abstract factory -- 创建相互关联的类

Function:
 Provide an interface for creating families of related or dependent objects without specifying their concrete classes.

Participants:

AbstractFactory  (ContinentFactory)

• declares an interface for operations that create abstract products

ConcreteFactory   (AfricaFactory, AmericaFactory)

• implements the operations to create concrete product objects

AbstractProduct   (Herbivore, Carnivore)

• declares an interface for a type of product object

Product  (Wildebeest, Lion, Bison, Wolf)

• defines a product object to be created by the corresponding concrete factory
• implements the AbstractProduct interface

Client  (AnimalWorld)

• uses interfaces declared by AbstractFactory and AbstractProduct classes