Java hashcode based on identity

Question

The default behavior of Object.hashCode() is to return essentially the "address" of the object so that a.hashCode() == b.hashCode() if and only if a == b. How can I get this behavior in a user-defined class if a superclass already defines hashCode()? For instance:

class A {
  public int hashCode() {
    return 0;
  }
}

class B extends A {
  public int hashCode() {
    // Now I want to return a unique hashcode for each object.
    // In pythonic terms, it'd look something like:
    return Object.hashCode(this);
  }
}

Ideas?

Answer 1

System.identityHashCode(Object) provides this behaviour.

You would write this:

class B extends A {
  public int hashCode() {
    return System.identityHashCode(this);
  }
}

Please check the equals-method, that it only returns true, if the two objects are the same. Otherwise it would break behaviour described for equals and hashCode. (To be correct, the equals-method has to return false, if you get different hashcodes for two objects.) To provide an implementation of equals() that comply with the given hashCode()-method:

public boolean equals(Object other){
   return this == other;
}

Answer 2

Use System.identityHashCode(). This is what IdentityHashMap uses.

You should be extremely wary of overriding an existing hashCode() with this though because you might break the hashCode contract, being that two objects that:

if a.equals(b) then a.hashCode() must equal b.hashCode()

You might break this by overriding the existing behaviour or you might need to override equals() too.