Factory Method

Intent

Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.

Structure

• Product - defines the interface for objects created by the factory method.
• Concrete Product - implements the Product interface.
• Creator - declares the factory method and may provide a default implementation.
• Concrete Creator - implements or overrides the factory method to return a Concrete Product.

Notes

• The Factory Method pattern, like the Template Method pattern, is a pattern for using implementation inheritance. It can be viewed as a Template Method pattern in which one or more of the algorithm steps that is deferred to subclasses involves creating objects. However the pattern terminology is different. The factory method itself corresponds to either a Primitive Operation or a Hook Operation in the Template Method pattern.
• The Prototype design pattern can be used as an alternative to the Factory Method design pattern. The Creator can declare a protected variable of type Product and the Concrete Creator subclasses can initialize this variable with different prototype objects.
• If the Creator classes use families of created objects then the resulting pattern is the Abstract Factory pattern. An Abstract Factory can use either Factory Methods or Prototypes.