工廠方法(Factory Method)

這個模式的定義:
定義一個用於創建物件的介面, 讓子類別決定要實例化哪一個類別. 工廠方法讓一個類別的實例化遞延到其子類別.
Define an interface for creating an object, but let subclasses decide wihich class to instantiate. Factory Method lets a class defer instantiation to subclasses.