Decorator is a structural design pattern that consists in dynamically augmenting the behavior of an existing object. It's different from classical inheritance, because the behavior is not added to all the objects of the same class, but only to the instances that are explicitly decorated.
Implementation-wise, it is very similar to the Proxy pattern, but instead of enhancing or modifying the behavior of the existing interface of an object, it augments it with new functionalities, as described in Figure 8.2:
Figure 8.2: Decorator pattern schematic
In Figure 8.2, the
Decorator object is extending the
Component object by adding the
methodC() operation. The existing methods are usually delegated to the decorated object without further processing but, in some cases, they might also be intercepted and augmented with extra behaviors.