Design Patterns Explained

Chapter 13 • The Principles and Strategies of Design Patterns 223
The Principle of Containing Variation
Several people have remarked about a certain similarity in all of the
designs they have seen me create: my inheritance hierarchies rarely
go more than two levels of classes deep. Those that do typically fit
into a design pattern structure that requires two levels as a base of
the abstract classes (the Decorator pattern, discussed in Chapter 15,
is an example that uses three levels).
The reason for this is that one of my design goals is never to have a
class contain two things that are varying that are somehow coupled
to each other. The patterns I have described so far do illustrate different
ways of containing variation effectively.
•
The Bridge pattern is an excellent example of contained variation.
The implementations present in the Bridge pattern are all different
but are accessed through a common interface. New implementations
can be accommodated by implementing them within this
interface.
The Abstract Factory contains the variation of which sets or families
of objects can be instantiated. There are many different ways of
implementing this pattern. It is useful to note that even if one
implementation is initially chosen and then it is determined
another way would have been better, the implementation can be
changed without affecting any other part of the system (since the
interface for the factory does not change, only the way it is imple -
mented). Thus, the notion of the Abstract Factory itself (implementing
to an interface) hides all of the variations of how to create
the objects.
The Adapter pattern is a tool to be used to take disparate objects
and give them a common interface. This is often needed now that I
am designing to interfaces as called for in many patterns.
A note on my designs
Containing
variation in the
Bridge pattern
Containing
variation in the
Abstract Factory
Containing variation
in the Adapter
pattern