Question

I have a very large Set of Immutable Objects so I am thinking of assigning a unique hashcode to them at construction time.

private static int counter = Integer.MIN_VALUE;

private final double foo;
private final double bar;
private final int hashCode;

public MyImmutableObject(double foo, double bar)
{
    counter++;
    this.foo = foo;
    this.bar = bar;
    this.hashCode = counter;
}

@Override
public int hashCode()
{
    return this.hashCode;
}

   /**
    * Unneeded override of equals since its the same as in
    * Object, but shown for demonstration purposes. 
    */
@Override
public boolean equals(final Object obj)
{
    return this == obj;
}

This way, I can achieve the highest possible dispersity of keys, since I will have 2³² unique keys. Of course, this also means that no two objects of this type will ever be equal, since an object will only be equal to itself.

edit: The Objects are also used as keys in Maps.

Is this possible to do? Are there hidden traps I missed?

Answer 1

Is this possible to do? Are there hidden traps I missed?

Thread safety, for one thing? You could use AtomicInteger to get round that, of course.

Then there’s also the pointlessness of it, really. The built-in hash code is going to be pretty close to unique without you doing any extra work (and taking an extra 4 bytes per object). Given that you’re not changing the meaning of equality (it’s still basically reference equality) I can’t see that there’s any good reason to do this.