7.1. Favoring Composition Over Inheritance

An object oriented design axiom known as "favor composition over inheritance" means that rather than always turning to inheritance as a tool for extending functionality of a given class, you should instead first try to solve the problem by using other classes compositionally within your class. For example, if you needed to implement a class that provided an interface between a network service and your application, instead of inheriting from a socket class (a class that gives you access to network resources), you should include another object in your class, which provides that network connectivity. You should "compose" your design from other reusable components. Building your designs in this way results in more flexibility, because those bits and pieces that you are composing together to solve a given problem can later be swapped out and changed to solve another problem. This is an incredibly powerful design philosophy and one which you should strive for in your code.

It is certainly possible to follow this design philosophy simply by using standard object oriented techniques. However, designing components for reuse causes complications.