Liskov substitution principle
里氏替换原则（Liskov Substitution principle）是对子类型的特别定义.
派生类（子类）对象能够替换其基类（超类）对象被使用.
A typical example that violates LSP is a Square class that derives from a Rectangle class, assuming getter and setter methods exist for both width and height.
setter violate Liskov principle in some context, because the dimensions of a Square cannot (or rather should not) be modified independently
getter no violation of LSP could occur.
