From: Olivier Renaud <o.renaud@...>
Date: 2007-08-19T11:17:07+09:00
Subject: Re: Ruby's equivalent to Java's "interface"

> > Steve Quezadas <steve@tripperjones.com> wrote:
> >   Please forgive this rather naive question, but it's a query that's
> > difficult to search for on google. In java, you can program to
> > interface. IE, in java, you can do:
> >
> > public interface QuakeBehavior {
> > public void quack();
> > }
> >
> > public class Quack implements QuckBehavior {
> > public void quack() {
> > System.out.println("Quack");
> > }
> > }
> >
> > public class Squeek implements QuackBehavior {
> > public void quack() {
> > System.out.println("Squeek");
> > }
> > }
> >
> > public class MuteQuack implements QuackBehavior {
> > public void quack() {
> > System.out.println("<< Silence >>");
> > }
> > }
> >
> > I am learning about Design Patterns (which is a wonderful concept), but
> > I want to know the Ruby equivalent.
> >
> > - steve

Le samedi 18 ao�t 2007 23:59, Deepak Vohra a �crit :
> Steve,
>
>   The Ruby equivalent of Java's interface is module.
>
>   Deepak
>

I disagree : the equivalent of java's interface is implicit interface. That 
is, two objects implement an interface if they can respond to a common set of 
methods. There is no need to explicitly declare, like in java with 
the "interface" construction, this set of methods. This is possible because 
of the dynamic typing of Ruby. You can search the keywords "duck typing", for 
more details.

It has the advantage of being more concise, faster to write, and avoids 
repetitions of method declarations . The disadvantage is that there is not 
dedicated places to formally define the interfaces.

Modules are constructs that allow to define a bunch of methods to be "copied" 
inside a class. Of course, every classes that include the same module share 
the same interface, since they copied the same methods. If you want to 
compare to java, modules are more similar to java's abstract classes. The 
difference is that java allows a class to inherit from only one abstract 
class, when a ruby class can include many modules.

If you really want to have a place to explicitly define an interface, you can 
declare fake methods in a module :

module QuakeBehavior
  def quack
    raise NotImplementedError.new
  end
end

class Squeek
  include QuakeBehavior
  def quack
    puts 'squeek'
  end
end

Note that the method 'quack' from class 'Squeek' replaces the definition of 
the method 'quack' that was first included from 'QuakeBehavior'.
If you use this technique, you will be warned by a NotImplementedError (so, at 
runtime), if you forgot to define a method in classes that include 
QuakeBehavior. I don't think it is a good design, but maybe it can help.

    
-- 
Olivier Renaud