How to compute the hashCode() from the object's address?

Question

In Java, I have a subclass Vertex of the Java3D class Point3f. Now Point3f computes equals() based on the values of its coordinates, but for my Vertex class I want to be stricter: two vertices are only equal if they are the same object. So far, so good:

class Vertex extends Point3f {

    // ...

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

I know this violates the contract of equals(), but since I'll only compare vertices to other vertices this is not a problem.

Now, to be able to put vertices into a HashMap, the hashCode() method must return results consistent with equals(). It currently does that, but probably bases its return value on the fields of the Point3f, and therefore will give hash collisions for different Vertex objects with the same coordinates.

Therefore I would like to base the hashCode() on the object's address, instead of computing it from the Vertex's fields. I know that the Object class does this, but I cannot call its hashCode() method because Point3f overrides it.

So, actually my question is twofold:

• Should I even want such a shallow equals()?
• If yes, then, how do I get the object's address to compute the hash code from?

Edit: I just thought of something... I could generate a random int value on object creation, and use that for the hash code. Is that a good idea? Why (not)?

Answer 1

Either use System.identityHashCode() or use an IdentityHashMap.

Answer 2

System.identityHashCode() returns the same hash code for the given object as would be returned by the default method hashCode(), whether or not the given object's class overrides hashCode().