Re: Using abstract class that implements interface

From:

Lew <noone@lewscanon.com>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 20 Apr 2009 10:03:40 -0400

Message-ID:

<gshvbt$1m4$1@news.albasani.net>

Mike Schilling wrote:

Tom Anderson wrote:

(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.)

Simpler than that: a class C which extends Foo will fail to load if
Foo is an interface.

The salient point is that when one writes an API, which comprises every class
a programmer writes except the top-layer invocation class, one has to plan
ahead for the maintenance needs of that API. Once released into the wild, the
contract of the API is a commitment and further changes have to be made
responsibly. The decision to implement a type as an interface or a class will
have to be honored through the useful life of that type, or the cost of a
change must be incurred. By exposing the type as an interface, one keeps
certain options open.

As usual, and as pointed out by many in this forum, that doesn't mean *always*
select interfaces as the exposed type or *never* do so with abstract classes.
It does mean understand the tradeoffs, and that generally interfaces are the
safer, cleaner choice.

--
Lew