Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!feeder.erje.net!eu.feeder.erje.net!newsreader4.netcologne.de!news.netcologne.de!.POSTED!not-for-mail
From: Patrick Roemer <sangamon@netcologne.de>
Newsgroups: comp.lang.java.programmer
Subject: Re: can this be done with generics?
Date: Mon, 25 Nov 2013 12:59:29 +0100
Organization: news.netcologne.de
Lines: 52
Distribution: world
Message-ID: <l6ve31$4r8$1@newsreader4.netcologne.de>
References: <slrnl9580s.42s.avl@gamma.logic.tuwien.ac.at>
NNTP-Posting-Host: xdsl-78-35-78-104.netcologne.de
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
X-Trace: newsreader4.netcologne.de 1385380769 4968 78.35.78.104 (25 Nov 2013 11:59:29 GMT)
X-Complaints-To: abuse@netcologne.de
NNTP-Posting-Date: Mon, 25 Nov 2013 11:59:29 +0000 (UTC)
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.24) Gecko/20100411 Thunderbird/2.0.0.24 Mnenhy/0.7.6.0
In-Reply-To: <slrnl9580s.42s.avl@gamma.logic.tuwien.ac.at>
Xref: csiph.com comp.lang.java.programmer:26608

Responding to Andreas Leitgeb:

>  class Foo {
>     public Foo foo1() {
>        /* do something very interesting ... */
>        return this;
>     }
>     // assume there were like 100 such foo#() methods here
>     //  (in practice they of course have more diverse names)
>  }
>  class Bar extends Foo {
>     public Bar bar() {
>        /* do something very interesting */
>        return this;
>     }
>  }
[...]
>   // but if I start with a Bar, and eventually after some foo#() calls
>   // want to call method bar() which is not in Foo, then I'm out of luck:
>   new Bar().foo1().foo42().foo84().bar(); // doesn't work, of course!
> 
> Is there a way to use *generics* for the methods of class Foo such that
> each foo#() returns the *static* type on which the compiler saw it
> applied?

Just a naive attempt... Not sure if this matches your use case and
requirements - this will only work for a one-level hierarchy with an
abstract root class and will probably require some ugly gymnastics
inside the Foo method implementations:

abstract class Foo<T extends Foo<T>> {
  protected abstract T self();
		
  public T foo1() {
    return self();
  }
}

class Bar extends Foo<Bar> {
  public Bar bar() {
    return this;
  }

  @Override
  protected Bar self() {
    return this;
  }
}

Best regards,
Patrick