Question

Map map = new HashMap();
System.out.println("hashCode:"+map.hashCode()); //hashcode==0 why?

map.put("test","test");
System.out.println("hashCode:"+map.hashCode()); //hashcode be okay here

How can I get hashCode after Map map = new HashMap(); ? (like: new Object().hashCode())

Answer 1

The hashCode for maps is defined as the sum of all hashcodes of the keys and values.

More precisely, Map.hashCode() is specified as:

Returns the hash code value for this map. The hash code of a map is defined to
be the sum of the hash codes of each entry in the map’s entrySet() view.
This ensures that m1.equals(m2) implies that m1.hashCode()==m2.hashCode() for any
two maps m1 and m2, as required by the general contract of Object.hashCode().

And Map.Entry.hashCode() is defined as

Returns the hash code value for this map entry. The hash code of a map entry e is defined to be:

(e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
(e.getValue()==null ? 0 : e.getValue().hashCode())

This ensures that e1.equals(e2) implies that e1.hashCode()==e2.hashCode() for any two
Entries e1 and e2, as required by the general contract of Object.hashCode.

Your new map has no entries yet, thus the sum is 0. Everything is at is should be.

If you want a number like the unoverridden Object.hashCode() would return, use System.identityHashCode(map). This is a (more or less unique) number for each map, which will not change when the contents of the map changes.