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Effective Java, 2nd Edition(新版)
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Praise for the First Edition
“I sure wish I had this book ten years ago. Some might think that I don’t need
any Java books, but I need this one.”
—James Gosling, fellow and vice president, Sun Microsystems, Inc., and
inventor of the Java programming language
“An excellent book, crammed with good advice on using the Java programming
language and object-oriented programming in general.”
—Gilad Bracha, distinguished engineer, Cadence Design Systems,
and coauthor of The Java™ Language Specification,
Third Edition (Addison-Wesley, 2005)
“10/10—anyone aspiring to write good Java code that others will appreciate
reading and maintaining should be required to own a copy of this book. This
is one of those rare books where the information won’t become obsolete with
subsequent releases of the JDK library.”
—Peter Tran, bartender, JavaRanch.com
“The best Java book yet written.... Really great; very readable and eminently
useful. I can’t say enough good things about this book. At JavaOne 2001,
James Gosling said, ‘Go buy this book!’ I’m glad I did, and I couldn’t agree
more.”
—Keith Edwards, senior member of research staff,
Computer Science Lab at the Palo Alto Research Center (PARC),
and author of Core JINI (Prentice Hall, 2000)
“This is a truly excellent book done by the guy who designed several of the
better recent Java platform APIs (including the Collections API).”
—James Clark, technical lead of the XML Working Group
during the creation of the XML 1.0 Recommendation;
editor of the XPath and XSLT Recommendations
“Great content. Analogous to Scott Meyers’s classic Effective C++. If you know
the basics of Java, this has to be your next book.”
—Gary K. Evans, OO mentor and consultant, Evanetics, Inc.
“Josh Bloch gives great insight into best practices that really can only be discovered
after years of study and experience.”
—Mark Mascolino, software engineer
“This is a superb book. It clearly covers many of the language/platform subtleties
and trickery you need to learn to become a real Java master.”
—Victor Wiewiorowski, vice president development and code quality manager,
ValueCommerce Co., Tokyo, Japan
“I like books that under-promise in their titles and over-deliver in their contents.
This book has 57 items of programming advice that are well chosen. Each item
reveals a clear, deep grasp of the language. Each one illustrates in simple, practical
terms the limits of programming on intuition alone, or taking the most direct path
to a solution without fully understanding what the language offers.”
—Michael Ernest, Inkling Research, Inc.
“I don’t find many programming books that make me want to read every page—
this is one of them.”
—Matt Tucker, chief technical officer, Jive Software
“Great how-to resource for the experienced developer.”
—John Zukowski, author of numerous Java books
“I picked this book up two weeks ago and can safely say I learned more about the
Java language in three days of reading than I did in three months of study! An
excellent book and a welcome addition to my Java library.”
—Jane Griscti, I/T advisory specialist
Effective Java™
Second Edition
The Java™ Series
Ken Arnold, James Gosling, David Holmes
The Java™ Programming Language, Fourth Edition
Joshua Bloch
Effective Java™ Programming Language Guide
Joshua Bloch
Effective Java,™ Second Edition
Stephanie Bodoff, Dale Green, Kim Haase, Eric Jendrock
The J2EE™ Tutorial, Second Edition
Mary Campione, Kathy Walrath, Alison Huml
The Java™ Tutorial, Third Edition: A Short Course on
the Basics
Mary Campione, Kathy Walrath, Alison Huml, The
Tutorial Team
The Java™ Tutorial Continued: The Rest of the JDK™
Patrick Chan
The Java™ Developers Almanac 1.4, Volume 1
Patrick Chan
The Java™ Developers Almanac 1.4, Volume 2
Patrick Chan, Rosanna Lee
The Java™ Class Libraries, Second Edition, Volume 2:
java.applet, java.awt, java.beans
Patrick Chan, Rosanna Lee, Doug Kramer
The Java™ Class Libraries, Second Edition, Volume 1:
Supplement for the Java™ 2 Platform, Standard Edition,
v1.2
Kirk Chen, Li Gong
Programming Open Service Gateways with Java™
Embedded Server
Zhiqun Chen
Java Card™ Technology for Smart Cards: Architecture
and Programmer’s Guide
Maydene Fisher, Jon Ellis, Jonathan Bruce
JDBC™ API Tutorial and Reference, Third Edition
Eric Freeman, Susanne Hupfer, Ken Arnold
JavaSpaces™ Principles, Patterns, and Practice
Li Gong, Gary Ellison, Mary Dageforde
Inside Java™ 2 Platform Security, Second Edition:
Architecture, API Design, and Implementation
James Gosling, Bill Joy, Guy Steele, Gilad Bracha
The Java™ Language Specification, Third Edition
Chet Haase, Romain Guy
Filthy Rich Clients: Developing Animated and Graphical
Effects for Desktop Java™ Applications
Mark Hapner, Rich Burridge, Rahul Sharma, Joseph
Fialli, Kim Haase
Java™ Message Service API Tutorial and Reference:
Messaging for the J2EE™ Platform
Eric Jendrock, Jennifer Ball
The Java™ EE 5 Tutorial, Third Edition
Jonni Kanerva
The Java™ FAQ
Jonathan Knudsen
Kicking Butt with MIDP and MSA: Creating Great
Mobile Applications
David Lambert
Smarter Selling: Consultative Selling Strategies to Meet
Your Buyer’s Needs Every Time
Doug Lea
Concurrent Programming in Java™, Second Edition:
Design Principles and Patterns
Rosanna Lee, Scott Seligman
JNDI API Tutorial and Reference: Building Directory-
Enabled Java™ Applications
Sheng Liang
The Java™ Native Interface: Programmer’s Guide and
Specification
Tim Lindholm, Frank Yellin
The Java™ Virtual Machine Specification, Second Edition
Roger Riggs, Antero Taivalsaari, Jim Van Peursem, Jyri
Huopaniemi, Mark Patel, Aleksi Uotila
Programming Wireless Devices with the Java™ 2
Platform, Micro Edition, Second Edition
Rahul Sharma, Beth Stearns, Tony Ng
J2EE™ Connector Architecture and Enterprise
Application Integration
Inderjeet Singh, Beth Stearns, Mark Johnson, Enterprise
Team
Designing Enterprise Applications with the J2EE™
Platform, Second Edition
Inderjeet Singh, Sean Brydon, Greg Murray, Vijay
Ramachandran, Thierry Violleau, Beth Stearns
Designing Web Services with the J2EE™ 1.4 Platform:
JAX-RPC, SOAP, and XML Technologies
Kathy Walrath, Mary Campione, Alison Huml, Sharon
Zakhour
The JFC Swing Tutorial, Second Edition: A Guide to
Constructing GUIs
SteveWilson, Jeff Kesselman
Java™ Platform Performance: Strategies and Tactics
Sharon Zakhour, Scott Hommel, Jacob Royal,
Isaac Rabinovitch, Tom Risser, Mark Hoeber
The Java™ Tutorial, Fourth Edition: A Short Course
on the Basics
Effective Java™
Second Edition
Joshua Bloch
Upper Saddle River, NJ • Boston • Indianapolis • San Francisco
New York • Toronto • Montreal London • Munich • Paris • Madrid
Capetown • Sydney • Tokyo • Singapore • Mexico City
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ISBN-13: 978-0-321-35668-0
ISBN-10: 0-321-35668-3
Text printed in the United States on recycled paper at Courier in Stoughton, Massachusetts.
First printing, May 2008
To my family: Cindy, Tim, and Matt
This page intentionally left blank
ix
Contents
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xv
Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xix
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
2 Creating and Destroying Objects. . . . . . . . . . . . . . . . . . .5
Item 1: Consider static factory methods instead of constructors . . . 5
Item 2: Consider a builder when faced with many constructor
parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Item 3: Enforce the singleton property with a private
constructor or an enum type . . . . . . . . . . . . . . . . . . . . . . . 17
Item 4: Enforce noninstantiability with a private constructor . . . . 19
Item 5: Avoid creating unnecessary objects . . . . . . . . . . . . . . . . . 20
Item 6: Eliminate obsolete object references . . . . . . . . . . . . . . . . . 24
Item 7: Avoid finalizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3 Methods Common to All Objects. . . . . . . . . . . . . . . . . .33
Item 8: Obey the general contract when overriding equals . . . . . 33
Item 9: Always override hashCode when you
override equals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Item 10: Always override toString . . . . . . . . . . . . . . . . . . . . . . . . 51
Item 11: Override clone judiciously . . . . . . . . . . . . . . . . . . . . . . . . 54
Item 12: Consider implementing Comparable . . . . . . . . . . . . . . . . 62
x CONTENTS
4 Classes and Interfaces. . . . . . . . . . . . . . . . . . . . . . . . . . .67
Item 13: Minimize the accessibility of classes and members . . . . . . 67
Item 14: In public classes, use accessor methods,
not public fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Item 15: Minimize mutability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Item 16: Favor composition over inheritance . . . . . . . . . . . . . . . . . . 81
Item 17: Design and document for inheritance or else prohibit it . . 87
Item 18: Prefer interfaces to abstract classes . . . . . . . . . . . . . . . . . . 93
Item 19: Use interfaces only to define types. . . . . . . . . . . . . . . . . . . 98
Item 20: Prefer class hierarchies to tagged classes . . . . . . . . . . . . . 100
Item 21: Use function objects to represent strategies . . . . . . . . . . . 103
Item 22: Favor static member classes over nonstatic . . . . . . . . . . . 106
5 Generics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .109
Item 23: Don’t use raw types in new code . . . . . . . . . . . . . . . . . . . 109
Item 24: Eliminate unchecked warnings. . . . . . . . . . . . . . . . . . . . . 116
Item 25: Prefer lists to arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
Item 26: Favor generic types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
Item 27: Favor generic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
Item 28: Use bounded wildcards to increase API flexibility . . . . . 134
Item 29: Consider typesafe heterogeneous containers . . . . . . . . . . 142
6 Enums and Annotations . . . . . . . . . . . . . . . . . . . . . . . .147
Item 30: Use enums instead of int constants. . . . . . . . . . . . . . . . . 147
Item 31: Use instance fields instead of ordinals . . . . . . . . . . . . . . . 158
Item 32: Use EnumSet instead of bit fields . . . . . . . . . . . . . . . . . . . 159
Item 33: Use EnumMap instead of ordinal indexing. . . . . . . . . . . . . 161
Item 34: Emulate extensible enums with interfaces . . . . . . . . . . . . 165
Item 35: Prefer annotations to naming patterns . . . . . . . . . . . . . . . 169
Item 36: Consistently use the Override annotation. . . . . . . . . . . . 176
Item 37: Use marker interfaces to define types . . . . . . . . . . . . . . . 179
7 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .181
Item 38: Check parameters for validity . . . . . . . . . . . . . . . . . . . . . 181
Item 39: Make defensive copies when needed . . . . . . . . . . . . . . . . 184
Item 40: Design method signatures carefully . . . . . . . . . . . . . . . . . 189
Item 41: Use overloading judiciously . . . . . . . . . . . . . . . . . . . . . . . 191
CONTENTS xi
Item 42: Use varargs judiciously . . . . . . . . . . . . . . . . . . . . . . . . . . 197
Item 43: Return empty arrays or collections, not nulls . . . . . . . . . 201
Item 44: Write doc comments for all exposed API elements . . . . 203
8 General Programming . . . . . . . . . . . . . . . . . . . . . . . . .209
Item 45: Minimize the scope of local variables . . . . . . . . . . . . . . . 209
Item 46: Prefer for-each loops to traditional for loops . . . . . . . . . 212
Item 47: Know and use the libraries . . . . . . . . . . . . . . . . . . . . . . . 215
Item 48: Avoid float and double if exact answers
are required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218
Item 49: Prefer primitive types to boxed primitives . . . . . . . . . . . 221
Item 50: Avoid strings where other types are more appropriate . . 224
Item 51: Beware the performance of string concatenation . . . . . . 227
Item 52: Refer to objects by their interfaces . . . . . . . . . . . . . . . . . 228
Item 53: Prefer interfaces to reflection . . . . . . . . . . . . . . . . . . . . . 230
Item 54: Use native methods judiciously. . . . . . . . . . . . . . . . . . . . 233
Item 55: Optimize judiciously . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234
Item 56: Adhere to generally accepted naming conventions . . . . . 237
9 Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .241
Item 57: Use exceptions only for exceptional conditions . . . . . . . 241
Item 58: Use checked exceptions for recoverable conditions
and runtime exceptions for programming errors . . . . . . . 244
Item 59: Avoid unnecessary use of checked exceptions . . . . . . . . 246
Item 60: Favor the use of standard exceptions. . . . . . . . . . . . . . . . 248
Item 61: Throw exceptions appropriate to the abstraction. . . . . . . 250
Item 62: Document all exceptions thrown by each method. . . . . . 252
Item 63: Include failure-capture information in
detail messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
Item 64: Strive for failure atomicity . . . . . . . . . . . . . . . . . . . . . . . 256
Item 65: Don’t ignore exceptions . . . . . . . . . . . . . . . . . . . . . . . . . 258
10 Concurrency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .259
Item 66: Synchronize access to shared mutable data. . . . . . . . . . . 259
Item 67: Avoid excessive synchronization . . . . . . . . . . . . . . . . . . 265
Item 68: Prefer executors and tasks to threads. . . . . . . . . . . . . . . . 271
Item 69: Prefer concurrency utilities to wait and notify. . . . . . . 273
xii CONTENTS
Item 70: Document thread safety . . . . . . . . . . . . . . . . . . . . . . . . . . 278
Item 71: Use lazy initialization judiciously . . . . . . . . . . . . . . . . . . 282
Item 72: Don’t depend on the thread scheduler . . . . . . . . . . . . . . . 286
Item 73: Avoid thread groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288
11 Serialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .289
Item 74: Implement Serializable judiciously. . . . . . . . . . . . . . . 289
Item 75: Consider using a custom serialized form . . . . . . . . . . . . . 295
Item 76: Write readObject methods defensively . . . . . . . . . . . . . 302
Item 77: For instance control, prefer enum types
to readResolve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308
Item 78: Consider serialization proxies instead of serialized
instances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312
Appendix: Items Corresponding to First Edition . . . . . .317
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .321
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .327
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