why hashcode same for String not for Class Object?

[Raza Mohd]

hi,

here I am having two String Obect . "s1" is not pointing to the address that "s2" is refering to and vice versa.
I know that hashcode gives the memory address of the object.
then why it gives same hashcode for object "s1" and "s2" and not for class(ArrayMap) object ?
please let me know the real difference.

following is code snippet.

 
public class ArrayMap
{
public static void main(String[] args)
{
String s1 = new String("raza");
String s2 = new String("raza");
 System.out.println(s1.hashCode());
  System.out.println(s2.hashCode());
System.out.println(new ArrayMap().hashCode());
 System.out.println(new ArrayMap().hashCode());
}
}

thanks
Raza!

[Matthew Brown]

The default hashCode() method gives the memory address. But plenty of classes override hashCode(). In particular, any class that overrides equals() should also override hashCode() to be consistent (if two objects are 'equal', they should have the same hash code, otherwise it will break the algorithms used in classes like HashMap).

String is a good example of this. String has overridden equals() so that two String objects are equal if they have the same string value. So it also overrides hashCode() as well.

Look at the documentation of hashCode() in the Object class for full details about the contract hashCode() ought to follow.

[Raza Mohd]

so why hashcode in String Class is overridden in such a way ..
that it gives same integer value for two different objects.

or I think.
if objects are equal (depending upon the way the equals is overridden in a particular class here , String)
then hashCode will be implemented as it should give the same integer value.

am i right?

[Matthew Brown]

That's right. The documentation says:

If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.

So if you ever override equals, you should override hashCode as well.