Re: Factory methods

From:

Tom Hawtin <usenet@tackline.plus.com>

Newsgroups:

comp.lang.java.help

Date:

Sun, 13 May 2007 16:26:27 +0100

Message-ID:

<46472d86$0$8735$ed2619ec@ptn-nntp-reader02.plus.net>

Jednostanicni organizam wrote:

class Box<E extends A> that can contain any element of type B, X, ...

class Box has method getABoxOfSomeNiceA() { .. } so that method would have
to ask B or X what is the nice B or X. And it would do so by calling the
B.someFactoryMethodToImplement().

I'm having some trouble following what you are attempting to do.

It seems like you want an abstract factory.

So we have your base product interface and implementations:

public interface Base {
     ...
}
public class Derived1 implements Base {
     ...
}
public class Derived2 implements Base {
     ...
}
....

You then need a factory interface for the base product type, and
implementations for each implementation:

public interface BaseFactory<E extends Base> {
     E create();
}
public final class Derived1Factory extends BaseFactory<Derived1> {
     public static final BaseFactory<Derived1> INSTANCE =
         new Derived1Factory();
     private Derived1Factory() {
     }
     public Derived1 create() {
         return new Derived1();
     }
}
....

Box is then constructed with a particular factory with which it can make
elements with:

public class Box<E extends A> {
     private final BaseFactory<E> factory;
     private final List<E> contents = new java.util.ArrayList<E>();
     public Box(BaseFactory<E> factory) {
         this.factory = factory;
     }
     public void grow() {
         contents.add(factory.create());
     }
     ...
}

Tom Hawtin