Map常用遍历方法

HashMap常用遍历方法

1.for循环中使用entries实现Map的遍历

@Test
    public void fun1(){
        Map<String, String> map = new HashMap<String, String>();
        map.put("Java入门教程", "http://c.biancheng.net/java/");
        map.put("C语言入门教程", "http://c.biancheng.net/c/");
        for (Map.Entry<String, String> entry : map.entrySet()) {
            String mapKey = entry.getKey();
            String mapValue = entry.getValue();
            System.out.println(mapKey + "：" + mapValue);
        }
    }

2.使用迭代器（Iterator）遍历

public void fun2(){
        Map<String, String> map = new HashMap<String, String>();
        map.put("Java入门教程", "http://c.biancheng.net/java/");
        map.put("C语言入门教程", "http://c.biancheng.net/c/");
        Iterator<Map.Entry<String, String>> entries = map.entrySet().iterator();
        while (entries.hasNext()) {
            Map.Entry<String, String> entry = entries.next();
            String key = entry.getKey();
            String value = entry.getValue();
            System.out.println(key + ":" + value);
        }
    }

二者耗时比较

@Test
    public void fun3(){
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        Random random =new Random();
        for (int a=0;a<1000000;a++){
            int b=random.nextInt(200);
            map.put(a,b);
        }
        long t1= System.currentTimeMillis();
        for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
            Integer mapKey = entry.getKey();
            Integer mapValue = entry.getValue();
            System.out.println(mapKey + "：" + mapValue);
        }
        long t2= System.currentTimeMillis();
        System.out.print("for循环中使用entries实现Map的遍历耗时为：");
        System.out.println((t2 - t1)+"毫秒");
    }

for循环中使用entries实现Map的遍历耗时为：9937毫秒

@Test
    public void fun4(){
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        Random random =new Random();
        for (int a=0;a<1000000;a++){
            int b=random.nextInt(200);
            map.put(a,b);
        }
        long t1= System.currentTimeMillis();
        Iterator<Map.Entry<Integer, Integer>> entries = map.entrySet().iterator();
        while (entries.hasNext()) {
            Map.Entry<Integer, Integer> entry = entries.next();
            Integer key = entry.getKey();
            Integer value = entry.getValue();
            System.out.println(key + ":" + value);
        }
        long t2= System.currentTimeMillis();
        System.out.print("使用迭代器（Iterator）遍历耗时为：");
        System.out.println((t2 - t1)+"毫秒");
    }

使用迭代器（Iterator）遍历耗时为：8554毫秒

经过代码测试，当数据量特别大时，迭代器的耗时相比较而言小一些