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From: shang@corp.mot.com (David L. Shang)
Subject: Re: Eiffel and Java
Date: 1996/10/31
Message-ID: <1996Oct31.162218.8386@schbbs.mot.com>#1/1
X-Deja-AN: 193626967
sender: news@schbbs.mot.com (SCHBBS News Account)
references: <55562c$nkd@mulga.cs.mu.OZ.AU>
organization: MOTOROLA 
reply-to: shang@corp.mot.com
newsgroups: 
 comp.lang.eiffel,comp.lang.ada,comp.lang.sather,comp.lang.java.advocacy,comp.object
Date: 1996-10-31T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <55562c$nkd@mulga.cs.mu.OZ.AU> 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.

But Sather's superclass construction is limited to non-parametrerized
classes only.

Transframe provides a more powerful superclass construction that 
can support parameterized classes.

Suppose we have two existing classes

	class Stack #(MemberType: type of any)
	{
		function push(MemberType);
		function pop:MemberType;
	}
	class Queue #(MemberType: type of any)
	{
		function push(MemberType);
		function pop:MemberType;
	}

and later, we decide to have a superclass from them, we can have

	class StackOrQueue #(MemberType: type of any)
			is super Stack, Queue
	{
		function push(MemberType);
		function pop:MemberType;
	}

without modifying the existing subclasses.

Now, we can write polymophic code like:

	x: StackOrQueue;
	if (some_condition) x = Stack#(T1)(); else x:=Queue#(T2)();
	y: any = GetObject();
	assume (y is x#.MemberType) x.push(y); otherwise do_nothing();

We can do the push without knowing whether "x" is a queue or
a stack; we don't know the exact member type either,
the type assurance statement will guarantee that the program
free of run-time type exceptions.

David Shang