What is composition as it relates to object oriented design?

Question

I hear (and read on this site) a lot about "favour composition over inheritance".

But what is Compositon? I understand inheritance from the point of Person : Mammal : Animal, but I can't really see the definition of Compostion anywhere.. Can somebody fill me in?

Answer 1

Composition refers to combining simple types to make more complex ones. In your example, composition could be:

Animal:     Skin animalSkin     Organs animalOrgans   Mammal::Animal:      Hair/fur mammalFur     warm-blooded-based_cirulation_system heartAndStuff  Person::Mammal:      string firstName     string lastName 

If you wanted to go totally composition (and get rid of all inheritance) it would look like this:

Animal:     Skin animalSkin     Organs animalOrgans  Mammal:     private Animal _animalRef     Hair/fur mammalFur     warm-blooded-based_cirulation_system heartAndStuff  Person:     private Mammal _mammalRef     string firstName     string lastName 

The advantage to this approach is that the types Mammal and Person do not have to conform to the interface of their previous parent. This could be a good thing because sometimes a change to the superclass can have serious effects on the subclasses. They still can have access to the properties and behaviours of these classes through their private instances of these classes, and if they want to expose these former-superclass behaviours, they can simply wrap them in a public method.

I found a good link with good examples here: http://www.artima.com/designtechniques/compoinh.html