Abstract Factory
效果：
1. Isolates concrete class
2. Makes exchanging product families easy
3. Promotes consistency among products
4. Supporting new kinds of products is difficult
实现：
1. Factories as singletons.
2. Createing the product.The most common way to do this is to define a factory method for each product. If many product families are possible, the concrete factory can be implemented using Prototype pattern.
3. Defining extensible factories. Abstract factory usually defines a different operation for each kind of product it can produce. A more flexible but less safe design is to add a parameter to operations that create objects.
效果：
1. Isolates concrete class
2. Makes exchanging product families easy
3. Promotes consistency among products
4. Supporting new kinds of products is difficult
实现：
1. Factories as singletons.
2. Createing the product.The most common way to do this is to define a factory method for each product. If many product families are possible, the concrete factory can be implemented using Prototype pattern.
3. Defining extensible factories. Abstract factory usually defines a different operation for each kind of product it can produce. A more flexible but less safe design is to add a parameter to operations that create objects.
