Favor composition over inheritance

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Composition should be used when possible because composition minimizes coupling, and therefore increases flexibility.

If you can achieve what you need by a composition relationship then do it that way. Inheritance should only be used when necessary, or when the benefits of inheritance can be fully utilized.

When a class overrides a method of its super-class, it needs to make assumptions about the state of that super-class when the method is called. It is simpler to "plug-in" the behaviour of another class via a composition relationship. Think of composition as inheriting only a small part of interface of another class, whereas inheritance forces you to inherit the entire interface. Flexibility is increased because following this rule should result in a greater number of smaller objects which can be interchanged and reused in various contexts.

A good example of this in practice is the Strategy pattern.

See also

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