Decorator pattern

Intent

Attaches additional responsibilities to an object dynamically and transparently.

decorator-pattern-structure

The Decorator can also be an interface or abstract class.
The Decorator conforms to the interface of its decorated Component to make it transparent to the clients of the Component.
The Decorator keeps a reference to its decorated Component, so that it can attach additional responsibilities to the object at run-time.
The Decorator does not alter the behaviors of its decorated object, but instead it attaches other behaviors to the object. In other words, the Decorator changes the /skin/ of an object, not its /guts/.

This pattern can be used when:

You want to add new behaviors to before or after performing an operation of the decorated object.
You want to introduce new behaviors but for some reasons you don’t want to do that with inheritance.

Because the decorator extends its decorated type, it might be expensive to introduce new decorators if the decorated type is heavy-weight.
Even thought the decorator extends its decorated type, the developers must be aware that the decorator does /not/ actually belong to the type hierarchy.