hash code

[christie xu]

Could somebody tell me what the hashcode exactly is?
Where to use it?
Is it anything related with the key value?
if I put:
HashMap hm = new HashMap();
hm.put( "one", new Integer( 1 ) );
System.out.println( HM.get( "one" ).hashCode() );
it prints out 1. but if I change Integer object to Float, it prints out 1065353216. what is that?

[Marilyn de Queiroz]

I think you have a problem unrelated to hashing. When I change my Integer to Float, I do not have that problem.

The Object class has a hashCode() method that the HashMap uses, but you do not normally need to be concerned with it.

[Gerald Dettling]

That's funny I get the same number (1065353216) as well.

[Greg Harris]

hmmm, me too... strange.

[Marilyn de Queiroz]

Try this one:

<font size=+0>import java.util.HashMap ;
  
public class Test
{
    
    public static void main( String[] args )
    {
        HashMap floatTest = new HashMap();
        floatTest.put( "one", new Float( 1 ) );
        System.out.println( floatTest.get( "one" ) );
    }
    
}

For more info on the hashCode() method, check it out in the Object class.

It is used a lot internally by the java libraries, but developers don't need to concern themselves with the innards of this method very often. The HashMap class uses it to associate the specified value with the specified key, but it is all hidden (internal) stuff.

It is also used in referencing objects, so occasionally you will see a toString() method that does not override the toString() method of Object return a value like test@310d42 which is actually a hashCode of the actual reference address (more specifically, the unsigned hexadecimal representation of the hash code of the object).
[This message has been edited by Marilyn deQueiroz (edited July 24, 2001).]

[Greg Harris]

oops, i copied christie's code and did not pay attention to the .hashCode() method... take that part out, christie, and your program will print "1".

[christie xu]

thanks a lot, every one. I know what is going on now.