java基础--关于map的遍历keySet和entrySet

首先是一个 关于遍历的小例子：

public static void main(String[] args) {
        // TODO Auto-generated method stub
        Map<String, String> maps = new HashMap<String, String>();
        maps.put("111", "111a");
        maps.put("222", "222b");
        maps.put("333", "333c");
        maps.put("444", "444d");
        maps.put("555", "555e");
        maps.put("666", "666f");

        for(String str : maps.keySet()){
            System.out.println(str + ":" + maps.get(str));
        }

        System.out.println("--------------");

        for(Entry<String, String> str : maps.entrySet()){
            System.out.println(str + "   " + str.getKey() + ":" + str.getValue());
        }

    }

至于这两者的性能：

通过测试发现,第二种方式的性能通常要比第一种方式高一倍。

例子如下：

import java.util.Calendar;
import java.util.Date;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map.Entry;

/**
 * 测试keySet()与entrySet()的迭代时间
 * keySet():迭代后只能通过get()取key
 * entrySet()：迭代后可以e.getKey()，e.getValue()取key和value。返回的是Entry接口
 * 最后说明下keySet()的速度比entrySet()慢了很多。看来以后要考虑用entrySet()了
 * @author YL
 * @date 2009.6.10
 */
public class HashMapTest 
{
public static void main(String[] args) 
{
HashMap<String,String> kmap = new HashMap<String,String>();
HashMap<String, String> emap = new HashMap<String, String>();

//装数据
for (int i = 0; i < 1000; i++) 
{
kmap.put(""+i, "YL");
}
for (int i = 0; i < 1000; i++) 
{
emap.put(""+i, "ZT");
}

long stimes = System.currentTimeMillis();
long ctimes = Calendar.getInstance().getTimeInMillis();
long dtimes = new Date().getTime();

//初始时间 这里我用了三种取值方式 最后发现System.currentTimeMillis();是最直接的取值方法
System.out.println(stimes+""+ctimes+""+dtimes);

Iterator<String> ktor = kmap.keySet().iterator();
while(ktor.hasNext())
{
System.out.println(ktor.next());
}

long stimes1 = System.currentTimeMillis();
long ctimes1 = Calendar.getInstance().getTimeInMillis();
long dtimes1 = new Date().getTime();

//结束世界并且也是entrySet的开始时间
System.out.println((stimes1-stimes)+""+(ctimes1-ctimes)+""+(dtimes1-dtimes));
System.out.println(stimes1+""+ctimes1+""+dtimes1);

Iterator<Entry<String, String>> itor = emap.entrySet().iterator();
while(itor.hasNext())
{
Entry<String, String> e = itor.next();
//System.out.println(e.getKey());
System.out.println(e.getValue());
}

long stimes2 = System.currentTimeMillis();
long ctimes2 = Calendar.getInstance().getTimeInMillis();
long dtimes2 = new Date().getTime();
System.out.println(stimes2+""+ctimes2+""+dtimes2);
System.out.println((stimes2-stimes1)+""+(ctimes2-ctimes1)+""+(dtimes2-dtimes1));
}
}