Java Reference
In-Depth Information
Decorator
Also known as Wrapper
Pattern Properties
Type: Structural, Object
Level: Component
Purpose
To provide a way to flexibly add or remove component functionality without changing its external appearance or
function.
Introduction
The Composite pattern example in the previous section added project functionality to the Personal Information
Manager, with a
Project
composed of a hierarchy of
Task
and
Deliverable
objects. All classes implemented
the
ProjectItem
interface, which identified them as classes that belonged to a project.
What if you wanted to extend the basic capabilities of the
Task
and
Deliverable
classes, adding extra features
like the following?
Dependent items - A
ProjectItem
that depends on another
Task
or
Deliverable
for completion.
Supporting documents - A
Task
or
Deliverable
that can reference additional reference documentation.
If you added these capabilities by subclassing, you would have to code a lot of classes. For instance, to make only
Deliverable
support these features, you would have to write four classes:
Deliverable
,
DependentDeliverable
,
SupportedDeliverable
, and
SupportedDependentDeliverable
.
Faced with this drawback, you might consider object composition as a way to add the new functionality. Coding
optional support into
Deliverable
and
Task
for both new features, however, can mean maintaining duplicate
code in multiple locations. At the very least, you increase the amount and complexity of the code.
What if, instead, you produce classes that have “plugin” capabilities? Instead of trying to add features to
Task
and
Deliverable
directly, you create dependent classes that can be attached to any ProjectItem to extend the basic functionality.
You could say it's the coding equivalent of adding a 3D sound set to your standard stereo. Your basic audio
capabilities remain the same, only now you have some extra feature to play with. For example, define a
DependentProjectItem
and a
SupportedProjectItem
. Each class has only the code needed to support its
optional capability, and a reference to the real
ProjectItem
that it extends. This means you have less code to
maintain, and the freedom to use any combination of these
Decorator
classes to add groups of capabilities to
ProjectItems
.
Applicability
Use the Decorator pattern when:
You want to make dynamic changes that are transparent to users, without the restrictions of subclassing.
Component capabilities can be added or withdrawn as the system runs.
There are a number of independently varying features that you should apply dynamically, and which you can use
in any combination on a component.
Description
Some objects have complex functionality and/or structure that can be added or removed in an accurate component
model. In the same way that overlays can be added to a map, showing additional features such as cities or
elevation, you might want the flexibility to add and remove certain features for an object.
