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The decorator pattern
works by wrapping the new "decorator" object around the
original object, which is typically achieved by passing the original
object as a parameter to the constructor of the decorator, with
the decorator implementing the new functionality. The interface
of the original object needs to be maintained by the decorator.
Decorators are alternatives to subclassing. Subclassing adds behaviour at compile time whereas decorators provide a new behaviour at runtime. This difference becomes most important when there are several independent ways of extending functionality. In some object-oriented programming languages, classes cannot be created at runtime, and it is typically not possible to predict what combinations of extensions will be needed at design time. This would mean that a new class would have to be made for every possible combination. By contrast, decorators are objects, created at runtime, and can be combined on a per-use basis. An example of the decorator pattern is the Java I/O Streams implementation. |
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Consider
a window in a windowing system. To allow scrolling of the window's
contents, we may wish to add horizontal or vertical scrollbars to
it, as appropriate. Assume windows are represented by instances
of the Window class, and assume this class has no functionality
for adding scrollbars. We could create a subclass ScrollingWindow
that provides them, or we could create a ScrollingWindowDecorator
that merely adds this functionality to existing Window objects.
At this point, either solution would be fine. Another solution is to simply modify the existing Window class, but this is not always possible we might not have access to its implementation, or we might be adding storage overhead for new functionality that the majority of objects will not use. |
Now let's assume
we also wish the option to add borders to our windows. Again, our
original Window class has no support. The ScrollingWindow subclass
now poses a problem, because it has effectively created a new kind
of window. If we wish to add border support to all windows, we must create subclasses WindowWithBorder and ScrollingWindowWithBorder. Obviously, this problem gets worse with every new feature to be added. For the decorator solution, we need merely create a new BorderedWindowDecorator at runtime, we can decorate existing windows with the ScrollingWindowDecorator or the BorderedWindowDecorator or both, as we see fit. A hotel is an establishment that provides paid lodging, usually on a short-term basis. Hotels often provide a number of additional guest services such as a restaurant, a swimming pool or childcare. Some hotels have conference services and meeting rooms and encourage groups to hold conventions and meetings at their location. A hotel chain is a collection or grouping of hotels under one recognizable brand operated by a management company. Best Western International claims to be the largest hotel chain in the world, in terms of the number of properties. |