Composition Over Inheritance

The Gang of Four [1] advocates favoring "composition over inheritance".

An outline of this concept in provided below as a refresher.

Outline

1. Favor "object composition" over "class inheritence".

Instead of inheriting from a parent class consider composing references to the parent class with the type or behavior desired.

2. The use of inheritence can be seen as a white-box reuse.

The reason being the sub-class commonly has visibility into the parent class.

3. Object composition resembles black-box reuse.

Through the use of composition, the implementation details are better encapsulated and enables delegating such behaviors.

UML quick notes

Composition is a stronger form of aggregation. A solid-diamond towards the composite class.

Aggregation is a stronger form of association. An un-filled diamond towards the composite class.

Reference

[1] Gang of Four 1995:20 [2]