Abstract class initialisation

Question

Hi,
I have the following problem. What do you suggest?
I have an interface A, abstract class B, B's subclasses C and D, and mainClass
interface A
abstract class B implements A {
    B b;
    public B(String desc)
     if (desc.equals("C")) b=new C();
     else if (desc.equals("D")) b=new D();
class C extends {
  public C() {}
class D extends {
  public D() {}
class mainClass{
  B b=new B("C"); // but B can not be initiated
}B is an abstract class and can not be initiated, I can not define B as normal class, because in B, I define some abstract methods
and want all its subclasses to define them.
What do you suggest me for enhancement?

Answer 1

Hi duffymo,
Your answer quite intersting, can you provide some concrete example!
Do you mean like this:
interface A
class B implements A{
B b;
public B(String desc) {
b=Class.newInstance(desc);
class C extends B
class main {
B=new B(desc);
It seems quite okey, but the problem is that I want subclasses (C and D) to impelement some certain methods. Thus I though B as abstract class
It's called a Factory class or virtual constructor -
that's how you create different runtime types. It's a
GOF Pattern - read about it.
I wouldn't build all that stuff into the abstract B
constructor. That'll force you to modify the B class
every time you add a new subtype - that's not a good
idea. Move that code out into a separate class named
AFactory and give it a static method that does that
creation stuff for you. I know - "That just moves the
changes out of B and into the AFactory" - yes, but I
think that's where they belong.
The java.lang.Class class has a factory method built
into it already - newInstance(). You can use that to
instantiate different subclasses:
A newInstance = Class.newInstance("C");
A anotherInstance = Class.newInstance("D");That's the way I'd do it. - MOD