Java Collections Framework详解与实用教程

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本篇教程深入探讨了Java集合框架（Java Collections Framework）的核心概念与实践，旨在为初学者和高级开发人员提供全面的学习指南。以下章节概述了教程的主要内容： 1. **教程提示**：本节提供了参加教程的适宜人群，以及对于快速掌握Java集合框架的入门示例。无论是编程新手还是经验丰富的开发者，都能找到适合自己的起点。 2. **集合框架简介**：这一部分介绍了Java集合框架的整体架构和设计理念，它为数据存储和操作提供了一套统一的接口和类，使得代码组织更加清晰和高效。 3. **接口和类**：涵盖了核心的集合接口如List、Set和Map，以及它们各自的实现类，如ArrayList、LinkedList、HashSet、TreeSet和HashMap等。这部分着重讲解这些接口的定义、功能和适用场景。 4. **特殊集合实现**：这部分详细讨论了一些特殊的集合实现，比如PriorityQueue、Stack和Queue，它们各自的特点和在特定情境下的应用。 5. **历史回顾**：讲述了Java集合框架的发展历程，澄清了由于版本迭代而产生的各种类库之间的关系，帮助读者理解框架演进背后的设计决策。 6. **算法支持**：介绍了Java集合框架如何支持常见的算法和数据结构操作，例如排序、查找、过滤等，以及如何优化性能。 7. **使用注意事项**：探讨了在实际项目中可能遇到的问题和最佳实践，包括内存管理、并发访问和性能调优等方面。 8. **替代选择**：分析了在特定场景下可能用到的其他数据结构或框架，以及它们与Java集合框架的对比。 9. **练习与习题**：为读者提供了实践环节，通过一系列的练习和问题解答，巩固所学知识并提升技能。 10. **总结与回顾**：教程的结尾部分总结了关键要点，并对未来的学习方向给出了建议，帮助读者深化理解和运用Java集合框架。通过阅读这篇教程，读者可以系统地了解Java集合框架，熟练掌握其使用方法，并能根据具体需求选择最合适的集合类型来设计和优化软件架构。

override the behavior.

Collections Framework design concerns

In the creation of the Collections Framework, the Sun development team needed to provide

flexible interfaces that manipulated groups of elements. To keep the design simple, instead

of providing separate interfaces for optional capabilities, the interfaces define all the methods

an implementation class may provide. However, some of the interface methods are optional.

Because an interface implementation must provide implementations for all the interface

methods, there needed to be a way for a caller to know if an optional method is not

supported. The manner the framework development team chose to signal callers when an

optional method is called was to throw an UnsupportedOperationException. If in the

course of using a collection an UnsupportedOperationException is thrown, then the

operation failed because it is not supported. To avoid having to place all collection operations

within a try-catch block, the UnsupportedOperationException class is an extension

of the RuntimeException class.

In addition to handling optional operations with a run-time exception, the iterators for the

concrete collection implementations are fail-fast . That means that if you are using an

Iterator to traverse a collection while the underlying collection is being modified by

another thread, then the Iterator fails immediately by throwing a

ConcurrentModificationException (another RuntimeException). That means the

next time an Iterator method is called, and the underlying collection has been modified,

the ConcurrentModificationException exception gets thrown.

Set interface

The Set interface extends the Collection interface and, by definition, forbids duplicates

within the collection. All the original methods are present and no new methods are

introduced. The concrete Set implementation classes rely on the equals() method of the

object added to check for equality.

HashSet and TreeSet classes

The Collections Framework provides two general-purpose implementations of the Set

interface: HashSet and TreeSet. More often than not, you will use a HashSet for storing

your duplicate-free collection. For efficiency, objects added to a HashSet need to implement

the hashCode() method in a manner that properly distributes the hash codes. While most

system classes override the default hashCode() implementation in Object, when creating

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your own classes to add to a HashSet remember to override hashCode(). The TreeSet

implementation is useful when you need to extract elements from a collection in a sorted

manner. In order to work properly, elements added to a TreeSet must be sortable. The

Collections Framework adds support for Comparable elements and will be covered in detail

in "Comparable interface" in Sorting on page 17. For now, just assume a tree knows how to

keep elements of the java.lang wrapper classes sorted. It is generally faster to add

elements to a HashSet, then convert the collection to a TreeSet for sorted traversal.

To optimize HashSet space usage, you can tune the initial capacity and load factor. The

TreeSet has no tuning options, as the tree is always balanced, ensuring log(n)

performance for insertions, deletions, and queries.

Both HashSet and TreeSet implement the Cloneable interface.

Set usage example

To demonstrate the use of the concrete Set classes, the following program creates a

HashSet and adds a group of names, including one name twice. The program then prints

out the list of names in the set, demonstrating the duplicate name isn't present. Next, the

program treats the set as a TreeSet and displays the list sorted.

import java.util.*;

public class SetExample {

public static void main(String args[]) {

Set set = new HashSet();

set.add("Bernadine");

set.add("Elizabeth");

set.add("Gene");

set.add("Elizabeth");

set.add("Clara");

System.out.println(set);

Set sortedSet = new TreeSet(set);

System.out.println(sortedSet);

}

Running the program produces the following output. Notice that the duplicate entry is only

present once, and the second list output is sorted alphabetically.

[Gene, Clara, Bernadine, Elizabeth]

[Bernadine, Clara, Elizabeth, Gene]

AbstractSet class

The AbstractSet class overrides the equals() and hashCode() methods to ensure

two equal sets return the same hash code. Two sets are equal if they are the same size and

contain the same elements. By definition, the hash code for a set is the sum of the hash

codes for the elements of the set. Thus, no matter what the internal ordering of the sets, two

equal sets will report the same hash code.

Exercises

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