java中Map的两种不同遍历、迭代方式及比较

方法分为两类：
一类是基于map的Entry；map.entrySet();
一类是基于map的key；map.keySet()
而每一类都有两种遍历方式：
a.利用迭代器 iterator；
b.利用for-each循环；

代码举例如下：

package cn.wzb;  

import java.util.ArrayList;  
import java.util.HashMap;  
import java.util.Iterator;  
import java.util.List;  
import java.util.Map;  
import java.util.Set;  
import java.util.Map.Entry;  

//循环遍历map的方法  
public class TestMap {  
    public static void main(String[] args) {  
        Map<String, Integer> map = new HashMap<String, Integer>();  
        map.put("a", 1);  
        map.put("b", 2);  
        map.put("c", 3);  

        // JDK1.4中  
        // 遍历方法一 hashMap.entrySet() 遍历  
        System.out.println("方法一");  
        Iterator<Entry<String, Integer>> it = map.entrySet().iterator();  
        while (it.hasNext()) {  
            Map.Entry<String, Integer> entry =  it.next();  
            String key = entry.getKey();  
             Integer value = entry.getValue();  
            System.out.println("key = " + key + "; value = " + value);  
        }  

        System.out.println("");  

        // JDK1.5中,应用新特性For-Each循环  // 遍历方法二  
        System.out.println("方法二");  
        for (Map.Entry<String, Integer> entry : map.entrySet()) {  
            String key = entry.getKey();  
            String value = entry.getValue().toString();  
            System.out.println("key =" + key + " value = " + value);  
        }  
        System.out.println("");  

        // 遍历方法三 hashMap.keySet().iterator() 遍历  
        System.out.println("方法三");  
        for (Iterator<String> i = map.keySet().iterator(); i.hasNext(); ) {  
             String key = i.next();           
            System.out.println(" key = " + key + "; value = " + map.get(key));  
        }  

       /* 
        for (Iterator<Integer> i = map.values().iterator(); i.hasNext(); ) { 
            Integer value = i.next(); 
            System.out.println(value));// 循环输出value 
        } 
        */  
        /* 
        for(Integer value : map.values() ) { 
            System.out.println("value = " + value); 
        } 
        */  


        System.out.println("");  
        // 遍历方法四  Hashmap.keySet()遍历  
        System.out.println("方法四");  
        for (String key : map.keySet()) {  
            System.out.println("key = " + key + "; value = " + map.get(key));  
        }  
        System.out.println();  

        // java如何遍历Map <String, ArrayList> map = new HashMap <String, ArrayList<Integer> >();  
        System.out.println("java  遍历Map <String, ArrayList<Integer> map = new HashMap <String, ArrayList<Integer>>();");  
        System.out.println("方法一：");  
        Map<String, List<Integer> > mapList  = new HashMap <String, List<Integer> >();  
        List<Integer> list = new ArrayList<Integer>();  
        list.add(1);  
        list.add(2);  
        list.add(3);  
        list.add(4);  
        list.add(5);  
        mapList.put("aaa", list);  
        mapList.put("bbb", list);  

        Set<String> keys = mapList.keySet();  
        Iterator<String> iterator = keys.iterator();  
        while (iterator.hasNext()) {  
            String key = iterator.next();  
            System.out.println(key + " : ");  

            List<Integer> arrayList = mapList.get(key);  
            for (Integer i : arrayList) {  
                System.out.print(i + ", ");  
            }  
            System.out.println();  
        }  

        System.out.println("方法二：");  

        for (Map.Entry<String, List<Integer> > entry : mapList.entrySet()) {  
            String key = entry.getKey();  
            System.out.println(key + " : ");  

            List<Integer> values = (List<Integer>) entry.getValue();  
            for (Integer value : values) {  
                System.out.print(value + ", ");  
            }  
            System.out.println();  
        }  
    }  
}