Is it a bad idea if equals(null) throws NullPointerException instead?

Question

The contract of equals with regards to null, is as follows:

For any non-null reference value x, x.equals(null) sho

Answer 1

This is a tricky question. For backward compatability you can't do so.

Imagine the following scenario

void m (Object o) {
 if (one.equals (o)) {}
 else if (two.equals (o)) {}
 else {}
}

Now with equals returning false else clause will get executed, but not when throwing an exception.

Also null is not really equal to say "2" so it makes perfect sense to return false. Then it is probably better to insist null.equals("b") to return also false :))

But this requirement does make a strange and non symmetric equals relation.