Inheritance vs Composition

From http://www.artima.com/designtechniques/compoinh5.html


a few guidelines that reflect how I tend to select between composition and inheritance.

Make sure inheritance models the is-a relationship
My main guiding philosophy is that inheritance should be used only when a subclass is-a superclass. In the example above, an Apple likely is-a Fruit, so I would be inclined to use inheritance.

An important question to ask yourself when you think you have an is-a relationship is whether that is-a relationship will be constant throughout the lifetime of the application and, with luck, the lifecycle of the code. For example, you might think that an Employee is-a Person, when really Employee represents a role that a Person plays part of the time. What if the person becomes unemployed? What if the person is both an Employee and a Supervisor? Such impermanent is-a relationships should usually be modelled with composition.

Don't use inheritance just to get code reuse
If all you really want is to reuse code and there is no is-a relationship in sight, use composition.

Don't use inheritance just to get at polymorphism
If all you really want is polymorphism, but there is no natural is-a relationship, use composition with interfaces. I'll be talking about this subject next month.