Re: Using abstract class that implements interface

From:

Tom Anderson <twic@urchin.earth.li>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 20 Apr 2009 13:03:53 +0100

Message-ID:

<alpine.DEB.1.10.0904201300370.20715@urchin.earth.li>

On Sun, 19 Apr 2009, Mike Schilling wrote:

Tom Anderson wrote:

On Sun, 19 Apr 2009, Mark Space wrote:

cbossens73@yahoo.fr wrote:

One of them being that it can make testing *very* difficult.

This.

It is true that an OO interface can be defined in Java both by
using a Java interface of a Java "class" (either abstract or not),
but the latter is often poor design...

Abstract base classes are good for providing a common implementation,
which is what the OP has done. But I agree that adding an interface
which also expresses the design is a no-brainer.

At minimum, what happens if you have only a abstract class, and want
to add your API to an existing class, or one that must extend another?

Then you refactor to separate an interface from the base class, which
is trivial to do, with or without a refactoring browser. Why on earth
would you do it before you need to?

If you're writing a 10-class application where you write all the code
yourself, no reason at all. If you're creating a framework that you're
going to distribute widely, create the interface now because changing
all the APIs in version 2 is impolite.

Another good reason to use an interface - if you're exposing the interface
to the world, where you can't refactor your users, then an interface gives
you some flexibility. But agaim, if you aren't, there's no need for it.

(Am i right in thinking that if i expose an abstract class Foo to the
world, i can't subseqeuently make Foo an interface and keep binary
compatibility? Since all those invokevirtual bytcodes would need to become
invokespecial, i assume not.)

tom

--
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