this commented out
with a //! comment marker. The subsequent expressions, however,
produce boolean values using relational comparisons, then use logical
operations on the results.
One output listing looked like this:
i = 85
j = 4
i > j is true
i < j is false
i >= j is true
i <= j is false
i == j is false
i != j is true
(i < 10) && (j < 10) is false
(i < 10) || (j < 10) is true
Note that a boolean value is automatically converted to an appropriate
text form if it’s used where a String is expected.
You can replace the definition for int in the above program with any other
primitive data type except boolean. Be aware, however, that the
comparison of floating-point numbers is very strict. A number that is the
tiniest fraction different from another number is still “not equal.” A
number that is the tiniest bit above zero is still nonzero.
Short-circuiting
When dealing with logical operators you run into a phenomenon called
“short circuiting.” This means that the expression will be evaluated only
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until the truth or falsehood of the entire expression can be unambiguously
determined. As a result, all the parts of a logical expression might not be
evaluated. Here’s an example that demonstrates short-circuiting:
//: c03:ShortCircuit.java
// Demonstrates short-circuiting behavior.
// with logical operators.
public class ShortCircuit {
static boolean test1(int val) {
System.out.println("test1(" + val + ")");
System.out.println("result: " + (val < 1));
return val < 1;
}
static boolean test2(int val) {
System.out.println("test2(" + val + ")");
System.out.println("result: " + (val < 2));
return val < 2;
}
static boolean test3(int val) {
System.out.println("test3(" + val + ")");
System.out.println("result: " + (val < 3));
return val < 3;
}
public static void main(String[] args) {
if(test1(0) && test2(2) && test3(2))
System.out.println("expression is true");
else
System.out.println("expression is false");
}
} ///:~
Each test performs a comparison against the argument and returns true
or false. It also prints information to show you that it’s being called. The
tests are used in the expression:
if(test1(0) && test2(2) && test3(2))
You might naturally think that all three tests would be executed, but the
output shows otherwise:
test1(0)
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result: true
test2(2)
result: false
expression is false
The first test produced a true result, so the expression evaluation
continues. However, the second test produced a false result. Since this
means that the whole expression must be false, why continue evaluating
the rest of the expression? It could be expensive. The reason for short-
circuiting, in fact, is precisely that; you can get a potential performance
increase if all the parts of a logical expression do not need to be evaluated.
Bitwise operators
The bitwise operators allow you to manipulate individual bits in an
integral primitive data type. Bitwise operators perform boolean algebra on
the corresponding bits in the two arguments to produce the result.
The bitwise operators come from C’s low-level orientation; you were often
manipulating hardware directly and had to set the bits in hardware
registers. Java was originally designed to be embedded in TV set-top
boxes, so this low-level orientation still made sense. However, you
probably won’t use the bitwise operators much.
The bitwise AND operator (&) produces a one in the output bit if both
input bits are one; otherwise it produces a zero. The bitwise OR operator
(|) produces a one in the output bit if either input bit is a one and
produces a zero only if both input bits are zero. The bitwise EXCLUSIVE
OR, or XOR (^), produces a one in the output bit if one or the other input
bit is a one, but not both. The bitwise NOT (~, also called the ones
complement operator) is a unary operator; it takes only one argument.
(All other bitwise operators are binary operators.) Bitwise NOT produces
the opposite of the input bit—a one if the input bit is zero, a zero if the
input bit is one.
The bitwise operators and logical operators use the same characters, so it
is helpful to have a mnemonic device to help you remember the meanings:
since bits are “small,” there is only one character in the bitwise operators.
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Bitwise operators can be combined with the = sign to unite the operation
and assignment: &=, |= and ^= are all legitimate. (Since ~ is a unary
operator it cannot be combined with the = sign.)
The boolean type is treated as a one-bit value so it is somewhat different.
You can perform a bitwise AND, OR and XOR, but you can’t perform a
bitwise NOT (presumably to prevent confusion with the logical NOT). For
booleans the bitwise operators have the same effect as the logical
operators except that they do not short circuit. Also, bitwise operations on
booleans include an XOR logical operator that is not included under the
list of “logical” operators. You’re prevented from using booleans in shift
expressions, which is described next.
Shift operators
The shift operators also manipulate bits. They can be used solely with
primitive, integral types. The left-shift operator (<<) produces the
operand to the left of the operator shifted to the left by the number of bits
specified after the operator (inserting zeroes at the lower-order bits). The
signed right-shift operator (>>) produces the operand to the left of the
operator shifted to the right by the number of bits specified after the
operator. The signed right shift >> uses sign extension: if the value is
positive, zeroes are inserted at the higher-order bits; if the value is
negative, ones are inserted at the higher-order bits. Java has also added
the unsigned right shift >>>, which uses zero extension: regardless of the
sign, zeroes are inserted at the higher-order bits. This operator does not
exist in C or C++.
If you shift a char, byte, or short, it will be promoted to int before the
shift takes place, and the result will be an int. Only the five low-order bits
of the right-hand side will be used. This prevents you from shifting more
than the number of bits in an int. If you’re operating on a long, you’ll get
a long result. Only the six low-order bits of the right-hand side will be
used so you can’t shift more than the number of bits in a long.
Shifts can be combined with the equal sign (<<= or >>= or >>>=). The
lvalue is replaced by the lvalue shifted by the rvalue. There is a problem,
however, with the unsigned right shift combined with assignment. If you
use it with byte or short you don’t get the correct results. Instead, these
are promoted to int and right shifted, but then truncated as they are
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assigned back into their variables, so you get -1 in those cases. The
following example demonstrates this:
//: c03:URShift.java
// Test of unsigned right shift.
public class URShift {
public static void main(String[] args) {
int i = -1;
i >>>= 10;
System.out.println(i);
long l = -1;
l >>>= 10;
System.out.println(l);
short s = -1;
s >>>= 10;
System.out.println(s);
byte b = -1;
b >>>= 10;
System.out.println(b);
b = -1;
System.out.println(b>>>10);
}
} ///:~
In the last line, the resulting value is not assigned back into b, but is
printed directly and so the correct behavior occurs.
Here’s an example that demonstrates the use of all the operators involving
bits:
//: c03:BitManipulation.java
// Using the bitwise operators.
import java.util.*;
public class BitManipulation {
public static void main(String[] args) {
Random rand = new Random();
int i = rand.nextInt();
int j = rand.nextInt();
pBinInt("-1", -1);
pBinInt("+1", +1);
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int maxpos = 2147483647;
pBinInt("maxpos", maxpos);
int maxneg = -2147483648;
pBinInt("maxneg", maxneg);
pBinInt("i", i);
pBinInt("~i", ~i);
pBinInt("-i", -i);
pBinInt("j", j);
pBinInt("i & j", i & j);
pBinInt("i | j", i | j);
pBinInt("i ^ j", i ^ j);
pBinInt("i << 5", i << 5);
pBinInt("i >> 5", i >> 5);
pBinInt("(~i) >> 5", (~i) >> 5);
pBinInt("i >>> 5", i >>> 5);
pBinInt("(~i) >>> 5", (~i) >>> 5);
long l = rand.nextLong();
long m = rand.nextLong();
pBinLong("-1L", -1L);
pBinLong("+1L", +1L);
long ll = 9223372036854775807L;
pBinLong("maxpos", ll);
long lln = -9223372036854775808L;
pBinLong("maxneg", lln);
pBinLong("l", l);
pBinLong("~l", ~l);
pBinLong("-l", -l);
pBinLong("m", m);
pBinLong("l & m", l & m);
pBinLong("l | m", l | m);
pBinLong("l ^ m", l ^ m);
pBinLong("l << 5", l << 5);
pBinLong("l >> 5", l >> 5);
pBinLong("(~l) >> 5", (~l) >> 5);
pBinLong("l >>> 5", l >>> 5);
pBinLong("(~l) >>> 5", (~l) >>> 5);
}
static void pBinInt(String s, int i) {
System.out.println(
s + ", int: " + i + ", binary: ");
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System.out.print(" ");
for(int j = 31; j >=0; j--)
if(((1 << j) & i) != 0)
System.out.print("1");
else
System.out.print("0");
System.out.println();
}
static void pBinLong(String s, long l) {
System.out.println(
s + ", long: " + l + ", binary: ");
System.out.print(" ");
for(int i = 63; i >=0; i--)
if(((1L << i) & l) != 0)
System.out.print("1");
else
System.out.print("0");
System.out.println();
}
} ///:~
The two methods at the end, pBinInt( ) and pBinLong( ) take an int or
a long, respectively, and print it out in binary format along with a
descriptive string. You can ignore the implementation of these for now.
You’ll note the use of System.out.print( ) instead of
System.out.println( ). The print( ) method does not emit a new line,
so it allows you to output a line in pieces.
As well as demonstrating the effect of all the bitwise operators for int and
long, this example also shows the minimum, maximum, +1 and -1 values
for int and long so you can see what they look like. Note that the high bit
represents the sign: 0 means positive and 1 means negative. The output
for the int portion looks like this:
-1, int: -1, binary:
11111111111111111111111111111111
+1, int: 1, binary:
00000000000000000000000000000001
maxpos, int: 2147483647, binary:
01111111111111111111111111111111
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maxneg, int: -2147483648, binary:
10000000000000000000000000000000
i, int: 59081716, binary:
00000011100001011000001111110100
~i, int: -59081717, binary:
11111100011110100111110000001011
-i, int: -59081716, binary:
11111100011110100111110000001100
j, int: 198850956, binary:
00001011110110100011100110001100
i & j, int: 58720644, binary:
00000011100000000000000110000100
i | j, int: 199212028, binary:
00001011110111111011101111111100
i ^ j, int: 140491384, binary:
00001000010111111011101001111000
i << 5, int: 1890614912, binary:
01110000101100000111111010000000
i >> 5, int: 1846303, binary:
00000000000111000010110000011111
(~i) >> 5, int: -1846304, binary:
11111111111000111101001111100000
i >>> 5, int: 1846303, binary:
00000000000111000010110000011111
(~i) >>> 5, int: 132371424, binary:
00000111111000111101001111100000
The binary representation of the numbers is referred to as signed two’s
complement.
Ternary if-else operator
This operator is unusual because it has three operands. It is truly an
operator because it produces a value, unlike the ordinary if-else statement
that you’ll see in the next section of this chapter. The expression is of the
form:
boolean-exp ? value0 : value1
If boolean-exp evaluates to true, value0 is evaluated and its result
becomes the value produced by the operator. If boolean-exp is false,
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value1 is evaluated and its result becomes the value produced by the
operator.
Of course, you could use an ordinary if-else statement (described later),
but the ternary operator is much terser. Although C (where this operator
originated) prides itself on being a terse language, and the ternary
operator might have been introduced partly for efficiency, you should be
somewhat wary of using it on an everyday basis—it’s easy to produce
unreadable code.
The conditional operator can be used for its side effects or for the value it
produces, but in general you want the value since that’s what makes the
operator distinct from the if-else. Here’s an example:
static int ternary(int i) {
return i < 10 ? i * 100 : i * 10;
}
You can see that this code is more compact than what you’d need to write
without the ternary operator:
static int alternative(int i) {
if (i < 10)
return i * 100;
else
return i * 10;
}
The second form is easier to understand, and doesn’t require a lot more
typing. So be sure to ponder your reasons when choosing the ternary
operator.
The comma operator
The comma is used in C and C++ not only as a separator in function
argument lists, but also as an operator for sequential evaluation. The sole
place that the comma operator is used in Java is in for loops, which will
be described later in this chapter.
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String operator +
There’s one special usage of an operator in Java: the + operator can be
used to concatenate strings, as you’ve already seen. It seems a natural use
of the + even though it doesn’t fit with the traditional way that + is used.
This capability seemed like a good idea in C++, so operator overloading
was added to C++ to allow the C++ programmer to add meanings to
almost any operator. Unfortunately, operator overloading combined with
some of the other restrictions in C++ turns out to be a fairly complicated
feature for programmers to design into their classes. Although operator
overloading would have been much simpler to implement in Java than it
was in C++, this feature was still considered too complex, so Java
programmers cannot implement their own overloaded operators as C++
programmers can.
The use of the String + has some interesting behavior. If an expression
begins with a String, then all operands that follow must be Strings
(remember that the compiler will turn a quoted sequence of characters
into a String):
int x = 0, y = 1, z = 2;
String sString = "x, y, z ";
System.out.println(sString + x + y + z);
Here, the Java compiler will convert x, y, and z into their String
representations instead of adding them together first. And if you say:
System.out.println(x + sString);
Java will turn x into a String.
Common pitfalls when using
operators
One of the pitfalls when using operators is trying to get away without
parentheses when you are even the least bit uncertain about how an
expression will evaluate. This is still true in Java.
An extremely common error in C and C++ looks like this:
while(x = y) {
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// ....
}
The programmer was trying to test for equivalence (==) rather than do an
assignment. In C and C++ the result of this assignment will always be
true if y is nonzero, and you’ll probably get an infinite loop. In Java, the
result of this expression is not a boolean, and the compiler expects a
boolean and won’t convert from an int, so it will conveniently give you a
compile-time error and catch the problem before you ever try to run the
program. So the pitfall never happens in Java. (The only time you won’t
get a compile-time error is when x and y are boolean, in which case x =
y is a legal expression, and in the above case, probably an error.)
A similar problem in C and C++ is using bitwise AND and OR instead of
the logical versions. Bitwise AND and OR use one of the characters (& or
|) while logical AND and OR use two (&& and ||). Just as with = and ==,
it’s easy to type just one character instead of two. In Java, the compiler
again prevents this because it won’t let you cavalierly use one type where
it doesn’t belong.
Casting operators
The word cast is used in the sense of “casting into a mold.” Java will
automatically change one type of data into another when appropriate. For
instance, if you assign an integral value to a floating-point variable, the
compiler will automatically convert the int to a float. Casting allows you
to make this type conversion explicit, or to force it when it wouldn’t
normally happen.
To perform a cast, put the desired data type (including all modifiers)
inside parentheses to the left of any value. Here’s an example:
void casts() {
int i = 200;
long l = (long)i;
long l2 = (long)200;
}
As you can see, it’s possible to perform a cast on a numeric value as well
as on a variable. In both casts shown here, however, the cast is
superfluous, since the compiler will automatically promote an int value to
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a long when necessary. However, you are allowed to use superfluous
casts in to make a point or to make your code more clear. In other
situations, a cast may be essential just to get the code to compile.
In C and C++, casting can cause some headaches. In Java, casting is safe,
with the exception that when you perform a so-called narrowing
conversion (that is, when you go from a data type that can hold more
information to one that doesn’t hold as much) you run the risk of losing
information. Here the compiler forces you to do a cast, in effect saying
“this can be a dangerous thing to do—if you want me to do it anyway you
must make the cast explicit.” With a widening conversion an explicit cast
is not needed because the new type will more than hold the information
from the old type so that no information is ever lost.
Java allows you to cast any primitive type to any other primitive type,
except for boolean, which doesn’t allow any casting at all. Class types do
not allow casting. To convert one to the other there must be special
methods. (String is a special case, and you’ll find out later in this book
that objects can be cast within a family of types; an Oak can be cast to a
Tree and vice-versa, but not to a foreign type such as a Rock.)
Literals
Ordinarily when you insert a literal value into a program the compiler
knows exactly what type to make it. Sometimes, however, the type is
ambiguous. When this happens you must guide the compiler by adding
some extra information in the form of characters associated with the
literal value. The following code shows these characters:
//: c03:Literals.java
class Literals {
char c = 0xffff; // max char hex value
byte b = 0x7f; // max byte hex value
short s = 0x7fff; // max short hex value
int i1 = 0x2f; // Hexadecimal (lowercase)
int i2 = 0X2F; // Hexadecimal (uppercase)
int i3 = 0177; // Octal (leading zero)
// Hex and Oct also work with long.
long n1 = 200L; // long suffix
long n2 = 200l; // long suffix
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long n3 = 200;
//! long l6(200); // not allowed
float f1 = 1;
float f2 = 1F; // float suffix
float f3 = 1f; // float suffix
float f4 = 1e-45f; // 10 to the power
float f5 = 1e+9f; // float suffix
double d1 = 1d; // double suffix
double d2 = 1D; // double suffix
double d3 = 47e47d; // 10 to the power
} ///:~
Hexadecimal (base 16), which works with all the integral data types, is
denoted by a leading 0x or 0X followed by 0—9 and a—f either in upper
or lowercase. If you try to initialize a variable with a value bigger than it
can hold (regardless of the numerical form of the value), the compiler will
give you an error message. Notice in the above code the maximum
possible hexadecimal values for char, byte, and short. If you exceed
these, the compiler will automatically make the value an int and tell you
that you need a narrowing cast for the assignment. You’ll know you’ve
stepped over the line.
Octal (base 8) is denoted by a leading zero in the number and digits from
0-7. There is no literal representation for binary numbers in C, C++ or
Java.
A trailing character after a literal value establishes its type. Upper or
lowercase L means long, upper or lowercase F means float and upper or
lowercase D means double.
Exponents use a notation that I’ve always found rather dismaying: 1.39 e-
47f. In science and engineering, ‘e’ refers to the base of natural
logarithms, approximately 2.718. (A more precise double value is
available in Java as Math.E.) This is used in exponentiation expressions
such as 1.39 x e-47, which means 1.39 x 2.718-47. However, when FORTRAN
was invented they decided that e would naturally mean “ten to the
power,” which is an odd decision because FORTRAN was designed for
science and engineering and one would think its designers would be
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sensitive about introducing such an ambiguity.1 At any rate, this custom
was followed in C, C++ and now Java. So if you’re used to thinking in
terms of e as the base of natural logarithms, you must do a mental
translation when you see an expression such as 1.39 e-47f in Java; it
means 1.39 x 10-47.
Note that you don’t need to use the trailing character when the compiler
can figure out the appropriate type. With
long n3 = 200;
there’s no ambiguity, so an L after the 200 would be superfluous.
However, with
float f4 = 1e-47f; // 10 to the power
the compiler normally takes exponential numbers as doubles, so without
the trailing f it will give you an error telling you that you must use a cast
to convert double to float.
Promotion
You’ll discover that if you perform any mathematical or bitwise operations
on primitive data types that are smaller than an int (that is, char, byte,
or short), those values will be promoted to int before performing the
operations, and the resulting value will be of type int. So if you want to
assign back into the smaller type, you must use a cast. (And, since you’re
assigning back into a smaller type, you might be losing information.) In
general, the largest data type in an expression is the one that determines
1 John Kirkham writes, “I started computing in 1962 using FORTRAN II on an IBM 1620.
At that time, and throughout the 1960s and into the 1970s, FORTRAN was an all
uppercase language. This probably started because many of the early input devices were
old teletype units that used 5 bit Baudot code, which had no lowercase capability. The ‘E’
in the exponential notation was also always upper case and was never confused with the
natural logarithm base ‘e’, which is always lowercase. The ‘E’ simply stood for exponential,
which was for the base of the number system used—usually 10. At the time octal was also
widely used by programmers. Although I never saw it used, if I had seen an octal number
in exponential notation I would have considered it to be base 8. The first time I remember
seeing an exponential using a lowercase ‘e’ was in the late 1970s and I also found it
confusing. The problem arose as lowercase crept into FORTRAN, not at its beginning. We
actually had functions to use if you really wanted to use the natural logarithm base, but
they were all uppercase.”
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the size of the result of that expression; if you multiply a float and a
double, the result will be double; if you add an int and a long, the
result will be long.
Java has no “sizeof”
In C and C++, the sizeof( ) operator satisfies a specific need: it tells you
the number of bytes allocated for data items. The most compelling need
for sizeof( ) in C and C++ is portability. Different data types might be
different sizes on different machines, so the programmer must find out
how big those types are when performing operations that are sensitive to
size. For example, one computer might store integers in 32 bits, whereas
another might store integers as 16 bits. Programs could store larger values
in integers on the first machine. As you might imagine, portability is a
huge headache for C and C++ programmers.
Java does not need a sizeof( ) operator for this purpose because all the
data types are the same size on all machines. You do not need to think
about portability on this level—it is designed into the language.
Precedence revisited
Upon hearing me complain about the complexity of remembering
operator precedence during one of my seminars, a student suggested a
mnemonic that is simultaneously a commentary: “Ulcer Addicts Really
Like C A lot.”
Mnemonic Operator
type
Operators
Ulcer Unary
+ - ++--
Addicts
Arithmetic (and shift)
* / % + - << >>
Really Relational
> < >= <= == !=
Like
Logical (and bitwise)
&& || & | ^
C Conditional
(ternary)
A > B ? X : Y
A Lot
Assignment
= (and compound
assignment like *=)
Of course, with the shift and bitwise operators distributed around the
table it is not a perfect mnemonic, but for non-bit operations it works.
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A compendium of operators
The following example shows which primitive data types can be used with
particular operators. Basically, it is the same example repeated over and
over, but using different primitive data types. The file will compile
without error because the lines that would cause errors are commented
out with a //!.
//: c03:AllOps.java
// Tests all the operators on all the
// primitive data types to show which
// ones are accepted by the Java compiler.
class AllOps {
// To accept the results of a boolean test:
void f(boolean b) {}
void boolTest(boolean x, boolean y) {
// Arithmetic operators:
//! x = x * y;
//! x = x / y;
//! x = x % y;
//! x = x + y;
//! x = x - y;
//! x++;
//! x--;
//! x = +y;
//! x = -y;
// Relational and logical:
//! f(x > y);
//! f(x >= y);
//! f(x < y);
//! f(x <= y);
f(x == y);
f(x != y);
f(!y);
x = x && y;
x = x || y;
// Bitwise operators:
//! x = ~y;
x = x & y;
x = x | y;
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x = x ^ y;
//! x = x << 1;
//! x = x >> 1;
//! x = x >>> 1;
// Compound assignment:
//! x += y;
//! x -= y;
//! x *= y;
//! x /= y;
//! x %= y;
//! x <<= 1;
//! x >>= 1;
//! x >>>= 1;
x &= y;
x ^= y;
x |= y;
// Casting:
//! char c = (char)x;
//! byte B = (byte)x;
//! short s = (short)x;
//! int i = (int)x;
//! long l = (long)x;
//! float f = (float)x;
//! double d = (double)x;
}
void charTest(char x, char y) {
// Arithmetic operators:
x = (char)(x * y);
x = (char)(x / y);
x = (char)(x % y);
x = (char)(x + y);
x = (char)(x - y);
x++;
x--;
x = (char)+y;
x = (char)-y;
// Relational and logical:
f(x > y);
f(x >= y);
f(x < y);
f(x <= y);
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f(x == y);
f(x != y);
//! f(!x);
//! f(x && y);
//! f(x || y);
// Bitwise operators:
x= (char)~y;
x = (char)(x & y);
x = (char)(x | y);
x = (char)(x ^ y);
x = (char)(x << 1);
x = (char)(x >> 1);
x = (char)(x >>> 1);
// Compound assignment:
x += y;
x -= y;
x *= y;
x /= y;
x %= y;
x <<= 1;
x >>= 1;
x >>>= 1;
x &= y;
x ^= y;
x |= y;
// Casting:
//! boolean b = (boolean)x;
byte B = (byte)x;
short s = (short)x;
int i = (int)x;
long l = (long)x;
float f = (float)x;
double d = (double)x;
}
void byteTest(byte x, byte y) {
// Arithmetic operators:
x = (byte)(x* y);
x = (byte)(x / y);
x = (byte)(x % y);
x = (byte)(x + y);
x = (byte)(x - y);
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x++;
x--;
x = (byte)+ y;
x = (byte)- y;
// Relational and logical:
f(x > y);
f(x >= y);
f(x < y);
f(x <= y);
f(x == y);
f(x != y);
//! f(!x);
//! f(x && y);
//! f(x || y);
// Bitwise operators:
x = (byte)~y;
x = (byte)(x & y);
x = (byte)(x | y);
x = (byte)(x ^ y);
x = (byte)(x << 1);
x = (byte)(x >> 1);
x = (byte)(x >>> 1);
// Compound assignment:
x += y;
x -= y;
x *= y;
x /= y;
x %= y;
x <<= 1;
x >>= 1;
x >>>= 1;
x &= y;
x ^= y;
x |= y;
// Casting:
//! boolean b = (boolean)x;
char c = (char)x;
short s = (short)x;
int i = (int)x;
long l = (long)x;
float f = (float)x;
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double d = (double)x;
}
void shortTest(short x, short y) {
// Arithmetic operators:
x = (short)(x * y);
x = (short)(x / y);
x = (short)(x % y);
x = (short)(x + y);
x = (short)(x - y);
x++;
x--;
x = (short)+y;
x = (short)-y;
// Relational and logical:
f(x > y);
f(x >= y);
f(x < y);
f(x <= y);
f(x == y);
f(x != y);
//! f(!x);
//! f(x && y);
//! f(x || y);
// Bitwise operators:
x = (short)~y;
x = (short)(x & y);
x = (short)(x | y);
x = (short)(x ^ y);
x = (short)(x << 1);
x = (short)(x >> 1);
x = (short)(x >>> 1);
// Compound assignment:
x += y;
x -= y;
x *= y;
x /= y;
x %= y;
x <<= 1;
x >>= 1;
x >>>= 1;
x &= y;
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x ^= y;
x |= y;
// Casting:
//! boolean b = (boolean)x;
char c = (char)x;
byte B = (byte)x;
int i = (int)x;
long l = (long)x;
float f = (float)x;
double d = (double)x;
}
void intTest(int x, int y) {
// Arithmetic operators:
x = x * y;
x = x / y;
x = x % y;
x = x + y;
x = x - y;
x++;
x--;
x = +y;
x = -y;
// Relational and logical:
f(x > y);
f(x >= y);
f(x < y);
f(x <= y);
f(x == y);
f(x != y);
//! f(!x);
//! f(x && y);
//! f(x || y);
// Bitwise operators:
x = ~y;
x = x & y;
x = x | y;
x = x ^ y;
x = x << 1;
x = x >> 1;
x = x >>> 1;
// Compound assignment:
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x += y;
x -= y;
x *= y;
x /= y;
x %= y;
x <<= 1;
x >>= 1;
x >>>= 1;
x &= y;
x ^= y;
x |= y;
// Casting:
//! boolean b = (boolean)x;
char c = (char)x;
byte B = (byte)x;
short s = (short)x;
long l = (long)x;
float f = (float)x;
double d = (double)x;
}
void longTest(long x, long y) {
// Arithmetic operators:
x = x * y;
x = x / y;
x = x % y;
x = x + y;
x = x - y;
x++;
x--;
x = +y;
x = -y;
// Relational and logical:
f(x > y);
f(x >= y);
f(x < y);
f(x <= y);
f(x == y);
f(x != y);
//! f(!x);
//! f(x && y);
//! f(x || y);
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// Bitwise operators:
x = ~y;
x = x & y;
x = x | y;
x = x ^ y;
x = x << 1;
x = x >> 1;
x = x >>> 1;
// Compound assignment:
x += y;
x -= y;
x *= y;
x /= y;
x %= y;
x <<= 1;
x >>= 1;
x >>>= 1;
x &= y;
x ^= y;
x |= y;
// Casting:
//! boolean b = (boolean)x;
char c = (char)x;
byte B = (byte)x;
short s = (short)x;
int i = (int)x;
float f = (float)x;
double d = (double)x;
}
void floatTest(float x, float y) {
// Arithmetic operators:
x = x * y;
x = x / y;
x = x % y;
x = x + y;
x = x - y;
x++;
x--;
x = +y;
x = -y;
// Relational and logical:
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f(x > y);
f(x >= y);
f(x < y);
f(x <= y);
f(x == y);
f(x != y);
//! f(!x);
//! f(x && y);
//! f(x || y);
// Bitwise operators:
//! x = ~y;
//! x = x & y;
//! x = x | y;
//! x = x ^ y;
//! x = x << 1;
//! x = x >> 1;
//! x = x >>> 1;
// Compound assignment:
x += y;
x -= y;
x *= y;
x /= y;
x %= y;
//! x <<= 1;
//! x >>= 1;
//! x >>>= 1;
//! x &= y;
//! x ^= y;
//! x |= y;
// Casting:
//! boolean b = (boolean)x;
char c = (char)x;
byte B = (byte)x;
short s = (short)x;
int i = (int)x;
long l = (long)x;
double d = (double)x;
}
void doubleTest(double x, double y) {
// Arithmetic operators:
x = x * y;
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x = x / y;
x = x % y;
x = x + y;
x = x - y;
x++;
x--;
x = +y;
x = -y;
// Relational and logical:
f(x > y);
f(x >= y);
f(x < y);
f(x <= y);
f(x == y);
f(x != y);
//! f(!x);
//! f(x && y);
//! f(x || y);
// Bitwise operators:
//! x = ~y;
//! x = x & y;
//! x = x | y;
//! x = x ^ y;
//! x = x << 1;
//! x = x >> 1;
//! x = x >>> 1;
// Compound assignment:
x += y;
x -= y;
x *= y;
x /= y;
x %= y;
//! x <<= 1;
//! x >>= 1;
//! x >>>= 1;
//! x &= y;
//! x ^= y;
//! x |= y;
// Casting:
//! boolean b = (boolean)x;
char c = (char)x;
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byte B = (byte)x;
short s = (short)x;
int i = (int)x;
long l = (long)x;
float f = (float)x;
}
} ///:~
Note that boolean is quite limited. You can assign to it the values true
and false, and you can test it for truth or falsehood, but you cannot add
booleans or perform any other type of operation on them.
In char, byte, and short you can see the effect of promotion with the
arithmetic operators. Each arithmetic operation on any of those types
results in an int result, which must be explicitly cast back to the original
type (a narrowing conversion that might lose information) to assign back
to that type. With int values, however, you do not need to cast, because
everything is already an int. Don’t be lulled into thinking everything is
safe, though. If you multiply two ints that are big enough, you’ll overflow
the result. The following example demonstrates this:
//: c03:Overflow.java
// Surprise! Java lets you overflow.
public class Overflow {
public static void main(String[] args) {
int big = 0x7fffffff; // max int value
prt("big = " + big);
int bigger = big * 4;
prt("bigger = " + bigger);
}
static void prt(String s) {
System.out.println(s);
}
} ///:~
The output of this is:
big = 2147483647
bigger = -4
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and you get no errors or warnings from the compiler, and no exceptions at
run-time. Java is good, but it’s not that good.
Compound assignments do not require casts for char, byte, or short,
even though they are performing promotions that have the same results
as the direct arithmetic operations. On the other hand, the lack of the cast
certainly simplifies the code.
You can see that, with the exception of boolean, any primitive type can
be cast to any other primitive type. Again, you must be aware of the effect
of a narrowing conversion when casting to a smaller type, otherwise you
might unknowingly lose information during the cast.
Execution control
Java uses all of C’s execution control statements, so if you’ve programmed
with C or C++ then most of what you see will be familiar. Most procedural
programming languages have some kind of control statements, and there
is often overlap among languages. In Java, the keywords include if-else,
while, do-while, for, and a selection statement called switch. Java
does not, however, support the much-maligned goto (which can still be
the most expedient way to solve certain types of problems). You can still
do a goto-like jump, but it is much more constrained than a typical goto.
true and false
All conditional statements use the truth or falsehood of a conditional
expression to determine the execution path. An example of a conditional
expression is A == B. This uses the conditional operator == to see if the
value of A is equivalent to the value of B. The expression returns true or
false. Any of the relational operators you’ve seen earlier in this chapter
can be used to produce a conditional statement. Note that Java doesn’t
allow you to use a number as a boolean, even though it’s allowed in C
and C++ (where truth is nonzero and falsehood is zero). If you want to use
a non-boolean in a boolean test, such as if(a), you must first convert it
to a boolean value using a conditional expression, such as if(a != 0).
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if-else
The if-else statement is probably the most basic way to control program
flow. The else is optional, so you can use if in two forms:
if(Boolean-expression)
statement
or
if(Boolean-expression)
statement
else
statement
The conditional must produce a boolean result. The statement means
either a simple statement terminated by a semicolon or a compound
statement, which is a group of simple statements enclosed in braces. Any
time the word “statement” is used, it always implies that the statement
can be simple or compound.
As an example of if-else, here is a test( ) method that will tell you
whether a guess is above, below, or equivalent to a target number:
//: c03:IfElse.java
public class IfElse {
static int test(int testval, int target) {
int result = 0;
if(testval > target)
result = +1;
else if(testval < target)
result = -1;
else
result = 0; // Match
return result;
}
public static void main(String[] args) {
System.out.println(test(10, 5));
System.out.println(test(5, 10));
System.out.println(test(5, 5));
}
} ///:~
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It is conventional to indent the body of a control flow statement so the
reader might easily determine where it begins and ends.
return
The return keyword has two purposes: it specifies what value a method
will return (if it doesn’t have a void return value) and it causes that value
to be returned immediately. The test( ) method above can be rewritten to
take advantage of this:
//: c03:IfElse2.java
public class IfElse2 {
static int test(int testval, int target) {
int result = 0;
if(testval > target)
return +1;
else if(testval < target)
return -1;
else
return 0; // Match
}
public static void main(String[] args) {
System.out.println(test(10, 5));
System.out.println(test(5, 10));
System.out.println(test(5, 5));
}
} ///:~
There’s no need for else because the method will not continue after
executing a return.
Iteration
while, do-while and for control looping and are sometimes classified as
iteration statements. A statement repeats until the controlling Boolean-
expression evaluates to false. The form for a while loop is
while(Boolean-expression)
statement
The Boolean-expression is evaluated once at the beginning of the loop
and again before each further iteration of the statement.
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Here’s a simple example that generates random numbers until a
particular condition is met:
//: c03:WhileTest.java
// Demonstrates the while loop.
public class WhileTest {
public static void main(String[] args) {
double r = 0;
while(r < 0.99d) {
r = Math.random();
System.out.println(r);
}
}
} ///:~
This uses the static method random( ) in the Math library, which
generates a double value between 0 and 1. (It includes 0, but not 1.) The
conditional expression for the while says “keep doing this loop until the
number is 0.99 or greater.” Each time you run this program you’ll get a
different-sized list of numbers.
do-while
The form for do-while is
do
statement
while(Boolean-expression);
The sole difference between while and do-while is that the statement of
the do-while always executes at least once, even if the expression
evaluates to false the first time. In a while, if the conditional is false the
first time the statement never executes. In practice, do-while is less
common than while.
for
A for loop performs initialization before the first iteration. Then it
performs conditional testing and, at the end of each iteration, some form
of “stepping.” The form of the for loop is:
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for(initialization; Boolean-expression; step)
statement
Any of the expressions initialization, Boolean-expression or step can be
empty. The expression is tested before each iteration, and as soon as it
evaluates to false execution will continue at the line following the for
statement. At the end of each loop, the step executes.
for loops are usually used for “counting” tasks:
//: c03:ListCharacters.java
// Demonstrates "for" loop by listing
// all the ASCII characters.
public class ListCharacters {
public static void main(String[] args) {
for( char c = 0; c < 128; c++)
if (c != 26 ) // ANSI Clear screen
System.out.println(
"value: " + (int)c +
" character: " + c);
}
} ///:~
Note that the variable c is defined at the point where it is used, inside the
control expression of the for loop, rather than at the beginning of the
block denoted by the open curly brace. The scope of c is the expression
controlled by the for.
Traditional procedural languages like C require that all variables be
defined at the beginning of a block so when the compiler creates a block it
can allocate space for those variables. In Java and C++ you can spread
your variable declarations throughout the block, defining them at the
point that you need them. This allows a more natural coding style and
makes code easier to understand.
You can define multiple variables within a for statement, but they must
be of the same type:
for(int i = 0, j = 1;
i < 10 && j != 11;
i++, j++)
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/* body of for loop */;
The int definition in the for statement covers both i and j. The ability to
define variables in the control expression is limited to the for loop. You
cannot use this approach with any of the other selection or iteration
statements.
The comma operator
Earlier in this chapter I stated that the comma operator (not the comma
separator, which is used to separate definitions and function arguments)
has only one use in Java: in the control expression of a for loop. In both
the initialization and step portions of the control expression you can have
a number of statements separated by commas, and those statements will
be evaluated sequentially. The previous bit of code uses this ability. Here’s
another example:
//: c03:CommaOperator.java
public class CommaOperator {
public static void main(String[] args) {
for(int i = 1, j = i + 10; i < 5;
i++, j = i * 2) {
System.out.println("i= " + i + " j= " + j);
}
}
} ///:~
Here’s the output:
i= 1 j= 11
i= 2 j= 4
i= 3 j= 6
i= 4 j= 8
You can see that in both the initialization and step portions the
statements are evaluated in sequential order. Also, the initialization
portion can have any number of definitions of one type.
break and continue
Inside the body of any of the iteration statements you can also control the
flow of the loop by using break and continue. break quits the loop
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without executing the rest of the statements in the loop. continue stops
the execution of the current iteration and goes back to the beginning of
the loop to begin the next iteration.
This program shows examples of break and continue within for and
while loops:
//: c03:BreakAndContinue.java
// Demonstrates break and continue keywords.
public class BreakAndContinue {
public static void main(String[] args) {
for(int i = 0; i < 100; i++) {
if(i == 74) break; // Out of for loop
if(i % 9 != 0) continue; // Next iteration
System.out.println(i);
}
int i = 0;
// An "infinite loop":
while(true) {
i++;
int j = i * 27;
if(j == 1269) break; // Out of loop
if(i % 10 != 0) continue; // Top of loop
System.out.println(i);
}
}
} ///:~
In the for loop the value of i never gets to 100 because the break
statement breaks out of the loop when i is 74. Normally, you’d use a
break like this only if you didn’t know when the terminating condition
was going to occur. The continue statement causes execution to go back
to the top of the iteration loop (thus incrementing i) whenever i is not
evenly divisible by 9. When it is, the value is printed.
The second portion shows an “infinite loop” that would, in theory,
continue forever. However, inside the loop there is a break statement
that will break out of the loop. In addition, you’ll see that the continue
moves back to the top of the loop without completing the remainder.
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(Thus printing happens in the second loop only when the value of i is
divisible by 10.) The output is:
0
9
18
27
36
45
54
63
72
10
20
30
40
The value 0 is printed because 0 % 9 produces 0.
A second form of the infinite loop is for(;;). The compiler treats both
while(true) and for(;;) in the same way so whichever one you use is a
matter of programming taste.
The infamous “goto”
The goto keyword has been present in programming languages from the
beginning. Indeed, goto was the genesis of program control in assembly
language: “if condition A, then jump here, otherwise jump there.” If you
read the assembly code that is ultimately generated by virtually any
compiler, you’ll see that program control contains many jumps. However,
a goto is a jump at the source-code level, and that’s what brought it into
disrepute. If a program will always jump from one point to another, isn’t
there some way to reorganize the code so the flow of control is not so
jumpy? goto fell into true disfavor with the publication of the famous
“Goto considered harmful” paper by Edsger Dijkstra, and since then goto-
bashing has been a popular sport, with advocates of the cast-out keyword
scurrying for cover.
As is typical in situations like this, the middle ground is the most fruitful.
The problem is not the use of goto, but the overuse of goto—in rare
situations goto is actually the best way to structure control flow.
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Although goto is a reserved word in Java, it is not used in the language;
Java has no goto. However, it does have something that looks a bit like a
jump tied in with the break and continue keywords. It’s not a jump but
rather a way to break out of an iteration statement. The reason it’s often
thrown in with discussions of goto is because it uses the same
mechanism: a label.
A label is an identifier followed by a colon, like this:
label1:
The only place a label is useful in Java is right before an iteration
statement. And that means right before—it does no good to put any other
statement between the label and the iteration. And the sole reason to put
a label before an iteration is if you’re going to nest another iteration or a
switch inside it. That’s because the break and continue keywords will
normally interrupt only the current loop, but when used with a label
they’ll interrupt the loops up to where the label exists:
label1:
outer-iteration {
inner-iteration {
//…
break; // 1
//…
continue; // 2
//…
continue label1; // 3
//…
break label1; // 4
}
}
In case 1, the break breaks out of the inner iteration and you end up in
the outer iteration. In case 2, the continue moves back to the beginning
of the inner iteration. But in case 3, the continue label1 breaks out of
the inner iteration and the outer iteration, all the way back to label1.
Then it does in fact continue the iteration, but starting at the outer
iteration. In case 4, the break label1 also breaks all the way out to
label1, but it does not re-enter the iteration. It actually does break out of
both iterations.
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Here is an example using for loops:
//: c03:LabeledFor.java
// Java’s "labeled for" loop.
public class LabeledFor {
public static void main(String[] args) {
int i = 0;
outer: // Can't have statements here
for(; true ;) { // infinite loop
inner: // Can't have statements here
for(; i < 10; i++) {
prt("i = " + i);
if(i == 2) {
prt("continue");
continue;
}
if(i == 3) {
prt("break");
i++; // Otherwise i never
// gets incremented.
break;
}
if(i == 7) {
prt("continue outer");
i++; // Otherwise i never
// gets incremented.
continue outer;
}
if(i == 8) {
prt("break outer");
break outer;
}
for(int k = 0; k < 5; k++) {
if(k == 3) {
prt("continue inner");
continue inner;
}
}
}
}
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// Can't break or continue
// to labels here
}
static void prt(String s) {
System.out.println(s);
}
} ///:~
This uses the prt( ) method that has been defined in the other examples.
Note that break breaks out of the for loop, and that the increment-
expression doesn’t occur until the end of the pass through the for loop.
Since break skips the increment expression, the increment is performed
directly in the case of i == 3. The continue outer statement in the case
of i == 7 also goes to the top of the loop and also skips the increment, so
it too is incremented directly.
Here is the output:
i = 0
continue inner
i = 1
continue inner
i = 2
continue
i = 3
break
i = 4
continue inner
i = 5
continue inner
i = 6
continue inner
i = 7
continue outer
i = 8
break outer
If not for the break outer statement, there would be no way to get out of
the outer loop from within an inner loop, since break by itself can break
out of only the innermost loop. (The same is true for continue.)
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Of course, in the cases where breaking out of a loop will also exit the
method, you can simply use a return.
Here is a demonstration of labeled break and continue statements with
while loops:
//: c03:LabeledWhile.java
// Java's "labeled while" loop.
public class LabeledWhile {
public static void main(String[] args) {
int i = 0;
outer:
while(true) {
prt("Outer while loop");
while(true) {
i++;
prt("i = " + i);
if(i == 1) {
prt("continue");
continue;
}
if(i == 3) {
prt("continue outer");
continue outer;
}
if(i == 5) {
prt("break");
break;
}
if(i == 7) {
prt("break outer");
break outer;
}
}
}
}
static void prt(String s) {
System.out.println(s);
}
} ///:~
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The same rules hold true for while:
1.
A plain continue goes to the top of the innermost loop and
continues.
2.
A labeled continue goes to the label and re-enters the loop right
after that label.
3.
A break “drops out of the bottom” of the loop.
4.
A labeled break drops out of the bottom of the end of the loop
denoted by the label.
The output of this method makes it clear:
Outer while loop
i = 1
continue
i = 2
i = 3
continue outer
Outer while loop
i = 4
i = 5
break
Outer while loop
i = 6
i = 7
break outer
It’s important to remember that the only reason to use labels in Java is
when you have nested loops and you want to break or continue through
more than one nested level.
In Dijkstra’s “goto considered harmful” paper, what he specifically
objected to was the labels, not the goto. He observed that the number of
bugs seems to increase with the number of labels in a program. Labels
and gotos make programs difficult to analyze statically, since it introduces
cycles in the program execution graph. Note that Java labels don’t suffer
from this problem, since they are constrained in their placement and can’t
be used to transfer control in an ad hoc manner. It’s also interesting to
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note that this is a case where a language feature is made more useful by
restricting the power of the statement.
switch
The switch is sometimes classified as a selection statement. The switch
statement selects from among pieces of code based on the value of an
integral expression. Its form is:
switch(integral-selector) {
case integral-value1 : statement; break;
case integral-value2 : statement; break;
case integral-value3 : statement; break;
case integral-value4 : statement; break;
case integral-value5 : statement; break;
// ...
default: statement;
}
Integral-selector is an expression that produces an integral value. The
switch compares the result of integral-selector to each integral-value. If
it finds a match, the corresponding statement (simple or compound)
executes. If no match occurs, the default statement executes.
You will notice in the above definition that each case ends with a break,
which causes execution to jump to the end of the switch body. This is the
conventional way to build a switch statement, but the break is optional.
If it is missing, the code for the following case statements execute until a
break is encountered. Although you don’t usually want this kind of
behavior, it can be useful to an experienced programmer. Note the last
statement, following the default, doesn’t have a break because the
execution just falls through to where the break would have taken it
anyway. You could put a break at the end of the default statement with
no harm if you considered it important for style’s sake.
The switch statement is a clean way to implement multi-way selection
(i.e., selecting from among a number of different execution paths), but it
requires a selector that evaluates to an integral value such as int or char.
If you want to use, for example, a string or a floating-point number as a
selector, it won’t work in a switch statement. For non-integral types, you
must use a series of if statements.
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Here’s an example that creates letters randomly and determines whether
they’re vowels or consonants:
//: c03:VowelsAndConsonants.java
// Demonstrates the switch statement.
public class VowelsAndConsonants {
public static void main(String[] args) {
for(int i = 0; i < 100; i++) {
char c = (char)(Math.random() * 26 + 'a');
System.out.print(c + ": ");
switch(c) {
case 'a':
case 'e':
case 'i':
case 'o':
case 'u':
System.out.println("vowel");
break;
case 'y':
case 'w':
System.out.println(
"Sometimes a vowel");
break;
default:
System.out.println("consonant");
}
}
}
} ///:~
Since Math.random( ) generates a value between 0 and 1, you need
only multiply it by the upper bound of the range of numbers you want to
produce (26 for the letters in the alphabet) and add an offset to establish
the lower bound.
Although it appears you’re switching on a character here, the switch
statement is actually using the integral value of the character. The singly-
quoted characters in the case statements also produce integral values
that are used for comparison.
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Notice how the cases can be “stacked” on top of each other to provide
multiple matches for a particular piece of code. You should also be aware
that it’s essential to put the break statement at the end of a particular
case, otherwise control will simply drop through and continue processing
on the next case.
Calculation details
The statement:
char c = (char)(Math.random() * 26 + 'a');
deserves a closer look. Math.random( ) produces a double, so the
value 26 is converted to a double to perform the multiplication, which
also produces a double. This means that ‘a’ must be converted to a
double to perform the addition. The double result is turned back into a
char with a cast.
What does the cast to char do? That is, if you have the value 29.7 and you
cast it to a char, is the resulting value 30 or 29? The answer to this can be
seen in this example:
//: c03:CastingNumbers.java
// What happens when you cast a float
// or double to an integral value?
public class CastingNumbers {
public static void main(String[] args) {
double
above = 0.7,
below = 0.4;
System.out.println("above: " + above);
System.out.println("below: " + below);
System.out.println(
"(int)above: " + (int)above);
System.out.println(
"(int)below: " + (int)below);
System.out.println(
"(char)('a' + above): " +
(char)('a' + above));
System.out.println(
"(char)('a' + below): " +
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(char)('a' + below));
}
} ///:~
The output is:
above: 0.7
below: 0.4
(int)above: 0
(int)below: 0
(char)('a' + above): a
(char)('a' + below): a
So the answer is that casting from a float or double to an integral value
always truncates.
A second question concerns Math.random( ). Does it produce a value
from zero to one, inclusive or exclusive of the value ‘1’? In math lingo, is it
(0,1), or [0,1], or (0,1] or [0,1)? (The square bracket means “includes”
whereas the parenthesis means “doesn’t include.”) Again, a test program
might provide the answer:
//: c03:RandomBounds.java
// Does Math.random() produce 0.0 and 1.0?
public class RandomBounds {
static void usage() {
System.out.println("Usage: \n\t" +
"RandomBounds lower\n\t" +
"RandomBounds upper");
System.exit(1);
}
public static void main(String[] args) {
if(args.length != 1) usage();
if(args[0].equals("lower")) {
while(Math.random() != 0.0)
; // Keep trying
System.out.println("Produced 0.0!");
}
else if(args[0].equals("upper")) {
while(Math.random() != 1.0)
; // Keep trying
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System.out.println("Produced 1.0!");
}
else
usage();
}
} ///:~
To run the program, you type a command line of either:
java RandomBounds lower
or
java RandomBounds upper
In both cases you are forced to break out of the program manually, so it
would appear that Math.random( ) never produces either 0.0 or 1.0.
But this is where such an experiment can be deceiving. If you consider2
that there are about 262 different double fractions between 0 and 1, the
likelihood of reaching any one value experimentally might exceed the
lifetime of one computer, or even one experimenter. It turns out that 0.0
is included in the output of Math.random( ). Or, in math lingo, it is
[0,1).
Summary
This chapter concludes the study of fundamental features that appear in
most programming languages: calculation, operator precedence, type
2 Chuck Allison writes: The total number of numbers in a floating-point number system is
2(M-m+1)b^(p-1) + 1
where b is the base (usually 2), p is the precision (digits in the mantissa), M is the largest
exponent, and m is the smallest exponent. IEEE 754 uses:
M = 1023, m = -1022, p = 53, b = 2
so the total number of numbers is
2(1023+1022+1)2^52
= 2((2^10-1) + (2^10-1))2^52
= (2^10-1)2^54
= 2^64 - 2^54
Half of these numbers (corresponding to exponents in the range [-1022, 0]) are less than 1
in magnitude (both positive and negative), so 1/4 of that expression, or 2^62 - 2^52 + 1
(approximately 2^62) is in the range [0,1). See my paper at
http://www.freshsources.com/1995006a.htm (last of text).
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casting, and selection and iteration. Now you’re ready to begin taking
steps that move you closer to the world of object-oriented programming.
The next chapter will cover the important issues of initialization and
cleanup of objects, followed in the subsequent chapter by the essential
concept of implementation hiding.
Exercises
Solutions to selected exercises can be found in the electronic document The Thinking in Java
Annotated Solution Guide, available for a small fee from www.BruceEckel.com.
1. There are two expressions in the section labeled “precedence”
early in this chapter. Put these expressions into a program and
demonstrate that they produce different results.
2. Put the methods ternary( ) and alternative( ) into a working
program.
3. From the sections labeled “if-else” and “return”, put the methods
test( ) and test2( ) into a working program.
4. Write a program that prints values from one to 100.
5. Modify Exercise 4 so that the program exits by using the break
keyword at value 47. Try using return instead.
6. Write a function that takes two String arguments, and uses all the
Boolean comparisons to compare the two Strings and print the
results. For the == and !=, also perform the equals( ) test. In
main( ), call your function with some different String objects.
7. Write a program that generates 25 random int values. For each
value, use an if-then-else statement to classify it as greater than,
less than or equal to a second randomly-generated value.
8. Modify Exercise 7 so that your code is surrounded by an “infinite”
while loop. It will then run until you interrupt it from the keyboard
(typically by pressing Control-C).
9. Write a program that uses two nested for loops and the modulus
operator (%) to detect and print prime numbers (integral numbers
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that are not evenly divisible by any other numbers except for
themselves and 1).
10. Create a switch statement that prints a message for each case, and
put the switch inside a for loop that tries each case. Put a break
after each case and test it, then remove the breaks and see what
happens.
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4: Initialization
& Cleanup
As the computer revolution progresses, “unsafe”
programming has become one of the major culprits that
makes programming expensive.
Two of these safety issues are initialization and cleanup. Many C bugs
occur when the programmer forgets to initialize a variable. This is
especially true with libraries when users don’t know how to initialize a
library component, or even that they must. Cleanup is a special problem
because it’s easy to forget about an element when you’re done with it,
since it no longer concerns you. Thus, the resources used by that element
are retained and you can easily end up running out of resources (most
notably, memory).
C++ introduced the concept of a constructor, a special method
automatically called when an object is created. Java also adopted the
constructor, and in addition has a garbage collector that automatically
releases memory resources when they’re no longer being used. This
chapter examines the issues of initialization and cleanup, and their
support in Java.
Guaranteed initialization
with the constructor
You can imagine creating a method called initialize( ) for every class you
write. The name is a hint that it should be called before using the object.
Unfortunately, this means the user must remember to call the method. In
Java, the class designer can guarantee initialization of every object by
providing a special method called a constructor. If a class has a
constructor, Java automatically calls that constructor when an object is
191
created, before users can even get their hands on it. So initialization is
guaranteed.
The next challenge is what to name this method. There are two issues. The
first is that any name you use could clash with a name you might like to
use as a member in the class. The second is that because the compiler is
responsible for calling the constructor, it must always know which
method to call. The C++ solution seems the easiest and most logical, so
it’s also used in Java: the name of the constructor is the same as the name
of the class. It makes sense that such a method will be called
automatically on initialization.
Here’s a simple class with a constructor:
//: c04:SimpleConstructor.java
// Demonstration of a simple constructor.
class Rock {
Rock() { // This is the constructor
System.out.println("Creating Rock");
}
}
public class SimpleConstructor {
public static void main(String[] args) {
for(int i = 0; i < 10; i++)
new Rock();
}
} ///:~
Now, when an object is created:
new Rock();
storage is allocated and the constructor is called. It is guaranteed that the
object will be properly initialized before you can get your hands on it.
Note that the coding style of making the first letter of all methods
lowercase does not apply to constructors, since the name of the
constructor must match the name of the class exactly.
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Like any method, the constructor can have arguments to allow you to
specify how an object is created. The above example can easily be changed
so the constructor takes an argument:
//: c04:SimpleConstructor2.java
// Constructors can have arguments.
class Rock2 {
Rock2(int i) {
System.out.println(
"Creating Rock number " + i);
}
}
public class SimpleConstructor2 {
public static void main(String[] args) {
for(int i = 0; i < 10; i++)
new Rock2(i);
}
} ///:~
Constructor arguments provide you with a way to provide parameters for
the initialization of an object. For example, if the class Tree has a
constructor that takes a single integer argument denoting the height of
the tree, you would create a Tree object like this:
Tree t = new Tree(12); // 12-foot tree
If Tree(int) is your only constructor, then the compiler won’t let you
create a Tree object any other way.
Constructors eliminate a large class of problems and make the code easier
to read. In the preceding code fragment, for example, you don’t see an
explicit call to some initialize( ) method that is conceptually separate
from definition. In Java, definition and initialization are unified
concepts—you can’t have one without the other.
The constructor is an unusual type of method because it has no return
value. This is distinctly different from a void return value, in which the
method returns nothing but you still have the option to make it return
something else. Constructors return nothing and you don’t have an
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option. If there was a return value, and if you could select your own, the
compiler would somehow need to know what to do with that return value.
Method overloading
One of the important features in any programming language is the use of
names. When you create an object, you give a name to a region of storage.
A method is a name for an action. By using names to describe your
system, you create a program that is easier for people to understand and
change. It’s a lot like writing prose—the goal is to communicate with your
readers.
You refer to all objects and methods by using names. Well-chosen names
make it easier for you and others to understand your code.
A problem arises when mapping the concept of nuance in human
language onto a programming language. Often, the same word expresses a
number of different meanings—it’s overloaded. This is useful, especially
when it comes to trivial differences. You say “wash the shirt,” “wash the
car,” and “wash the dog.” It would be silly to be forced to say, “shirtWash
the shirt,” “carWash the car,” and “dogWash the dog” just so the listener
doesn’t need to make any distinction about the action performed. Most
human languages are redundant, so even if you miss a few words, you can
still determine the meaning. We don’t need unique identifiers—we can
deduce meaning from context.
Most programming languages (C in particular) require you to have a
unique identifier for each function. So you could not have one function
called print( ) for printing integers and another called print( ) for
printing floats—each function requires a unique name.
In Java (and C++), another factor forces the overloading of method
names: the constructor. Because the constructor’s name is predetermined
by the name of the class, there can be only one constructor name. But
what if you want to create an object in more than one way? For example,
suppose you build a class that can initialize itself in a standard way or by
reading information from a file. You need two constructors, one that takes
no arguments (the default constructor, also called the no-arg
constructor), and one that takes a String as an argument, which is the
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name of the file from which to initialize the object. Both are constructors,
so they must have the same name—the name of the class. Thus, method
overloading is essential to allow the same method name to be used with
different argument types. And although method overloading is a must for
constructors, it’s a general convenience and can be used with any method.
Here’s an example that shows both overloaded constructors and
overloaded ordinary methods:
//: c04:Overloading.java
// Demonstration of both constructor
// and ordinary method overloading.
import java.util.*;
class Tree {
int height;
Tree() {
prt("Planting a seedling");
height = 0;
}
Tree(int i) {
prt("Creating new Tree that is "
+ i + " feet tall");
height = i;
}
void info() {
prt("Tree is " + height
+ " feet tall");
}
void info(String s) {
prt(s + ": Tree is "
+ height + " feet tall");
}
static void prt(String s) {
System.out.println(s);
}
}
public class Overloading {
public static void main(String[] args) {
for(int i = 0; i < 5; i++) {
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Tree t = new Tree(i);
t.info();
t.info("overloaded method");
}
// Overloaded constructor:
new Tree();
}
} ///:~
A Tree object can be created either as a seedling, with no argument, or as
a plant grown in a nursery, with an existing height. To support this, there
are two constructors, one that takes no arguments (we call constructors
that take no arguments default constructors1) and one that takes the
existing height.
You might also want to call the info( ) method in more than one way. For
example, with a String argument if you have an extra message you want
printed, and without if you have nothing more to say. It would seem
strange to give two separate names to what is obviously the same concept.
Fortunately, method overloading allows you to use the same name for
both.
Distinguishing overloaded methods
If the methods have the same name, how can Java know which method
you mean? There’s a simple rule: each overloaded method must take a
unique list of argument types.
If you think about this for a second, it makes sense: how else could a
programmer tell the difference between two methods that have the same
name, other than by the types of their arguments?
Even differences in the ordering of arguments are sufficient to distinguish
two methods: (Although you don’t normally want to take this approach, as
it produces difficult-to-maintain code.)
1 In some of the Java literature from Sun they instead refer to these with the clumsy but
descriptive name “no-arg constructors.” The term “default constructor” has been in use for
many years and so I will use that.
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//: c04:OverloadingOrder.java
// Overloading based on the order of
// the arguments.
public class OverloadingOrder {
static void print(String s, int i) {
System.out.println(
"String: " + s +
", int: " + i);
}
static void print(int i, String s) {
System.out.println(
"int: " + i +
", String: " + s);
}
public static void main(String[] args) {
print("String first", 11);
print(99, "Int first");
}
} ///:~
The two print( ) methods have identical arguments, but the order is
different, and that’s what makes them distinct.
Overloading with primitives
A primitive can be automatically promoted from a smaller type to a larger
one and this can be slightly confusing in combination with overloading.
The following example demonstrates what happens when a primitive is
handed to an overloaded method:
//: c04:PrimitiveOverloading.java
// Promotion of primitives and overloading.
public class PrimitiveOverloading {
// boolean can't be automatically converted
static void prt(String s) {
System.out.println(s);
}
void f1(char x) { prt("f1(char)"); }
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void f1(byte x) { prt("f1(byte)"); }
void f1(short x) { prt("f1(short)"); }
void f1(int x) { prt("f1(int)"); }
void f1(long x) { prt("f1(long)"); }
void f1(float x) { prt("f1(float)"); }
void f1(double x) { prt("f1(double)"); }
void f2(byte x) { prt("f2(byte)"); }
void f2(short x) { prt("f2(short)"); }
void f2(int x) { prt("f2(int)"); }
void f2(long x) { prt("f2(long)"); }
void f2(float x) { prt("f2(float)"); }
void f2(double x) { prt("f2(double)"); }
void f3(short x) { prt("f3(short)"); }
void f3(int x) { prt("f3(int)"); }
void f3(long x) { prt("f3(long)"); }
void f3(float x) { prt("f3(float)"); }
void f3(double x) { prt("f3(double)"); }
void f4(int x) { prt("f4(int)"); }
void f4(long x) { prt("f4(long)"); }
void f4(float x) { prt("f4(float)"); }
void f4(double x) { prt("f4(double)"); }
void f5(long x) { prt("f5(long)"); }
void f5(float x) { prt("f5(float)"); }
void f5(double x) { prt("f5(double)"); }
void f6(float x) { prt("f6(float)"); }
void f6(double x) { prt("f6(double)"); }
void f7(double x) { prt("f7(double)"); }
void testConstVal() {
prt("Testing with 5");
f1(5);f2(5);f3(5);f4(5);f5(5);f6(5);f7(5);
}
void testChar() {
char x = 'x';
prt("char argument:");
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f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
}
void testByte() {
byte x = 0;
prt("byte argument:");
f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
}
void testShort() {
short x = 0;
prt("short argument:");
f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
}
void testInt() {
int x = 0;
prt("int argument:");
f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
}
void testLong() {
long x = 0;
prt("long argument:");
f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
}
void testFloat() {
float x = 0;
prt("float argument:");
f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
}
void testDouble() {
double x = 0;
prt("double argument:");
f1(x);f2(x);f3(x);f4(x);f5(x);f6(x);f7(x);
}
public static void main(String[] args) {
PrimitiveOverloading p =
new PrimitiveOverloading();
p.testConstVal();
p.testChar();
p.testByte();
p.testShort();
p.testInt();
p.testLong();
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p.testFloat();
p.testDouble();
}
} ///:~
If you view the output of this program, you’ll see that the constant value 5
is treated as an int, so if an overloaded method is available that takes an
int it is used. In all other cases, if you have a data type that is smaller than
the argument in the method, that data type is promoted. char produces a
slightly different effect, since if it doesn’t find an exact char match, it is
promoted to int.
What happens if your argument is bigger than the argument expected by
the overloaded method? A modification of the above program gives the
answer:
//: c04:Demotion.java
// Demotion of primitives and overloading.
public class Demotion {
static void prt(String s) {
System.out.println(s);
}
void f1(char x) { prt("f1(char)"); }
void f1(byte x) { prt("f1(byte)"); }
void f1(short x) { prt("f1(short)"); }
void f1(int x) { prt("f1(int)"); }
void f1(long x) { prt("f1(long)"); }
void f1(float x) { prt("f1(float)"); }
void f1(double x) { prt("f1(double)"); }
void f2(char x) { prt("f2(char)"); }
void f2(byte x) { prt("f2(byte)"); }
void f2(short x) { prt("f2(short)"); }
void f2(int x) { prt("f2(int)"); }
void f2(long x) { prt("f2(long)"); }
void f2(float x) { prt("f2(float)"); }
void f3(char x) { prt("f3(char)"); }
void f3(byte x) { prt("f3(byte)"); }
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void f3(short x) { prt("f3(short)"); }
void f3(int x) { prt("f3(int)"); }
void f3(long x) { prt("f3(long)"); }
void f4(char x) { prt("f4(char)"); }
void f4(byte x) { prt("f4(byte)"); }
void f4(short x) { prt("f4(short)"); }
void f4(int x) { prt("f4(int)"); }
void f5(char x) { prt("f5(char)"); }
void f5(byte x) { prt("f5(byte)"); }
void f5(short x) { prt("f5(short)"); }
void f6(char x) { prt("f6(char)"); }
void f6(byte x) { prt("f6(byte)"); }
void f7(char x) { prt("f7(char)"); }
void testDouble() {
double x = 0;
prt("double argument:");
f1(x);f2((float)x);f3((long)x);f4((int)x);
f5((short)x);f6((byte)x);f7((char)x);
}
public static void main(String[] args) {
Demotion p = new Demotion();
p.testDouble();
}
} ///:~
Here, the methods take narrower primitive values. If your argument is
wider then you must cast to the necessary type using the type name in
parentheses. If you don’t do this, the compiler will issue an error message.
You should be aware that this is a narrowing conversion, which means
you might lose information during the cast. This is why the compiler
forces you to do it—to flag the narrowing conversion.
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Overloading on return values
It is common to wonder “Why only class names and method argument
lists? Why not distinguish between methods based on their return
values?” For example, these two methods, which have the same name and
arguments, are easily distinguished from each other:
void f() {}
int f() {}
This works fine when the compiler can unequivocally determine the
meaning from the context, as in int x = f( ). However, you can call a
method and ignore the return value; this is often referred to as calling a
method for its side effect since you don’t care about the return value but
instead want the other effects of the method call. So if you call the method
this way:
f();
how can Java determine which f( ) should be called? And how could
someone reading the code see it? Because of this sort of problem, you
cannot use return value types to distinguish overloaded methods.
Default constructors
As mentioned previously, a default constructor (a.k.a. a “no-arg”
constructor) is one without arguments, used to create a “vanilla object.” If
you create a class that has no constructors, the compiler will automatically
create a default constructor for you. For example:
//: c04:DefaultConstructor.java
class Bird {
int i;
}
public class DefaultConstructor {
public static void main(String[] args) {
Bird nc = new Bird(); // default!
}
} ///:~
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The line
new Bird();
creates a new object and calls the default constructor, even though one
was not explicitly defined. Without it we would have no method to call to
build our object. However, if you define any constructors (with or without
arguments), the compiler will not synthesize one for you:
class Bush {
Bush(int i) {}
Bush(double d) {}
}
Now if you say:
new Bush();
the compiler will complain that it cannot find a constructor that matches.
It’s as if when you don’t put in any constructors, the compiler says “You
are bound to need some constructor, so let me make one for you.” But if
you write a constructor, the compiler says “You’ve written a constructor so
you know what you’re doing; if you didn’t put in a default it’s because you
meant to leave it out.”
The this keyword
If you have two objects of the same type called a and b, you might wonder
how it is that you can call a method f( ) for both those objects:
class Banana { void f(int i) { /* ... */ } }
Banana a = new Banana(), b = new Banana();
a.f(1);
b.f(2);
If there’s only one method called f( ), how can that method know whether
it’s being called for the object a or b?
To allow you to write the code in a convenient object-oriented syntax in
which you “send a message to an object,” the compiler does some
undercover work for you. There’s a secret first argument passed to the
method f( ), and that argument is the reference to the object that’s being
manipulated. So the two method calls above become something like:
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Banana.f(a,1);
Banana.f(b,2);
This is internal and you can’t write these expressions and get the compiler
to accept them, but it gives you an idea of what’s happening.
Suppose you’re inside a method and you’d like to get the reference to the
current object. Since that reference is passed secretly by the compiler,
there’s no identifier for it. However, for this purpose there’s a keyword:
this. The this keyword—which can be used only inside a method—
produces the reference to the object the method has been called for. You
can treat this reference just like any other object reference. Keep in mind
that if you’re calling a method of your class from within another method
of your class, you don’t need to use this; you simply call the method. The
current this reference is automatically used for the other method. Thus
you can say:
class Apricot {
void pick() { /* ... */ }
void pit() { pick(); /* ... */ }
}
Inside pit( ), you could say this.pick( ) but there’s no need to. The
compiler does it for you automatically. The this keyword is used only for
those special cases in which you need to explicitly use the reference to the
current object. For example, it’s often used in return statements when
you want to return the reference to the current object:
//: c04:Leaf.java
// Simple use of the "this" keyword.
public class Leaf {
int i = 0;
Leaf increment() {
i++;
return this;
}
void print() {
System.out.println("i = " + i);
}
public static void main(String[] args) {
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Leaf x = new Leaf();
x.increment().increment().increment().print();
}
} ///:~
Because increment( ) returns the reference to the current object via the
this keyword, multiple operations can easily be performed on the same
object.
Calling constructors from constructors
When you write several constructors for a class, there are times when
you’d like to call one constructor from another to avoid duplicating code.
You can do this using the this keyword.
Normally, when you say this, it is in the sense of “this object” or “the
current object,” and by itself it produces the reference to the current
object. In a constructor, the this keyword takes on a different meaning
when you give it an argument list: it makes an explicit call to the
constructor that matches that argument list. Thus you have a
straightforward way to call other constructors:
//: c04:Flower.java
// Calling constructors with "this."
public class Flower {
int petalCount = 0;
String s = new String("null");
Flower(int petals) {
petalCount = petals;
System.out.println(
"Constructor w/ int arg only, petalCount= "
+ petalCount);
}
Flower(String ss) {
System.out.println(
"Constructor w/ String arg only, s=" + ss);
s = ss;
}
Flower(String s, int petals) {
this(petals);
//! this(s); // Can't call two!
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this.s = s; // Another use of "this"
System.out.println("String & int args");
}
Flower() {
this("hi", 47);
System.out.println(
"default constructor (no args)");
}
void print() {
//! this(11); // Not inside non-constructor!
System.out.println(
"petalCount = " + petalCount + " s = "+ s);
}
public static void main(String[] args) {
Flower x = new Flower();
x.print();
}
} ///:~
The constructor Flower(String s, int petals) shows that, while you can
call one constructor using this, you cannot call two. In addition, the
constructor call must be the first thing you do or you’ll get a compiler
error message.
This example also shows another way you’ll see this used. Since the name
of the argument s and the name of the member data s are the same,
there’s an ambiguity. You can resolve it by saying this.s to refer to the
member data. You’ll often see this form used in Java code, and it’s used in
numerous places in this book.
In print( ) you can see that the compiler won’t let you call a constructor
from inside any method other than a constructor.
The meaning of static
With the this keyword in mind, you can more fully understand what it
means to make a method static. It means that there is no this for that
particular method. You cannot call non-static methods from inside
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static methods2 (although the reverse is possible), and you can call a
static method for the class itself, without any object. In fact, that’s
primarily what a static method is for. It’s as if you’re creating the
equivalent of a global function (from C). Except global functions are not
permitted in Java, and putting the static method inside a class allows it
access to other static methods and to static fields.
Some people argue that static methods are not object-oriented since they
do have the semantics of a global function; with a static method you
don’t send a message to an object, since there’s no this. This is probably a
fair argument, and if you find yourself using a lot of static methods you
should probably rethink your strategy. However, statics are pragmatic
and there are times when you genuinely need them, so whether or not
they are “proper OOP” should be left to the theoreticians. Indeed, even
Smalltalk has the equivalent in its “class methods.”
Cleanup: finalization and
garbage collection
Programmers know about the importance of initialization, but often
forget the importance of cleanup. After all, who needs to clean up an int?
But with libraries, simply “letting go” of an object once you’re done with it
is not always safe. Of course, Java has the garbage collector to reclaim the
memory of objects that are no longer used. Now consider a very unusual
case. Suppose your object allocates “special” memory without using new.
The garbage collector knows only how to release memory allocated with
new, so it won’t know how to release the object’s “special” memory. To
handle this case, Java provides a method called finalize( ) that you can
define for your class. Here’s how it’s supposed to work. When the garbage
collector is ready to release the storage used for your object, it will first
call finalize( ), and only on the next garbage-collection pass will it
2 The one case in which this is possible occurs if you pass a reference to an object into the
static method. Then, via the reference (which is now effectively this), you can call non-
static methods and access non-static fields. But typically if you want to do something like
this you’ll just make an ordinary, non-static method.
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reclaim the object’s memory. So if you choose to use finalize( ), it gives
you the ability to perform some important cleanup at the time of garbage
collection.
This is a potential programming pitfall because some programmers,
especially C++ programmers, might initially mistake finalize( ) for the
destructor in C++, which is a function that is always called when an object
is destroyed. But it is important to distinguish between C++ and Java
here, because in C++ objects always get destroyed (in a bug-free
program), whereas in Java objects do not always get garbage-collected.
Or, put another way:
Garbage collection is not destruction.
If you remember this, you will stay out of trouble. What it means is that if
there is some activity that must be performed before you no longer need
an object, you must perform that activity yourself. Java has no destructor
or similar concept, so you must create an ordinary method to perform this
cleanup. For example, suppose in the process of creating your object it
draws itself on the screen. If you don’t explicitly erase its image from the
screen, it might never get cleaned up. If you put some kind of erasing
functionality inside finalize( ), then if an object is garbage-collected, the
image will first be removed from the screen, but if it isn’t, the image will
remain. So a second point to remember is:
Your objects might not get garbage-collected.
You might find that the storage for an object never gets released because
your program never nears the point of running out of storage. If your
program completes and the garbage collector never gets around to
releasing the storage for any of your objects, that storage will be returned
to the operating system en masse as the program exits. This is a good
thing, because garbage collection has some overhead, and if you never do
it you never incur that expense.
What is finalize( ) for?
You might believe at this point that you should not use finalize( ) as a
general-purpose cleanup method. What good is it?
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A third point to remember is:
Garbage collection is only about memory.
That is, the sole reason for the existence of the garbage collector is to
recover memory that your program is no longer using. So any activity that
is associated with garbage collection, most notably your finalize( )
method, must also be only about memory and its deallocation.
Does this mean that if your object contains other objects finalize( )
should explicitly release those objects? Well, no—the garbage collector
takes care of the release of all object memory regardless of how the object
is created. It turns out that the need for finalize( ) is limited to special
cases, in which your object can allocate some storage in some way other
than creating an object. But, you might observe, everything in Java is an
object so how can this be?
It would seem that finalize( ) is in place because of the possibility that
you’ll do something C-like by allocating memory using a mechanism other
than the normal one in Java. This can happen primarily through native
methods, which are a way to call non-Java code from Java. (Native
methods are discussed in Appendix B.) C and C++ are the only languages
currently supported by native methods, but since they can call
subprograms in other languages, you can effectively call anything. Inside
the non-Java code, C’s malloc( ) family of functions might be called to
allocate storage, and unless you call free( ) that storage will not be
released, causing a memory leak. Of course, free( ) is a C and C++
function, so you’d need to call it in a native method inside your
finalize( ).
After reading this, you probably get the idea that you won’t use
finalize( ) much. You’re correct; it is not the appropriate place for
normal cleanup to occur. So where should normal cleanup be performed?
You must perform cleanup
To clean up an object, the user of that object must call a cleanup method
at the point the cleanup is desired. This sounds pretty straightforward,
but it collides a bit with the C++ concept of the destructor. In C++, all
objects are destroyed. Or rather, all objects should be destroyed. If the
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C++ object is created as a local (i.e., on the stack—not possible in Java),
then the destruction happens at the closing curly brace of the scope in
which the object was created. If the object was created using new (like in
Java) the destructor is called when the programmer calls the C++
operator delete (which doesn’t exist in Java). If the C++ programmer
forgets to call delete, the destructor is never called and you have a
memory leak, plus the other parts of the object never get cleaned up. This
kind of bug can be very difficult to track down.
In contrast, Java doesn’t allow you to create local objects—you must
always use new. But in Java, there’s no “delete” to call to release the
object since the garbage collector releases the storage for you. So from a
simplistic standpoint you could say that because of garbage collection,
Java has no destructor. You’ll see as this book progresses, however, that
the presence of a garbage collector does not remove the need for or utility
of destructors. (And you should never call finalize( ) directly, so that’s
not an appropriate avenue for a solution.) If you want some kind of
cleanup performed other than storage release you must still explicitly call
an appropriate method in Java, which is the equivalent of a C++
destructor without the convenience.
One of the things finalize( ) can be useful for is observing the process of
garbage collection. The following example shows you what’s going on and
summarizes the previous descriptions of garbage collection:
//: c04:Garbage.java
// Demonstration of the garbage
// collector and finalization
class Chair {
static boolean gcrun = false;
static boolean f = false;
static int created = 0;
static int finalized = 0;
int i;
Chair() {
i = ++created;
if(created == 47)
System.out.println("Created 47");
}
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public void finalize() {
if(!gcrun) {
// The first time finalize() is called:
gcrun = true;
System.out.println(
"Beginning to finalize after " +
created + " Chairs have been created");
}
if(i == 47) {
System.out.println(
"Finalizing Chair #47, " +
"Setting flag to stop Chair creation");
f = true;
}
finalized++;
if(finalized >= created)
System.out.println(
"All " + finalized + " finalized");
}
}
public class Garbage {
public static void main(String[] args) {
// As long as the flag hasn't been set,
// make Chairs and Strings:
while(!Chair.f) {
new Chair();
new String("To take up space");
}
System.out.println(
"After all Chairs have been created:\n" +
"total created = " + Chair.created +
", total finalized = " + Chair.finalized);
// Optional arguments force garbage
// collection & finalization:
if(args.length > 0) {
if(args[0].equals("gc") ||
args[0].equals("all")) {
System.out.println("gc():");
System.gc();
}
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if(args[0].equals("finalize") ||
args[0].equals("all")) {
System.out.println("runFinalization():");
System.runFinalization();
}
}
System.out.println("bye!");
}
} ///:~
The above program creates many Chair objects, and at some point after
the garbage collector begins running, the program stops creating Chairs.
Since the garbage collector can run at any time, you don’t know exactly
when it will start up, so there’s a flag called gcrun to indicate whether the
garbage collector has started running yet. A second flag f is a way for
Chair to tell the main( ) loop that it should stop making objects. Both of
these flags are set within finalize( ), which is called during garbage
collection.
Two other static variables, created and finalized, keep track of the
number of Chairs created versus the number that get finalized by the
garbage collector. Finally, each Chair has its own (non-static) int i so it
can keep track of what number it is. When Chair number 47 is finalized,
the flag is set to true to bring the process of Chair creation to a stop.
All this happens in main( ), in the loop
while(!Chair.f) {
new Chair();
new String("To take up space");
}
You might wonder how this loop could ever finish, since there’s nothing
inside the loop that changes the value of Chair.f. However, the
finalize( ) process will, eventually, when it finalizes number 47.
The creation of a String object during each iteration is simply extra
storage being allocated to encourage the garbage collector to kick in,
which it will do when it starts to get nervous about the amount of memory
available.
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When you run the program, you provide a command-line argument of
“gc,” “finalize,” or “all.” The “gc” argument will call the System.gc( )
method (to force execution of the garbage collector). Using the “finalize”
argument calls System.runFinalization( ) which—in theory—will
cause any unfinalized objects to be finalized. And “all” causes both
methods to be called.
The behavior of this program and the version in the first edition of this
book shows that the whole issue of garbage collection and finalization has
been evolving, with much of the evolution happening behind closed doors.
In fact, by the time you read this, the behavior of the program may have
changed once again.
If System.gc( ) is called, then finalization happens to all the objects. This
was not necessarily the case with previous implementations of the JDK,
although the documentation claimed otherwise. In addition, you’ll see
that it doesn’t seem to make any difference whether
System.runFinalization( ) is called.
However, you will see that only if System.gc( ) is called after all the
objects are created and discarded will all the finalizers be called. If you do
not call System.gc( ), then only some of the objects will be finalized. In
Java 1.1, a method System.runFinalizersOnExit( ) was introduced
that caused programs to run all the finalizers as they exited, but the
design turned out to be buggy and the method was deprecated. This is yet
another clue that the Java designers were thrashing about trying to solve
the garbage collection and finalization problem. We can only hope that
things have been worked out in Java 2.
The preceding program shows that the promise that finalizers will always
be run holds true, but only if you explicitly force it to happen yourself. If
you don’t cause System.gc( ) to be called, you’ll get an output like this:
Created 47
Beginning to finalize after 3486 Chairs have been
created
Finalizing Chair #47, Setting flag to stop Chair
creation
After all Chairs have been created:
total created = 3881, total finalized = 2684
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bye!
Thus, not all finalizers get called by the time the program completes. If
System.gc( ) is called, it will finalize and destroy all the objects that are
no longer in use up to that point.
Remember that neither garbage collection nor finalization is guaranteed.
If the Java Virtual Machine (JVM) isn’t close to running out of memory,
then it will (wisely) not waste time recovering memory through garbage
collection.
The death condition
In general, you can’t rely on finalize( ) being called, and you must create
separate “cleanup” functions and call them explicitly. So it appears that
finalize( ) is only useful for obscure memory cleanup that most
programmers will never use. However, there is a very interesting use of
finalize( ) which does not rely on it being called every time. This is the
verification of the death condition3 of an object.
At the point that you’re no longer interested in an object—when it’s ready
to be cleaned up—that object should be in a state whereby its memory can
be safely released. For example, if the object represents an open file, that
file should be closed by the programmer before the object is garbage-
collected. If any portions of the object are not properly cleaned up, then
you have a bug in your program that could be very difficult to find. The
value of finalize( ) is that it can be used to discover this condition, even
if it isn’t always called. If one of the finalizations happens to reveal the
bug, then you discover the problem, which is all you really care about.
Here’s a simple example of how you might use it:
//: c04:DeathCondition.java
// Using finalize() to detect an object that
// hasn't been properly cleaned up.
class Book {
3 A term coined by Bill Venners (www.artima.com) during a seminar that he and I were
giving together.
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boolean checkedOut = false;
Book(boolean checkOut) {
checkedOut = checkOut;
}
void checkIn() {
checkedOut = false;
}
public void finalize() {
if(checkedOut)
System.out.println("Error: checked out");
}
}
public class DeathCondition {
public static void main(String[] args) {
Book novel = new Book(true);
// Proper cleanup:
novel.checkIn();
// Drop the reference, forget to clean up:
new Book(true);
// Force garbage collection & finalization:
System.gc();
}
} ///:~
The death condition is that all Book objects are supposed to be checked
in before they are garbage-collected, but in main( ) a programmer error
doesn’t check in one of the books. Without finalize( ) to verify the death
condition, this could be a difficult bug to find.
Note that System.gc( ) is used to force finalization (and you should do
this during program development to speed debugging). But even if it isn’t,
it’s highly probable that the errant Book will eventually be discovered
through repeated executions of the program (assuming the program
allocates enough storage to cause the garbage collector to execute).
How a garbage collector works
If you come from a programming language where allocating objects on the
heap is expensive, you may naturally assume that Java’s scheme of
allocating everything (except primitives) on the heap is expensive.
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However, it turns out that the garbage collector can have a significant
impact on increasing the speed of object creation. This might sound a bit
odd at first—that storage release affects storage allocation—but it’s the
way some JVMs work and it means that allocating storage for heap
objects in Java can be nearly as fast as creating storage on the stack in
other languages.
For example, you can think of the C++ heap as a yard where each object
stakes out its own piece of turf. This real estate can become abandoned
sometime later and must be reused. In some JVMs, the Java heap is quite
different; it’s more like a conveyor belt that moves forward every time you
allocate a new object. This means that object storage allocation is
remarkably rapid. The “heap pointer” is simply moved forward into virgin
territory, so it’s effectively the same as C++’s stack allocation. (Of course,
there’s a little extra overhead for bookkeeping but it’s nothing like
searching for storage.)
Now you might observe that the heap isn’t in fact a conveyor belt, and if
you treat it that way you’ll eventually start paging memory a lot (which is
a big performance hit) and later run out. The trick is that the garbage
collector steps in and while it collects the garbage it compacts all the
objects in the heap so that you’ve effectively moved the “heap pointer”
closer to the beginning of the conveyor belt and further away from a page
fault. The garbage collector rearranges things and makes it possible for
the high-speed, infinite-free-heap model to be used while allocating
storage.
To understand how this works, you need to get a little better idea of the
way the different garbage collector (GC) schemes work. A simple but slow
GC technique is reference counting. This means that each object contains
a reference counter, and every time a reference is attached to an object the
reference count is increased. Every time a reference goes out of scope or is
set to null, the reference count is decreased. Thus, managing reference
counts is a small but constant overhead that happens throughout the
lifetime of your program. The garbage collector moves through the entire
list of objects and when it finds one with a reference count of zero it
releases that storage. The one drawback is that if objects circularly refer to
each other they can have nonzero reference counts while still being
garbage. Locating such self-referential groups requires significant extra
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work for the garbage collector. Reference counting is commonly used to
explain one kind of garbage collection but it doesn’t seem to be used in
any JVM implementations.
In faster schemes, garbage collection is not based on reference counting.
Instead, it is based on the idea that any nondead object must ultimately be
traceable back to a reference that lives either on the stack or in static
storage. The chain might go through several layers of objects. Thus, if you
start in the stack and the static storage area and walk through all the
references you’ll find all the live objects. For each reference that you find,
you must trace into the object that it points to and then follow all the
references in that object, tracing into the objects they point to, etc., until
you’ve moved through the entire web that originated with the reference on
the stack or in static storage. Each object that you move through must still
be alive. Note that there is no problem with detached self-referential
groups—these are simply not found, and are therefore automatically
garbage.
In the approach described here, the JVM uses an adaptive garbage-
collection scheme, and what it does with the live objects that it locates
depends on the variant currently being used. One of these variants is stop-
and-copy. This means that—for reasons that will become apparent—the
program is first stopped (this is not a background collection scheme).
Then, each live object that is found is copied from one heap to another,
leaving behind all the garbage. In addition, as the objects are copied into
the new heap they are packed end-to-end, thus compacting the new heap
(and allowing new storage to simply be reeled off the end as previously
described).
Of course, when an object is moved from one place to another, all
references that point at (i.e., that reference) the object must be changed.
The reference that goes from the heap or the static storage area to the
object can be changed right away, but there can be other references
pointing to this object that will be encountered later during the “walk.”
These are fixed up as they are found (you could imagine a table that maps
old addresses to new ones).
There are two issues that make these so-called “copy collectors”
inefficient. The first is the idea that you have two heaps and you slosh all
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the memory back and forth between these two separate heaps,
maintaining twice as much memory as you actually need. Some JVMs deal
with this by allocating the heap in chunks as needed and simply copying
from one chunk to another.
The second issue is the copying. Once your program becomes stable it
might be generating little or no garbage. Despite that, a copy collector will
still copy all the memory from one place to another, which is wasteful. To
prevent this, some JVMs detect that no new garbage is being generated
and switch to a different scheme (this is the “adaptive” part). This other
scheme is called mark and sweep, and it’s what earlier versions of Sun’s
JVM used all the time. For general use, mark and sweep is fairly slow, but
when you know you’re generating little or no garbage it’s fast.
Mark and sweep follows the same logic of starting from the stack and
static storage and tracing through all the references to find live objects.
However, each time it finds a live object that object is marked by setting a
flag in it, but the object isn’t collected yet. Only when the marking process
is finished does the sweep occur. During the sweep, the dead objects are
released. However, no copying happens, so if the collector chooses to
compact a fragmented heap it does so by shuffling objects around.
The “stop-and-copy” refers to the idea that this type of garbage collection
is not done in the background; instead, the program is stopped while the
GC occurs. In the Sun literature you’ll find many references to garbage
collection as a low-priority background process, but it turns out that the
GC was not implemented that way, at least in earlier versions of the Sun
JVM. Instead, the Sun garbage collector ran when memory got low. In
addition, mark-and-sweep requires that the program be stopped.
As previously mentioned, in the JVM described here memory is allocated
in big blocks. If you allocate a large object, it gets its own block. Strict
stop-and-copy requires copying every live object from the source heap to a
new heap before you could free the old one, which translates to lots of
memory. With blocks, the GC can typically use dead blocks to copy objects
to as it collects. Each block has a generation count to keep track of
whether it’s alive. In the normal case, only the blocks created since the
last GC are compacted; all other blocks get their generation count bumped
if they have been referenced from somewhere. This handles the normal
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case of lots of short-lived temporary objects. Periodically, a full sweep is
made—large objects are still not copied (just get their generation count
bumped) and blocks containing small objects are copied and compacted.
The JVM monitors the efficiency of GC and if it becomes a waste of time
because all objects are long-lived then it switches to mark-and-sweep.
Similarly, the JVM keeps track of how successful mark-and-sweep is, and
if the heap starts to become fragmented it switches back to stop-and-copy.
This is where the “adaptive” part comes in, so you end up with a
mouthful: “adaptive generational stop-and-copy mark-and-sweep.”
There are a number of additional speedups possible in a JVM. An
especially important one involves the operation of the loader and Just-In-
Time (JIT) compiler. When a class must be loaded (typically, the first time
you want to create an object of that class), the .class file is located and
the byte codes for that class are brought into memory. At this point, one
approach is to simply JIT all the code, but this has two drawbacks: it takes
a little more time, which, compounded throughout the life of the program,
can add up; and it increases the size of the executable (byte codes are
significantly more compact than expanded JIT code) and this might cause
paging, which definitely slows down a program. An alternative approach
is lazy evaluation, which means that the code is not JIT compiled until
necessary. Thus, code that never gets executed might never get JIT
compiled.
Member initialization
Java goes out of its way to guarantee that variables are properly initialized
before they are used. In the case of variables that are defined locally to a
method, this guarantee comes in the form of a compile-time error. So if
you say:
void f() {
int i;
i++;
}
you’ll get an error message that says that i might not have been initialized.
Of course, the compiler could have given i a default value, but it’s more
likely that this is a programmer error and a default value would have
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covered that up. Forcing the programmer to provide an initialization
value is more likely to catch a bug.
If a primitive is a data member of a class, however, things are a bit
different. Since any method can initialize or use that data, it might not be
practical to force the user to initialize it to its appropriate value before the
data is used. However, it’s unsafe to leave it with a garbage value, so each
primitive data member of a class is guaranteed to get an initial value.
Those values can be seen here:
//: c04:InitialValues.java
// Shows default initial values.
class Measurement {
boolean t;
char c;
byte b;
short s;
int i;
long l;
float f;
double d;
void print() {
System.out.println(
"Data type Initial value\n" +
"boolean " + t + "\n" +
"char [" + c + "] "+ (int)c +"\n"+
"byte " + b + "\n" +
"short " + s + "\n" +
"int " + i + "\n" +
"long " + l + "\n" +
"float " + f + "\n" +
"double " + d);
}
}
public class InitialValues {
public static void main(String[] args) {
Measurement d = new Measurement();
d.print();
/* In this case you could also say:
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new Measurement().print();
*/
}
} ///:~
The output of this program is:
Data type Initial value
boolean false
char [ ] 0
byte 0
short 0
int 0
long 0
float 0.0
double 0.0
The char value is a zero, which prints as a space.
You’ll see later that when you define an object reference inside a class
without initializing it to a new object, that reference is given a special
value of null (which is a Java keyword).
You can see that even though the values are not specified, they
automatically get initialized. So at least there’s no threat of working with
uninitialized variables.
Specifying initialization
What happens if you want to give a variable an initial value? One direct
way to do this is simply to assign the value at the point you define the
variable in the class. (Notice you cannot do this in C++, although C++
novices always try.) Here the field definitions in class Measurement are
changed to provide initial values:
class Measurement {
boolean b = true;
char c = 'x';
byte B = 47;
short s = 0xff;
int i = 999;
long l = 1;
float f = 3.14f;
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double d = 3.14159;
//. . .
You can also initialize nonprimitive objects in this same way. If Depth is
a class, you can insert a variable and initialize it like so:
class Measurement {
Depth o = new Depth();
boolean b = true;
// . . .
If you haven’t given o an initial value and you try to use it anyway, you’ll
get a run-time error called an exception (covered in Chapter 10).
You can even call a method to provide an initialization value:
class CInit {
int i = f();
//...
}
This method can have arguments, of course, but those arguments cannot
be other class members that haven’t been initialized yet. Thus, you can do
this:
class CInit {
int i = f();
int j = g(i);
//...
}
But you cannot do this:
class CInit {
int j = g(i);
int i = f();
//...
}
This is one place in which the compiler, appropriately, does complain
about forward referencing, since this has to do with the order of
initialization and not the way the program is compiled.
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This approach to initialization is simple and straightforward. It has the
limitation that every object of type Measurement will get these same
initialization values. Sometimes this is exactly what you need, but at other
times you need more flexibility.
Constructor initialization
The constructor can be used to perform initialization, and this gives you
greater flexibility in your programming since you can call methods and
perform actions at run-time to determine the initial values. There’s one
thing to keep in mind, however: you aren’t precluding the automatic
initialization, which happens before the constructor is entered. So, for
example, if you say:
class Counter {
int i;
Counter() { i = 7; }
// . . .
then i will first be initialized to 0, then to 7. This is true with all the
primitive types and with object references, including those that are given
explicit initialization at the point of definition. For this reason, the
compiler doesn’t try to force you to initialize elements in the constructor
at any particular place, or before they are used—initialization is already
guaranteed4.
Order of initialization
Within a class, the order of initialization is determined by the order that
the variables are defined within the class. The variable definitions may be
scattered throughout and in between method definitions, but the
variables are initialized before any methods can be called—even the
constructor. For example:
//: c04:OrderOfInitialization.java
// Demonstrates initialization order.
4 In contrast, C++ has the constructor initializer list that causes initialization to occur
before entering the constructor body, and is enforced for objects. See Thinking in C++, 2nd
edition (available on this book’s CD ROM and at www.BruceEckel.com).
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// When the constructor is called to create a
// Tag object, you'll see a message:
class Tag {
Tag(int marker) {
System.out.println("Tag(" + marker + ")");
}
}
class Card {
Tag t1 = new Tag(1); // Before constructor
Card() {
// Indicate we're in the constructor:
System.out.println("Card()");
t3 = new Tag(33); // Reinitialize t3
}
Tag t2 = new Tag(2); // After constructor
void f() {
System.out.println("f()");
}
Tag t3 = new Tag(3); // At end
}
public class OrderOfInitialization {
public static void main(String[] args) {
Card t = new Card();
t.f(); // Shows that construction is done
}
} ///:~
In Card, the definitions of the Tag objects are intentionally scattered
about to prove that they’ll all get initialized before the constructor is
entered or anything else can happen. In addition, t3 is reinitialized inside
the constructor. The output is:
Tag(1)
Tag(2)
Tag(3)
Card()
Tag(33)
f()
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Thus, the t3 reference gets initialized twice, once before and once during
the constructor call. (The first object is dropped, so it can be garbage-
collected later.) This might not seem efficient at first, but it guarantees
proper initialization—what would happen if an overloaded constructor
were defined that did not initialize t3 and there wasn’t a “default”
initialization for t3 in its definition?
Static data initialization
When the data is static the same thing happens; if it’s a primitive and you
don’t initialize it, it gets the standard primitive initial values. If it’s a
reference to an object, it’s null unless you create a new object and attach
your reference to it.
If you want to place initialization at the point of definition, it looks the
same as for non-statics. There’s only a single piece of storage for a static,
regardless of how many objects are created. But the question arises of
when the static storage gets initialized. An example makes this question
clear:
//: c04:StaticInitialization.java
// Specifying initial values in a
// class definition.
class Bowl {
Bowl(int marker) {
System.out.println("Bowl(" + marker + ")");
}
void f(int marker) {
System.out.println("f(" + marker + ")");
}
}
class Table {
static Bowl b1 = new Bowl(1);
Table() {
System.out.println("Table()");
b2.f(1);
}
void f2(int marker) {
System.out.println("f2(" + marker + ")");
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}
static Bowl b2 = new Bowl(2);
}
class Cupboard {
Bowl b3 = new Bowl(3);
static Bowl b4 = new Bowl(4);
Cupboard() {
System.out.println("Cupboard()");
b4.f(2);
}
void f3(int marker) {
System.out.println("f3(" + marker + ")");
}
static Bowl b5 = new Bowl(5);
}
public class StaticInitialization {
public static void main(String[] args) {
System.out.println(
"Creating new Cupboard() in main");
new Cupboard();
System.out.println(
"Creating new Cupboard() in main");
new Cupboard();
t2.f2(1);
t3.f3(1);
}
static Table t2 = new Table();
static Cupboard t3 = new Cupboard();
} ///:~
Bowl allows you to view the creation of a class, and Table and
Cupboard create static members of Bowl scattered through their class
definitions. Note that Cupboard creates a non-static Bowl b3 prior to
the static definitions. The output shows what happens:
Bowl(1)
Bowl(2)
Table()
f(1)
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Bowl(4)
Bowl(5)
Bowl(3)
Cupboard()
f(2)
Creating new Cupboard() in main
Bowl(3)
Cupboard()
f(2)
Creating new Cupboard() in main
Bowl(3)
Cupboard()
f(2)
f2(1)
f3(1)
The static initialization occurs only if it’s necessary. If you don’t create a
Table object and you never refer to Table.b1 or Table.b2, the static
Bowl b1 and b2 will never be created. However, they are initialized only
when the first Table object is created (or the first static access occurs).
After that, the static objects are not reinitialized.
The order of initialization is statics first, if they haven’t already been
initialized by a previous object creation, and then the non-static objects.
You can see the evidence of this in the output.
It’s helpful to summarize the process of creating an object. Consider a
class called Dog:
1.
The first time an object of type Dog is created, or the first time a
static method or static field of class Dog is accessed, the Java
interpreter must locate Dog.class, which it does by searching
through the classpath.
2.
As Dog.class is loaded (creating a Class object, which you’ll learn
about later), all of its static initializers are run. Thus, static
initialization takes place only once, as the Class object is loaded for
the first time.
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3.
When you create a new Dog( ), the construction process for a
Dog object first allocates enough storage for a Dog object on the
heap.
4.
This storage is wiped to zero, automatically setting all the
primitives in that Dog object to their default values (zero for
numbers and the equivalent for boolean and char) and the
references to null.
5.
Any initializations that occur at the point of field definition are
executed.
6.
Constructors are executed. As you shall see in Chapter 6, this might
actually involve a fair amount of activity, especially when
inheritance is involved.
Explicit static initialization
Java allows you to group other static initializations inside a special
“static construction clause” (sometimes called a static block) in a class. It
looks like this:
class Spoon {
static int i;
static {
i = 47;
}
// . . .
It appears to be a method, but it’s just the static keyword followed by a
method body. This code, like other static initializations, is executed only
once, the first time you make an object of that class or the first time you
access a static member of that class (even if you never make an object of
that class). For example:
//: c04:ExplicitStatic.java
// Explicit static initialization
// with the "static" clause.
class Cup {
Cup(int marker) {
System.out.println("Cup(" + marker + ")");
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}
void f(int marker) {
System.out.println("f(" + marker + ")");
}
}
class Cups {
static Cup c1;
static Cup c2;
static {
c1 = new Cup(1);
c2 = new Cup(2);
}
Cups() {
System.out.println("Cups()");
}
}
public class ExplicitStatic {
public static void main(String[] args) {
System.out.println("Inside main()");
Cups.c1.f(99); // (1)
}
// static Cups x = new Cups(); // (2)
// static Cups y = new Cups(); // (2)
} ///:~
The static initializers for Cups run when either the access of the static
object c1 occurs on the line marked (1), or if line (1) is commented out and
the lines marked (2) are uncommented. If both (1) and (2) are commented
out, the static initialization for Cups never occurs. Also, it doesn’t matter
if one or both of the lines marked (2) are uncommented; the static
initialization only occurs once.
Non-static instance initialization
Java provides a similar syntax for initializing non-static variables for
each object. Here’s an example:
//: c04:Mugs.java
// Java "Instance Initialization."
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class Mug {
Mug(int marker) {
System.out.println("Mug(" + marker + ")");
}
void f(int marker) {
System.out.println("f(" + marker + ")");
}
}
public class Mugs {
Mug c1;
Mug c2;
{
c1 = new Mug(1);
c2 = new Mug(2);
System.out.println("c1 & c2 initialized");
}
Mugs() {
System.out.println("Mugs()");
}
public static void main(String[] args) {
System.out.println("Inside main()");
Mugs x = new Mugs();
}
} ///:~
You can see that the instance initialization clause:
{
c1 = new Mug(1);
c2 = new Mug(2);
System.out.println("c1 & c2 initialized");
}
looks exactly like the static initialization clause except for the missing
static keyword. This syntax is necessary to support the initialization of
anonymous inner classes (see Chapter 8).
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Array initialization
Initializing arrays in C is error-prone and tedious. C++ uses aggregate
initialization to make it much safer5. Java has no “aggregates” like C++,
since everything is an object in Java. It does have arrays, and these are
supported with array initialization.
An array is simply a sequence of either objects or primitives, all the same
type and packaged together under one identifier name. Arrays are defined
and used with the square-brackets indexing operator [ ]. To define an
array you simply follow your type name with empty square brackets:
int[] a1;
You can also put the square brackets after the identifier to produce exactly
the same meaning:
int a1[];
This conforms to expectations from C and C++ programmers. The former
style, however, is probably a more sensible syntax, since it says that the
type is “an int array.” That style will be used in this book.
The compiler doesn’t allow you to tell it how big the array is. This brings
us back to that issue of “references.” All that you have at this point is a
reference to an array, and there’s been no space allocated for the array. To
create storage for the array you must write an initialization expression.
For arrays, initialization can appear anywhere in your code, but you can
also use a special kind of initialization expression that must
