Re: Is this code "proper" use of extend?
Arne VajhQj wrote:
Lew wrote:
In his superb /Effective Java/, Joshua Bloch has a chapter (Item 14)
devoted to the principle that one should "Favor composition over
inheritance", with a detailed explanation as to why, and when the
choice of inheritance is indicated.
Yes, but his point does not apply for this case.
If I were to summarize his point with my words it would be: overriding
some methods but not all methods makes the new class dependent
of the implementation instead of just the interface of the old
class because methods may call each other.
But in this case no existing methods are overridden so there
are no problem.
Arne
If no existing method is overridden, then why have this class extend the
other class at all?
Why not do something like this:
public class SpecialThing {
final List list = new ArrayList();
public List getList() { return list; }
// Do special things here...
}
This doesn't couple your SpecialThing to a particular inheritance
structure, and clearly separates the List operations from the Special
operations.
If SpecialThing should indeed be a "List with more functionality", then
I really do prefer delegation over inheritance. Implement "List", and
delegate to the backing list.
In reality, this is similar to what happens in inheritance anyway, it
just hides what the "parent" class is from those who shouldn't care.
Hmm... Now that I've written this down, I need to spend some time to
consider when its appropriate to use composition vs inheritance... I
know there are really good reasons to use each for certain things, but I
usually use intuition to tell me which...
One feature that I wish Java had would be to "list" methods for
delegation. A special construct:
class Foo implements List {
List myList;
delegates<List.*>() {myList};
}
Or some way to do easy grouping/pattern matching for names. Kind of
like a compile-time proxy. It would help with AOP programming a bit.
--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>