Java HashMap和对象键

Question

Example

LinkedHashMap<Long, String> myHashMap = new LinkedHashMap<>();
myHashMap.put(new Long(1), "A Value");

Questions

1. Is the key a reference or a copy?
2. If I write String aValue = myHashMap.get(new Long(1)); , will I get "A Value" back? Or have I just queried for a different object (reference) and therefore I'll get an error?

Answer 1

1. The map stores aa copy of the reference to the object passed as argument. No copy of objects is made.
2. Yes, you will get "A Value" back, as documented. A Map compares its keys with equals(), not == (except for IdentityHashMap). You could test that pretty easily, BTW.

Answer 2

1. The key is a reference to the same instance.

2. You will get "A Value" back, because Long has overridden

   • equals() ( return value == obj.longValue() ),
   • hashCode() ( return Long.hashCode(value) ).