How do I select a class dynamically?

Question

I have two classes that extends a third class, i.e.

public class class_a extends parent_class

and

public class class_b extends parent_class

My question is it possible to have a third class to create a reference to a class based on condition? i.e.

public void test() {
  parent_class b;
  if (cond)
    b = new class_a();
  else
    b = new class_b();
}

Is there a way to do that?

I don't want to create variables per type of class, I will only use one throughout the life time of this function.

Answer 1

That is exactly what the factory design pattern is for.

http://en.wikipedia.org/wiki/Factory_method_pattern

This might also be of use Factory Pattern. When to use factory methods?

Answer 2

Yes. Polymorphism allows you threat subclass as base class. So, you can write method with parent_class return value type, like so:

parent_class create(boolean condition)
{
   return condition ? new class_a() : new class_b();      
}

As @John answered, it is called Factory method.

P.S. In Java, you better should name classes using CamelCase, like ClassA and ParentClass. Code style.