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From: Sebastian <nowhere@nirvana.org>
Newsgroups: comp.lang.java.programmer
Subject: Re: "implements" does not mean "implements"
Date: Sun, 13 Nov 2016 22:30:58 +0100
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Am 12.11.2016 um 14:50 schrieb Arne Vajhøj:
> On 11/12/2016 5:22 AM, Sebastian wrote:
>> I like to say that interfaces specify behaviors of objects, not
>> properties of classes. Saying that C implements A amounts to saying that
>> any o that is an instance of C behaves like an A. It has nothing to do
>> with what methods get implemented in C.
>
> Behavior of objects and properties of classes should have some
> relationship.
>
> But just realized something that I think matches well
> with you way of thinking.
>
> interface A { void m(); }
>
> abstract class C implements A { }
>
> class Foobar extends C { public void m() { } }
>
> actually compiles - it is not necessary to have an abstract m in C.
>
> Arne

yes, indeed, the methods are already abstract in C by virtue of the 
interface.

By the way, a typical use of such an idiom may be where C is some 
abstract class that offers some help in implementing A, or adds related 
behavior, so that C wouldn't really be an abstraction in its own right, 
but would exist only for purposes of code-reuse. In Java 8, such usage 
could be replaced with default methods in interfaces.

-- Sebastian