Re: When would you use abstract classes over interfaces

Lew <com.lewscanon@lew>
Mon, 28 Jul 2008 21:56:49 -0400
sasha wrote:
(Repeating the question from the subject:

When would you use abstract classes over interfaces[?]

I am talking about in practice, when writing production code?

Umm, yea-ah - this /is/ clj.programmer. It's all about real code here (except
for the arguments about closures).

I still think the only good point about abstract classes is
ability to add some implementation.

Isn't that enough?

The key to the power of abstract classes is in their limitations:

An abstract class

* must be inherited to be used.
   ** Beware: inheritance is dangerous.

* forms the single root of a hierarchy,
   because Java doesn't have multiple class inheritance.

* defines a concrete type.

* must have a constructor.

The permissions that augment those features:

An abstract class

* may have implementation.

* may declare all kinds of members - methods, fields, interfaces, classes -
   static or instance, with arbitrary accessibility.

The combination leads one to use abstract classes where the implementation is
what is common, and the overridden methods will be used in very specific ways
for that implementation.

'assert' is your friend when designing abstract classes.


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