Design Patterns Explained

Chapter 21 • Design Patterns Reviewed 3O7
The value of hiding the implementations is that the patterns allow
for easily adding new implementations, since the client objects do
not know how the current implementation works.
Commonality/Variability Analysis and
Design Patterns
In Chapter 9, "The Bridge Pattern," I showed how the Bridge pattern
can be derived using commonality/variability analysis. Many other
patterns can be derived as well, including the Strategy, Iterator,
Proxy, State, Visitor, Template Method, and Abstract Factory. What
is more important, however, is how many patterns are imple mented
by using commonality/variability analysis. Looking for
commonalities can help us discover that a pattern is present in our
problem domain.
For example, in the Bridge pattern, I may start with several special
cases:
• Draw a square with drawing program one.
• Draw a circle with drawing program two.
• Draw a rectangle with drawing program one.
Knowing the Bridge helps me see these as special cases of two commonalities:
• Drawing programs
• Shapes to draw
The Strategy pattern is similar in that when I see several different
rules, I know to look for a commonality amongst the rules so I can
encapsulate them.
Commonality/
variability analysis