Re: Using abstract class that implements interface

From:

Patricia Shanahan <pats@acm.org>

Newsgroups:

comp.lang.java.programmer

Date:

Sat, 18 Apr 2009 11:37:47 -0700

Message-ID:

<K-ednfOHmLpjgHfUnZ2dnUVZ_oydnZ2d@earthlink.com>

Zuisman Moshe wrote:

Hi dear All...
It is a kind of philosophical question (since finally - it works bot
ways)... But - since I am quite new in JAVA (I comes from PERL and
shell programming) - it is interesting for me...
So - story is this:
I have family of classes - that share same interface (methods - that
they declare to "external world" as public) - "foo_interface"... They
also have large part of internal implementation - shared by all of
them ( attributes and protected methods)... So - I defined also
foo_interface and foo_abstract_class - that implements it... All
"real" classes - inherit from this foo_abstract_class... When I keep
references in arrays of objects , and serialize/desirealize them - I
keep them as instances of foo_interface... For me it looks logical...
Abstract class keep common part of internal implementation, but for
"external world" they are all instances of foo_interface... But - my
colleges - that come from JAVA programming - says - it is redundant
and interface is unnecessary... Is my implementation really something
fishy , or - it "does not break" rules og "JAVA world"???

The combination of interface and abstract class is very useful, for
exactly the reason you state. If your colleagues need examples of this,
point them to the java.util collections. List is an interface.
AbstractList is an abstract class that provides default implementations
for most of the List methods. The concrete class ArrayList is declared as:

public class ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable

I would reference AbstractList directly if, and only if, I were writing
a new List implementation. Uses of List implementations should use type
List, not AbstractList.

Patricia