HashMap的keySet遍历和entrySet遍历时间效率比较

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

public class HashMapTest {
	public static void main(String[] args) {
		
		Map<Integer, String> map = new HashMap<Integer, String>();
		for (int i = 0; i <= 5000000; i++) {
			map.put(i, "a" + i);
		}
		
		//遍历方法1
		Set<Integer>set = map.keySet();
		Iterator iterator1 = set.iterator();		
		long startTime1 = Calendar.getInstance().getTimeInMillis(); //开始时间		
		while (iterator1.hasNext()) {
			Object key = iterator1.next();
			Object value = map.get(key);
		}
		long endTime1 = Calendar.getInstance().getTimeInMillis();
		System.out.println("costTime1 = " + (endTime1 - startTime1)); //结束时间
		
		//遍历方法2
		Iterator iterator2 = map.entrySet().iterator();
		long startTime2 = Calendar.getInstance().getTimeInMillis(); //开始时间
		while (iterator2.hasNext()) {
			Map.Entry<Integer, String> entry = (Map.Entry<Integer, String>) iterator2.next();
			Object key = entry.getKey();
			Object val = entry.getValue();
		}
		long endTime2 = Calendar.getInstance().getTimeInMillis(); //结束时间
		System.out.println("costTime2 = " + (endTime2 - startTime2));
	}
}

输出结果：

costTime1 = 138
costTime2 = 77