Composition Over Inheritance

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

Favor "object composition" over "class inheritence".

What this means is do not extend from a parent class. Consider instead to compose references to the parent class. When using inheritence it can be seen as a white-box reuse. The reason being the sub-class commonly has visibility into the parent class. 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.

One way to think about the has-a relationship between 2 classes is also like a weight. Okay, technically, never seen it referenced in this way, but, I think a good illustration of the depicted line diagram. You have a association (line), aggregation (line, un-filled diamond), and composition (line, filled diamond). Association, no class "owns" the other. In aggregation a parent class has other classes, but these other classes are "less attached" in that they can also exist independently whether or not the parent class exists. In composition, we are stating a stronger form of aggregation, such that they all exist together and get destroyed together.

Reference

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