Editing Decorator pattern (section)

==Motivation==
[[Image:UML2 Decorator Pattern.png|thumb|400px|UML diagram for the window example]]
As an example, consider a window in a [[windowing system]]. To allow [[scrolling]] of the window's contents, one may wish to add horizontal or vertical [[scrollbar]]s to it, as appropriate. Assume windows are represented by instances of the ''Window'' interface, and assume this class has no functionality for adding scrollbars. One could create a subclass ''ScrollingWindow'' that provides them, or create a ''ScrollingWindowDecorator'' that adds this functionality to existing ''Window'' objects. At this point, either solution would be fine.

Now, assume one also desires the ability to add borders to windows. Again, the original ''Window'' class has no support. The ''ScrollingWindow'' subclass now poses a problem, because it has effectively created a new kind of window. If one wishes to add border support to many but not ''all'' windows, one must create subclasses ''WindowWithBorder'' and ''ScrollingWindowWithBorder'', etc. This problem gets worse with every new feature or window subtype to be added. For the decorator solution, a new ''BorderedWindowDecorator'' is created. Any combination of ''ScrollingWindowDecorator'' or ''BorderedWindowDecorator'' can decorate existing windows. If the functionality needs to be added to all Windows, the base class can be modified. On the other hand, sometimes (e.g., using external frameworks) it is not possible, legal, or convenient to modify the base class.

In the previous example, the ''SimpleWindow'' and ''WindowDecorator'' classes implement the ''Window'' interface, which defines the ''draw()'' method and the ''getDescription()'' method that are required in this scenario, in order to decorate a window control.