Java Reference
In-Depth Information
In addition, the coding requirements for the Bridge give you an overall savings in the number of classes written as
you increase the number of variations.
Table 3-2
shows the number of terminal classes required using strict
inheritance compared with the Bridge pattern.
Table 3-2.
Class coding requirements
External representations
I
mplementations
Classes required with inheritance
Classes required with Bridge
2
2
4
4
3
2
6
5
4
4
1
6
8
5
4
20
9
Comparison of Inheritance Pattern and Bridge Pattern
The Bridge design allows you to
multiplex
the external representation and internal implementation choices for the
component. Multiplexing simply means associating any combination of external and internal elements, to get a
greater range of options.
Dividing the component according to its two differentiating concepts also tends to produce a component that is
easier to understand and maintain. This is because each inheritance chain revolves around a single concept,
abstraction or implementation.
The Bridge is a useful pattern for any system that should display localized flexibility during runtime. An example
is GUI systems that must be portable among platforms, requiring that an underlying implementation be applied
when the application is started in a different operating system. Systems that change their representation of data
depending on locale (for example, altering date, language, or monetary representation) are often good candidates
for the Bridge, as well. Similarly, the Bridge is often effective for business entities that can potentially map back
to a number of different database sources.
A conceptual example for the Bridge is a technical support switchboard. A number of pre-established phone lines
connect a user with a variety of technical support personnel. Naturally, the response will be markedly different
depending on the experience of the technical support representative who is on the line. The response probably
varies according to the question as well; there will be a somewhat different response to users complaining about
broken cup-holders on their PCs.
Implementation
Figure 3.3. Bridge class diagram
Implementing the Bridge pattern requires the following classes:
Abstraction -
The
Abstraction
class defines the functional abstraction for the
Bridge
, providing standard
behavior and structure. It contains a reference to an
Implementation
instance. This
Implementation
instance is
usually set with either a setter method (to allow modification at run-time) or through the constructor.
RefineAbstraction -
The
RefineAbstraction
class extends the Abstraction class and provides additional or
modified behavior.
