主题：[转帖]Java容器类List和Set的分析
txdnet
[专家分：0] 发布于 2008-06-21 15:21:00
Java2容器类类库的用途是"保存对象",它分为两类:
Collection一组独立的元素,通常这些元素都服从某种规则.List必须保持元素特定的顺序,而Set不能有重复元素.
Map一组成对的"键值对"对象,即其元素是成对的对象,最典型的应用就是数据字典,并且还有其它广泛的应用.另外,Map可以返回其所有键组成的Set和其所有值组成的Collection,或其键值对组成的Set,并且还可以像数组一样扩展多维Map,只要让Map中键值对的每个"值"是一个Map即可.
1.迭代器
迭代器是一种设计模式,它是一个对象,它可以遍历并选择序列中的对象,而开发人员不需要了解该序列的底层结构.迭代器通常被称为"轻量级"对象,因为创建它的代价小.
Java中的Iterator功能比较简单,并且只能单向移动:
(1)使用方法iterator()要求容器返回一个Iterator.第一次调用Iterator的next()方法时,它返回序列的第一个元素.
(2) 使用next()获得序列中的下一个元素.
(3) 使用hasNext()检查序列中是否还有元素.
(4) 使用remove()将迭代器新返回的元素删除.
Iterator是Java迭代器最简单的实现,为List设计的ListIterator具有更多的功能,它可以从两个方向遍历List,也可以从List中插入和删除元素.
2.List的功能方法
List(interface):次序是List最重要的特点;它确保维护元素特定的顺序.List为Collection添加了许多方法,使得能够向List中间插入与移除元素(只推荐LinkedList使用).一个List可以生成ListIterator,使用它可以从两个方向遍历List,也可以从List中间插入和删除元素.
ArrayList:由数组实现的List.它允许对元素进行快速随机访问,但是向List中间插入与移除元素的速度很慢.ListIterator只应该用来由后向前遍历ArrayList,而不是用来插入和删除元素,因为这比LinkedList开销要大很多.
LinkedList:对顺序访问进行了优化,向List中间插入与删除得开销不大,随机访问则相对较慢(可用ArrayList代替).它具有方法addFirst(),addLast(),getFirst(),getLast(),removeFirst(),removeLast(),这些方法(没有在任何接口或基类中定义过)使得LinkedList可以当作堆栈,队列和双向队列使用.
3.Set的功能方法
Set(interface):存入Set的每个元素必须是唯一的,因为Set不保存重复元素.加入Set的t必须定义equals()方法以确保对象的唯一性.Set与Collection有完全一样的接口.Set接口不保证维护元素的次序.
HashSet: 为快速查找而设计的Set.存入HashSet的对象必须定义hashCode().
TreeSet: 保持次序的Set,底层为树结构.使用它可以从Set中提取有序的序列.
LinkedHashSet:具有HashSet的查询速度,且内部使用链表维护元素的顺序(插入的次序).于是在使用迭代器遍历Set时,结果会按元素插入的次序显示.
HashSet采用散列函数对元素进行排序,这是专门为快速查询而设计的;TreeSet采用红黑树的数据结构进行排序元素;LinkedHashSet内部使用散列以加快查询速度,同时使用链表维护元素的次序,使得看起来元素是以插入的顺序保存的.需要注意的是,生成自己的类时,Set需要维护元素的存储顺序,因此要实现Comble接口并定义compareTo()方法.
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沙发
nana001 [专家分：10] 发布于 2008-06-27 12:55:00
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