单一职责原则 Single Responsibility Principle
There should never be more than one reason for a class to change.
应该有且仅有一个原因引起类的变化。
里氏替换原则 Liskov Substitution Principle
Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it.
只要父类能够出现的地方子类都能够出现，而且替换为子类也不会产生任何异常或错误，使用者根本就不需要知道是父类还是子类。但是，反过来就不戏行了。子类出现的地方，父类就不一定能够适应。
依赖倒置原则 Dependence Inversion Principle
High level modules should not depend upon low level modules. Both should depend upon abstractions. Abstractions should not depends upon details. Details should depend upon abstractions.
高层模块不应该依赖底层模块，两者都应该依赖其抽象；抽象不应该依赖细节，细节应该依赖抽象。
接口分离原则 Interface Segregation Principle
Client should not be forced to depend upon interfaces that they don’t use; The dependency of one class to another one should depend on the smallest possible interface.
建立单一的接口，不要建立臃肿庞大的接口。即接口尽量细化，同时接口中的方法尽量少。
迪米特原则 Law of Demeter / Least Knowledge Principle
Only talk to your immediate friends。
也称最少知识原则。一个类应该对自己需要耦合或者调用的类知道的越少越好。
开闭原则 Open Closed Principle
Software entities like classes, modules and functions should be open for extension but close for modifications.
一个软件实体如类，模块和函数应该对扩展开放，对修改关闭。