Re: Composition vs. inheritance

From:
=?ISO-8859-1?Q?Arne_Vajh=F8j?= <arne@vajhoej.dk>
Newsgroups:
comp.lang.java.programmer
Date:
Wed, 23 Apr 2008 20:55:11 -0400
Message-ID:
<480fda65$0$90264$14726298@news.sunsite.dk>
Tim Smith wrote:

In article <480e9e90$0$90264$14726298@news.sunsite.dk>,
 Arne VajhHj <arne@vajhoej.dk> wrote:

Note the word "favor".

It does not mean that extend is always bad. It mean that extend is
often bad.

If the class is intended to be extended then it is OK to extend it.

A very good indication that the class is intended to be extended is if
it is abstract.

:-)

So go ahead and extend abstract classes as you want.

Be very careful about extending non abstract classes. It is very
easy to make your class depend on some implementation detail
in the class it extends.

If there is a comment in the tops saying "extend this class to ...",
then it is probably OK to extend.

But if not then think twice.


That's decent advice for the case where one is dealing with someone
else's class, but the impression I got was that he's asking a more
general question, so that if he were designing both classes, he would
know when to design using inheritance and when to use composition.


Then it becomes a little bit more blurred. But I still think the
general ideas hold.

If you have an "is a" relation ship and the parent class can be
written as an abstract class clearly intended to be extended, then
that is a fine design.

If not default to composition.

Obviously writing the parent class yourself do give some
freedom to tweak it either way. But if it is designed
with a view on future usage, then the problem domain can
often direct what is the best fit.

Arne

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