Java : In ConcurrentHashMap , 如果我更改 key ，为什么会有不同的输出

Question

第一次迭代:-

class ConcurrentHashMapBehaviour
{       
    public static void main(String[] args) 
    {
        ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<String, Integer>();
        map.put("ONE", 1);
        map.put("TWO", 2);
        map.put("THREE", 3);
        map.put("FOUR", 4);
        Iterator<String> it = map.keySet().iterator();
        while (it.hasNext()){
            String key = (String) it.next();
            System.out.println(key+" : "+map.get(key));
            map.put("SEVEN", 7);
        }
    }
}

输出是:

ONE : 1
FOUR : 4
TWO : 2
THREE : 3
SEVEN : 7

更改 key 后的第二次迭代

class ConcurrentHashMapBehaviour
{       
    public static void main(String[] args) 
    {
        ConcurrentHashMap<String, Integer> map = new 
        ConcurrentHashMap<String, Integer>();

        map.put("ONE", 1);
        map.put("TWO", 2);
        map.put("THREE", 3);
        map.put("FOUR", 4);
        Iterator<String> it = map.keySet().iterator();
        while (it.hasNext()){
            String key = (String) it.next();
            System.out.println(key+" : "+map.get(key));
            map.put("FIVE", 5);
        }           
    }
} 

输出是:

ONE : 1
FOUR : 4
TWO : 2
THREE : 3

所以我的问题是为什么第一次迭代包含 7 作为输出，但在第二次迭代中不包含 5？

Answer 1

Javadoc(我强调):

Similarly, Iterators, Spliterators and Enumerations return elements reflecting the state of the hash table at some point at or since the creation of the iterator/enumeration.

换句话说，对于迭代器可用的 View 没有任何保证。在创建 Iterator 之后，它可能会或可能不会意识到映射的更改。唯一的保证是

They do not throw ConcurrentModificationException.