集合类
集合类是Java基础技术中十分重要的内容。Java集合类封装了集合中常用数据结构和算法，它犹如哪咤的烽火轮，功能强大，而且不用亲自制造(Made in America)。首先看下面这张表，本文即通过它展开相关内容。
Implementations
Hash Table
Resizable Array
Balanced Tree
Linked List
Hash Table + LinkedList
Interfaces
Set
HashSet
TreeSet
LinkedHashSet
List
ArrayList
LinkedList
Map
HashMap
TreeMap
LinkedHashMap
图1
Java集合类的种类
注意图1第二列，Java在设计集合结构时，把集合划成3类：第一种Set，普通的无序集；第二种List，偏序集；第三种Map，有序对的集合。其中Set和List都继承自Collection接口，Map没有从Collection继承。这样做也是没办法的事，因为Map并不是一个通常意义的集合，它的元素是一个key-value-pair，而Collection根本就不是Map，它还没有key的概念。虽然Collection和Map彼此没有继承关系，但它们有大量同名的方法。值得注意的是，Map提供了keySet()和values()两个方法，分别返回一个key的集合和值的集合。
示例1：通过对一个HashSet对象和一个ArrayList对象中元素使用跌代器可以显示Set与List中元素之间的顺序关系：Set中元素是无序的，List不是。
<div style="padding-right: 5.4pt; padding-left: 5.4pt; background: #e6e6e6; padding-bottom: 4px; width: 95%; padding-top: 4px;">http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gifimport java.util.ArrayList;
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gifimport java.util.HashSet;
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gifimport java.util.Iterator;
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gifimport java.util.List;
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gifimport java.util.Set;
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockStart.gifhttp://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedBlock.gifpublic class ElementOrder ...{
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockStart.gifhttp://images.csdn.net/syntaxhighlighting/OutliningIndicators/ContractedSubBlock.gif public static void main(String[] args) ...{
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif Iterator it = null;
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif Set ht = new HashSet();
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif ht.add("htA");
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif ht.add("htB");
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif ht.add("htC");
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif ht.add("htD");
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif it = ht.iterator();
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http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif System.out.println(it.next());
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif }
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif List al = new ArrayList();
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif al.add("alA");
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif al.add("alB");
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif al.add("alC");
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif al.add("adD");
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif it = al.iterator();
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif
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http://images.csdn.net/syntaxhighlighting/OutliningIndicators/InBlock.gif System.out.println(it.next());
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif }
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedSubBlockEnd.gif }
http://images.csdn.net/syntaxhighlighting/OutliningIndicators/ExpandedBlockEnd.gif}
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