Re: Eiffel and Java

["Marcos F. F. de Macedo" <marcosm@sede.unijui.tche.br>]

Alexander Asteroth wrote:
> 
> Marcos F. F. de Macedo wrote:

> > This kind of interface/implementation in Sather is diferent of the usual
> > concept. This is subtype/subclass independence. That is a good thing in
> > Sather that I have not yet thinked of the exact consequences.
> >
> > Interfaces in Java have nothing to do with this concepts. Interface in
> > Java is an implementation restriction (not a conceptual one) to avoid
> > multiple-inheritance and work yet work with static typing. Interface is
> > java is just a deferred class with just deferred routines. Nothing more,
> > nothing less.
> 
> I don't see your point. What is missing in Java's interfaces.
> 
> If it's that you don't have to touch existing code (as F.H. wrote)
> that has to do with supertyping, not with the abstract classes concept
> of Sather (Or am I completetly wrong here?).
> 
> Can you give an example of what you mean and what you think
> is wrong about Java's interfaces?
> 
Of course I can. Let's make myself clear. When I talk about abstract
classes I talk about the general concept that Yes common to most object
oriented classes (pure virtual in C++, defferred in Eiffel, abstract
class in Smalltalk, virtual in Simula,etc). An abstract class is a
classe that has one or more routines that dont have implementations but
just interface. Such class can not have instances because its behavior
is not defined. Its used to model general class for example. I can have
an abstract class ANIMAL with sub-classes Dog, Cat, Bird. No object will
be direct an animal but one of Dog, Cat or Bird. The abstract serves as
the general model an in a Framework its the root of the main
hierarquies. In static typed languages the abstract class is used to
defined a minimal behavior of subclasses.
Its common to abstract classes to not have all methods abstract but do
define other that class abstract methods over self to give an special
behavior. Such methods are called template and are in the soul of OOP.

The Java interface are just an abstract class with all methods abstract
(no implementation). Its is used because its hard to implement multiple
inheritance when there are attributes. So you can inherit from one class
but multiple inheritance. In languages such as Sather, Eiffel, Beta,
etc... You can made abstract classes with just abstract methods and you
will have the same behavior. But templates can only be done if some
methods are not abstract. The good thing in Java is that you can inherit
one parent (single inheritance) but can inherit multiple interface
(multiple subtyping). In Eiffel you have (multiple-inheritance and
multiple sub-typing).

The type hierarquie in Sather is diferent in that you can put an
interface in a class that was not declared to "implements" it. That can
be a good thing to correct bad design but can be a good thing to make
bad design too. You will have to work with two diferent hierarquies. Its
that necessary? 
(if you want to know how this diferent implementations are done send me
an e-mail).

I hope thar this was not too long but too clear :-)


-- 
Marcos F. F. de Macedo
marcosm@sede.unijui.tche.br
UNIJUI - BRAZIL