Re: Casting an object in equals Java 5

Andreas Leitgeb wrote:


     One of the few places where .getClass() seems essential is in
implementing .equals() for non-final classes:

    class Super {
        public boolean equals(Object obj) {
            if (! (obj instanceof Super)) // broken!
                return false;
            ...
        }
    }

    class Sub extends Super {
        public boolean equals(Object obj) {
            if (! (obj instanceof Sub)) // broken!
                return false;
            ...
        }
    }

The problem is that since a Sub object passes the instanceof test
in Super's .equals() but a Super object fails the test in Sub, you
can get aSuper.equals(aSub) == true but aSub.equals(aSuper) == false.

     The only fix I know of is to use .getClass() instead of relying
on instanceof:

    class Super {
        public boolean equals(Object obj) {
            if (obj.getClass() != Super.class)
                return false;
            ...
        }
    }

Now a Sub instance will fail the test and will not be considered
equal to any Super (which is what one almost always wants).

     Does anyone know of a smoother way to do this?

--
Eric.Sosman@sun.com