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Interfaces are NOT MI.



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Interfaces are NOT MI.

Posted by Ralph M. Prescott on 17 Nov 1998, 10:54 AM

Sorry Mike,

I have to disagree with some of your points.

> Clearly, an interface is not a class, otherwise it wouldn't have been given its own special construct, right? Wrong!
> Therein lies the problem: A Java interface IS a class - a purely abstract class.
A class is a class too. It's also an Object. Does that make it NOT a class? This is flawed logic.

> When we use interfaces we are using nothing other than multiple inheritance itself.
Yes all behavior and zero implementation.
To paraphrase Bill: it's the separation of concerns between "what" is done (the interface) and "how" it is done (the implementation).

> Oooo, but I thought multiple inheritance was sooo BAD! Not so. If it were, only BAD programmers would use Java interfaces, and clearly this is not the case.
> MI has, in fact, just one problem: A bad reputation, which stems almost solely from C++�s appalling support for MI.
That and its common misuse by blurring implementation and interface.
C++ supports interfaces. They're called pure virtual base classes.
They aren't often used when designing except by OO purists.

> Java interface advantages:
> - Provides (limited) support for MI
It limits MI of implementation not MI of behavior.

> - Simplifies compiler and runtime design (an advantage for James Gosling, infact, but not for me)
> (Can anyone think of any more?)
Forces you to reduces unneeded class coupling by separating concerns. Oh bother.

> Java interface disadvantages:
> - Two language constructs required for what amounts to same thing
They aren't the same thing.

> - Inconsistencies across language constructs (e.g. method modifiers in class vs. in interface)
Yes, the goal of minimal keywords can be a drag.

> - Does not map intuitively to domain object models and visa versa (lack of seamlessness)
> - Only solves naming conflict from a compiler perspective - not from a modelling perspective
I disagree strongly here as well. You simply have to switch to
modeling things by the "roles" they fill -> contracts they keep -> interfaces they implement
(it's all the same thing) separatly from their implementation.
Check out Peter Coad's recent work. He explains this better than I can in
this short space. Also consider why the UML provide a very concise
shorthand for interfaces -- the lollypop. It's not JUST for COM anymore ;-)

> Unlike some other notable kludges of the Java language (primitive types, fixed package names), Java interfaces actually work surprisingly well in practice. Nevertheless, I could easily envisage seeing negative consequences of interfaces at a later stage of Java�s evolution.
fixed package names? What's the problem here?
Well, I could see the need to play games with dynamic interfaces.
Roles that come and go over time. But you could roll your own mechanism for that.

[DBC stuff snipped]
Won't touch that one.

> Anyone feel differently?
Anyone else?



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