Re: Chained call pattern with inheritance, polymorphism and generics...

From:
Piotr Kobzda <pikob@gazeta.pl>
Newsgroups:
comp.lang.java.programmer
Date:
Sun, 30 Sep 2007 15:16:36 +0200
Message-ID:
<fdo7jk$qt6$1@inews.gazeta.pl>
Daniel Pitts wrote:

The point is, I shouldn't have to delegate to super just to return the
same reference as a different type.

While that IS currently the case, I wish it werent.


As already mentioned, you can still achieve that today.

Having a base builder declared as follows:

public abstract class BaseBuilder<T extends BaseBuilder<T>> {

     protected abstract T nextBuilder();

     public T something(String s) {
         return nextBuilder();
     }
}

One way to achieve your goal would be:

public abstract class SpecificBuilder<T extends SpecificBuilder<T>>
extends BaseBuilder<T> {

     public static final class Instance extends SpecificBuilder<Instance> {

         protected Instance nextBuilder() {
             return this;
         }
     }

     public T other() {
         return nextBuilder();
     }
}

public abstract class MoreSpecificBuilder<T extends
MoreSpecificBuilder<T>> extends SpecificBuilder<T> {

     public static final class Instance extends
MoreSpecificBuilder<Instance> {

         protected Instance nextBuilder() {
             return this;
         }
     }

     public T doMore() {
         return nextBuilder();
     }
}

With the following usage:

new SpecificBuilder.Instance().something("test").other();
new MoreSpecificBuilder.Instance().something("test").other().doMore();

Or use another way (the same as above, but more "classical" approach),
which exactly matches your usage scheme:

public abstract class SpecificBuilderBase<T extends
SpecificBuilderBase<T>> extends BaseBuilder<T> {

     public T other() {
         return thisBuilder();
     }
}

public final class SpecificBuilder extends
SpecificBuilderBase<SpecificBuilder> {

     protected SpecificBuilder thisBuilder() {
         return this;
     }
}

public abstract class MoreSpecificBuilderBase<T extends
MoreSpecificBuilderBase<T>> extends SpecificBuilderBase<T> {

     public T doMore() {
         return thisBuilder();
     }
}

public final class MoreSpecificBuilder extends
MoreSpecificBuilderBase<MoreSpecificBuilder> {

     protected MoreSpecificBuilder thisBuilder() {
         return this;
     }
}

Everything in that "pattern" depends on your needs. When you need
extendability of your concrete builder, you do provide an abstract
specialization with a default final realization as an option, otherwise
you just implement it as final class only (still preserving some
extendability of it in a future via delegation to super).

HTH,
piotr

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