dependency inversion principle

In a bad design the high level class uses directly the low level classes. In this case if we want to change the design to direct the output to a new FileWriter class we have to change the Copy class. (Let's assume that it is a very complex class, with a lot of logic and realy hard to test).

In order to avoid such problems we can introduce an abstraction layer between the high level classes and low level classes. Since the high level modules contains the complex logic they should not depend on the low level modules and that the new abstraction layer should not be created based on low level modules. The low level modules are created based on the abstraction layer.

According to this principle the way of designing a class structure is to start from high level modules to the low level modules:
High Level Classes --> Abstraction Layer --> Low Level Classes

Intent

• High-level modules should not depend on low-level modules. Both should depend on abstractions.
• Abstractions should not depend on details. Details should depend on abstractions.