HashMap的7种遍历方式_hashmap遍历

注意：首先要弄清楚EntrySet和KeySet的区别，HashMap遍历中，keySet是键的集合，Set里面的类型是key类型，entrySet是键-值对的集合，Set里面的类型是Map.Entry

1.keySet是键的集合

Map map=new HashMap();

Iterator it=map.keySet().iterator();

Object key ,value;

while(it.hasNext()){

key=it.next();

value=map.get(key);

System.out.println(key+":"+value);

}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

2.entrySet是键-值对的集合

Map map=new HashMap();

Iterator it=map.entrySet().iterator();

Object key ,value;

while(it.hasNext()){

Map.Entry entry = (Map.Entry)it.next();

key=entry.getKey();

value=entry.getValue();

System.out.println(key+"="+value);

}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19

1.iterator迭代器进行Entry遍历(此方法效率最佳)

Iterator<Map.Entry<Integer, String>> iterator = map.entrySet().iterator();
    while (iterator.hasNext()){
        Map.Entry<Integer, String> next = iterator.next();
        System.out.println(next.getKey());
        System.out.println(next.getValue());
    }
1
2
3
4
5
6

2.iterator迭代器进行KeySet遍历

Iterator<Integer> iterator = map.keySet().iterator();
    while (iterator.hasNext()){
        Integer key = iterator.next();
        System.out.print(key);
        System.out.print(map.get(key));
    }
1
2
3
4
5
6

3.ForEach的EntrySet方式进行遍历

 for (Map.Entry<Integer,String> entry: map.entrySet()) {
        System.out.println("entry.getKey() = " + entry.getKey());
        System.out.println("entry.getValue() = " + entry.getValue());
    }
1
2
3
4

4.ForEach的KeySet方式进行遍历

 for (Interger key: map.keySet()) {
     	System.out.print(key);
        System.out.print(map.get(key));
    }
1
2
3
4

5.Lambda 表达式进行遍历

    map.forEach((key,value) -> {
        System.out.print(key);
        System.out.print(value);
    });
1
2
3
4

6. Streams API 单线程的方式进行遍历

    map.entrySet().stream().forEach((integerStringEntry -> {
        System.out.println(integerStringEntry.getKey());
        System.out.println(integerStringEntry.getValue());
    }));
1
2
3
4

7.Streams API 多线程的方式进行遍历

    map.entrySet().parallelStream().forEach((integerStringEntry -> {
        System.out.println(integerStringEntry.getKey());
        System.out.println(integerStringEntry.getValue());
    }));
1
2
3
4