HashMap.KeySet（）在Java 7中以不同顺序返回键Java 8

Question

public class TestClass {    

    public static void main(String[] args)
    {
        HashMap<String,Integer> testMap = new HashMap<String,Integer>();
        testMap.put("Key1", 1);
        testMap.put("Key2", 2);
        testMap.put("Key3", 3);
        testMap.put("Key4", 4);
        testMap.put("Key5", 5);
        //[Key2, Key1, Key4, Key3, Key5] //java7
        //[Key2, Key1, Key5, Key4, Key3] //java8
        System.out.println(testMap.keySet().toString());
    }

}

Why there is difference in the order of the keys?

Answer 1

why there is difference in the order of the keys ?

Because:

1. the Java specs (ie the javadocs) do not specify the order of a HashMap's keyset, and

2. there were major changes to the implementation of HashMap between Java 7 and Java 8.

Those implementation changes (which gave significant performance improvements) resulted in the order of the keyset changing.

But that is not considered a "breaking" change because the keyset order has always been clearly noted as unspecified .... meaning that you should not rely on it.

Answer 2

Ordering is not guaranteed as per spec. Individual VM's are free to implement whatever they choose.

Answer 3

Hashmap不会保持顺序，如果要顺序插入，则可以使用linkedhashmap :)

Answer 4

Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.