Java泛型与集合深度解析

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"Java泛型和集合" Java泛型是Java编程语言中的一种特性，它允许程序员在定义类、接口和方法时指定参数类型。这一特性引入于Java 5.0，大大增强了代码的类型安全性和可读性，减少了运行时的类型检查和强制类型转换。泛型的主要目标是实现泛型集合，即可以容纳任何类型对象的容器，但又限制了这些对象的类型，确保了数据的纯净性。集合框架是Java中用于存储和操作对象的统一接口和类的集合，包括List、Set、Queue和Map等接口，以及ArrayList、LinkedList、HashSet、HashMap等实现类。Java泛型与集合框架的结合使得开发者能够创建强类型化的集合，避免了类型转换的繁琐和潜在的ClassCastException风险。在Java中，泛型的使用主要体现在以下几个方面： 1. 泛型类：如ArrayList<T>，这里的T代表一个类型参数，可以在实例化时指定具体类型，如ArrayList<String>或ArrayList<Integer>。 2. 泛型接口：例如Comparator<T>接口，可以比较指定类型的对象。 3. 泛型方法：如Collections.sort(List<T> list)，这个方法可以对任何实现了Comparable接口的List进行排序。 4. 泛型通配符：如"? extends Number"，表示列表中的元素可以是Number或其子类，提供了更灵活的类型约束。 5. 类型擦除：Java泛型在编译后会进行类型擦除，也就是说在运行时，泛型类和非泛型类是相同的。这是为了保持向后兼容性，但意味着不能在运行时检查泛型类型。 6. 泛型边界：例如<T extends Comparable<T>>，限制了T必须是Comparable的实例，这样就可以直接在泛型类或方法中调用compareTo方法。集合框架的泛型使用也遵循以上原则，比如HashMap<K, V>中的K和V分别代表键和值的类型，通过泛型可以确保键和值之间的类型一致性，避免插入不匹配的键值对。在实际开发中，理解并熟练运用Java泛型和集合框架，能够编写出更加安全、高效且易于维护的代码。此外，对于面试和项目开发，掌握泛型和集合的知识也是必不可少的，它们是Java程序员的基础技能之一。

• We often include code that corresponds to the body of an appropriate main method:

Stack<Integer> stack = new ArrayStack<Integer>();

for (int i = 0; i<4; i++) stack.push(i);

assert stack.toString().equals("stack[0, 1, 2, 3]");

• Code fragments are printed in fixed-width font when they appear within a para-

graph (as when we referred to a main method in the preceding item).

• We often omit standard imports. Code that uses the Java Collection Framework

or other utility classes should be preceded by the line:

import java.util.*;

• Sample interactive sessions, showing command-line input and corresponding out-

put, are shown in constant-width font, with user-supplied input preceded by a

percent sign:

% javac g/Stack.java g/ArrayStack.java g/Stacks.java l/Client.java

Note: Client.java uses unchecked or unsafe operations.

Note: Recompile with -Xlint:unchecked for details.

• When user-supplied input is two lines long, the first line is ended with a backslash:

% javac -Xlint:unchecked g/Stack.java g/ArrayStack.java \

% g/Stacks.java l/Client.java

l/Client.java:4: warning: [unchecked] unchecked call

to push(E) as a member of the raw type Stack

for (int i = 0; i<4; i++) stack.push(new Integer(i));

Using Code Examples

This book is here to help you get your job done. In general, you may use the code in

this book in your programs and documentation. You do not need to contact us for

permission unless you’re reproducing a significant portion of the code. For example,

writing a program that uses several chunks of code from this book does not require

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require permission. Answering a question by citing this book and quoting example

code does not require permission. Incorporating a significant amount of example code

from this book into your product’s documentation does require permission.

We appreciate, but do not require, attribution. An attribution usually includes the title,

author, publisher, and ISBN. For example: "Java Generics and Collections, by Maurice

If you feel your use of code examples falls outside fair use or the permission given above,

feel free to contact us at permissions@oreilly.com.

xiv | Preface

PART I

Generics

Generics are a powerful, and sometimes controversial, new feature of the Java pro-

gramming language. This part of the book describes generics, using the Collections

Framework as a source of examples. A comprehensive introduction to the Collections

Framework appears in the second part of the book.

The first five chapters focus on the fundamentals of generics. Chapter 1 gives an over-

view of generics and other new features in Java 5, including boxing, foreach loops, and

functions with a variable number of arguments. Chapter 2 reviews how subtyping

works and explains how wildcards let you use subtyping in connection with generics.

Chapter 3 describes how generics work with the Comparable interface, which requires

a notion of bounds on type variables. Chapter 4 looks at how generics work with various

declarations, including constructors, static members, and nested classes. Chapter 5

explains how to evolve legacy code to exploit generics, and how ease of evolution is a

key advantage of the design of generics in Java. Once you have these five chapters under

your belt, you will be able to use generics effectively in most basic situations.

The next four chapters treat advanced topics. Chapter 6 explains how the same design

that leads to ease of evolution also necessarily leads to a few rough edges in the treat-

ment of casts, exceptions, and arrays. The fit between generics and arrays is the worst

rough corner of the language, and we formulate two principles to help you work around

the problems. Chapter 7 explains new features that relate generics and reflection, in-

cluding the newly generified type Class<T> and additions to the Java library that support

reflection of generic types. Chapter 8 contains advice on how to use generics effectively

in practical coding. We consider checked collections, security issues, specialized

classes, and binary compatibility. Chapter 9 presents five extended examples, looking

at how generics affect five well-known design patterns: Visitor, Interpreter, Function,

Strategy, and Subject-Observer.

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