Re: Casting an object in equals Java 5

From:
Eric Sosman <Eric.Sosman@sun.com>
Newsgroups:
comp.lang.java.programmer
Date:
Tue, 30 Sep 2008 11:51:10 -0400
Message-ID:
<1222789739.524006@news1nwk>
Andreas Leitgeb wrote:

[...]
PS: I think, that "normal" programs should never have a need
  for .getClass(). (Application servers are not "normal" and
  neither are class viewers in this context :-)


     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

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