Map.keySet 和 Map.entrySet 的效率问题

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class Test {

 public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<String, Integer>();
        for(int i=0;i<10000;i++){
            map.put(i+"", i);
        }
        long begin1 = System.currentTimeMillis();
        Set<String> keyset = map.keySet();
        Iterator<String> it = keyset.iterator();
        while(it.hasNext()){
        String k  = it.next();
            System.out.println(map.get(k));
        }
        long end1 = System.currentTimeMillis();

        long begin2 = System.currentTimeMillis();
        Set<Map.Entry<String, Integer>> entries = map.entrySet();
        Iterator<Map.Entry<String, Integer>> it2 = entries.iterator();
        while(it2.hasNext()){
            Map.Entry<String, Integer> next = it2.next();
            System.out.println(next.getKey()+"---"+next.getValue());
        }
        long end2 = System.currentTimeMillis();

        System.out.println((end1-begin1)+"---"+(end2-begin2));;
    }

}

执行时间，用keySet方式遍历要比entrySet的方式用时大两倍！

时间差在，keySet方式要再从Map里面获取一遍。