Re: why is multiple inheritance not implemented in java?

From:

Lasse Reichstein Nielsen <lrn@hotpop.com>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 17 Jan 2008 18:15:10 +0100

Message-ID:

<sl0wnzkx.fsf@hotpop.com>

Eric Sosman <esosman@ieee-dot-org.invalid> writes:

     interface Ownable {
        /** Return true iff the Ownable belongs to me. */
        boolean mine();
    }

    interface Diggable {
        /** Send diggers into the mine to dig up the Diggable. */
        void mine();
    }

    class Diamond implements Ownable, Diggable {
        // ???
    }

(I'm not a language theorist and my understanding of the "diamond
problem" may well be imperfect, but to my inexpert eye this looks
a lot like it. Or maybe like some related problem?)

It is *a* diamond problem (cute too!), but not the diamond problem.

Try instead

abstract class AA {
  public int foo();
}
class AB extends AA {
  public int foo() { return 42; }
}
class BA extends AA {
  public int foo() { return 37; }
}
class BB extends AB,BA /*multiple inheritance, if it existsed */ {
}
{
  AA a = new BB();
  System.out.println(a.foo());
}

The class inheritance diagram forms a diamond, with the BB class
inheriting two implementations of the *same* method.

/L
--
Lasse Reichstein Nielsen - lrn@hotpop.com
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