[1711] in java-interest
Re: Beta interface blues
daemon@ATHENA.MIT.EDU (Arthur van Hoff)
Thu Sep 14 22:45:29 1995
Date: Thu, 14 Sep 1995 16:02:33 -0800 (PDT)
From: Arthur van Hoff <Arthur.Vanhoff@Eng.Sun.COM>
Reply-To: Arthur van Hoff <Arthur.Vanhoff@Eng.Sun.COM>
To: "John D. Ramsdell" <ramsdell@linus.mitre.org>
Cc: java-interest@java.Eng.Sun.COM
Hi John,
> While updating a program to the Beta level syntax, I ran into a
> problem with interfaces. I've constructed a general package for
> manipulating sets of elements. The elements of the set have the
> requirement that they be ordered and testable for sameness. There is
> no requirement that elements inherit from a common object other than
> Object. In a version of sets for Alpha3, these requirements were
> defined using the following interface:
>
> ------------
> package set;
> public interface Element {
> boolean same(Element e);
> boolean less(Element e); // True when comparable and less than
> }
> ------------
>
> Since a set can be an element of a set, the class Set implements an
> Element.
> ------------
> package set;
> public final class Set implements Element {
> private Pair p; // Store elements in a list of pairs.
> ....
> public boolean same(Element e) {
> return e != null
> && e instanceof Set
> && pair_same(p, ((Set)e).p); // <- Cast in question
> }
>
> private boolean pair_same(Pair p0, Pair p1) { .. }
>
> public boolean less(Element e) { ... }
> }
> -----------
>
> The beta Java compiler rejects this code claiming it is illegal to
> cast an element to a set. One solution is to define Element as an
> abstract class and require that all elements inherit from this class.
> However, I would like to allow sets of AppletElements, which inherit
> from Applet, simply by extending the Applet class with same and less
> methods. My reading of The Java Language Specification suggests that
> the cast should be allowed. Is this a bug? If not, how does one
> construct a general set package?
This is a bug. I'll make sure it get fixed. Thanks for pointing it out.
Have fun,
Arthur vanHoff
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