The Factory Method Pattern

The factory method pattern is used when a class needs to instantiate a derivation of another class, but does not know which one. The factory method allows a derived class to make this decision.

Product is the interface for the type of object that the factory creates. Creator is the interface that defines the Factory Method.  Clients will need to subclass Creator to make a particular ConcreteProduct.  The abstract method FactoryMethod() is implemented in the concrete class and it knows the rules to enable it to create and return the appropriate concrete product.