Re: Eiffel and Java

[fjh@murlibobo.cs.mu.OZ.AU (Fergus Henderson)]

jsa@alexandria (Jon S Anthony) writes:

]shang@corp.mot.com (David L. Shang) writes:
]
]> fjh@mundook.cs.mu.OZ.AU (Fergus Henderson) writes:
]> > Suppose I have two existing library classes (perhaps supplied by different
]> > vendors) which have some commonality, but don't inherit from a common
]> > base class.  In Sather, one can simply create a new interface and
]> > declare these classes to be instances of this interface, without
]> > modifying the existing code.
]> > 
]> > (Is that possible in Java?)
]> 
]> No. Java supports only top-down class hierarchy construction
]> (from superclass to subclasses), but not bottom-up: from
]> subclasses to superclass.

Why not?  I would have thought that since Java has separate interface
inheritence already, it would not be difficult to provide this feature.

]But in general it is _trivial_ to do in Ada.

I'm skeptical.

]with Stacks;  -- Generic
]with Queues;  -- Generic
]generic
]    type Any_Obj is tagged private;
]package Stack_Or_Queue
]
]    package S is new Stacks(Any_Obj);
]    package Q is new Queues(Any_Obj);
]
]    type Any is access all Any_Obj'Class;
]    type Stack_Queue ( S : access S.Stack; Q : access Q.Queue ) is private;
]...
]procedure Push (Element : Any;  Onto : Stack_Queue);
]function  Pop  (SQ : Stack_Queue) return Any;
]...
]private
]    type Stack_Queue ...
]...
]end Stack_Or_Queue;

Can I use this stack_or_queue interface to access different
implementations of the stack type (e.g. using arrays or using
linked lists?)  How about priority_queues?
With the Sather version, anything that implements push and pop can be
declared to implement the stack_or_queue interface.  It looks like
your stack_or_queue interface has exactly two implementations and
can't be extended.  (And that's despite being much more verbose
and complicated than the Sather version.)

--
Fergus Henderson <fjh@cs.mu.oz.au>   |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>   |  of excellence is a lethal habit"
PGP: finger fjh@128.250.37.3         |     -- the last words of T. S. Garp.