简单工厂
问题描述
把不同类型的 pizza 的创建过程从 Pizza 类中抽出来封装成 SimplePizzaFactory 类，其他统一的操作放在 Pizza 类中
工厂方法
The Factory Method Pattern defines an interface for creating an object, but lets subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.
工厂方法的目的是允许一个类将实例化延迟到它的子类。
抽象工厂
The Abstract Factory Pattern provides an interface for creating families of related or dependent objects without specifying their concrete classes.
