How do i use the same factory method for a subclass?

I need a object B, but i get a object A when i execute 'B.GetByID()'

public class A 
{
    public A()
    {

    }

    public static A GetSelf()
    {
        return new A();
    }

    public static A GetByID()
    {
        return GetSelf();
    }
}


public class B extends A
{
    public B()
    {
        super();
    }
开发者_开发技巧

    public static B GetSelf()
    {
        return new B();
    }
}


B.GetByID(); //returns A, i need B

You can only do that by also creating a B GetByID() method in B. That's somewhat ugly though...

Basically your B.GetByID() call will be resolved to A.GetByID(); nothing in the compiled code will indicate that it was originally B.GetByID(), and the call to GetSelf() within GetByID() will be resolved to A.GetSelf() anyway.

Basically, static methods don't allow for polymorphism in the way you want. I suggest you create an AFactory and a BFactory subclass, and use method overriding in the normal way, with instance methods.

You could add a GetByID method to B, like so:

public class B ... {

    public static B GetByID()
    {
        return GetSelf();
    }

}

Your factory method (no matter in which class it's declared) needs to be aware of both, A and B and then make a decision on which class to instantiate. If the caller knows it needs a B, it just can do a new B() anyway, so the factory method has no value there anyway.