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 depend upon details.Details should depend upon abstractions.
高层模块不应该依赖低层模块，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 。