The intent of the Design Pattern Decorator is to add responsibilities statically or dynamically to an object.

Implementation

The figure below shows a UML class diagram for the Decorator Pattern:

Component

Interface for objects that can have responsibilities added to them dynamically.

ConcreteComponent

Defines an object to which additional responsibilities can be added.

Decorator

Maintains a reference to a Component object and defines an interface that conforms to Component’s interface.

Concrete Decorators

Concrete Decorators extend the functionality of the component by adding state or adding behaviour.

Description

The decorator pattern applies when there is a need to dynamically add as well as remove responsibilities to a class, and when subclassing would be impossible due to a large number of subclasses that could result.