说明：  工厂方法模式是类的创建模式，又叫做虚拟构造子模式或多态性工厂模式。它的用意是定义一个创建产品对象的工厂接口，将实际创建工作推迟到子类中。
(factory method model is the creation of Class model, also called virtual-tectonic model or polymorphism factory model. Its definition is intended to create a product targeted at the factory interface, the actual work to create deferred to the child class.)