Design Patterns Explained

218 Part IV • Putting It All Together: Thinking in Patterns
Extend software
capabilities without
changing it
Patterns are
microcosms of
Alexander's
philosophy
The Open-Closed Principle
Software clearly needs to be extensible. However, making changes to
software runs the risk of introducing problems. This dilemma led
Bertrand Meyer to propose the open-closed principle. 1 To paraphrase
this principle, the modules, methods, and classes should be open for
extension, while closed for modification. 2 In other words, we must
design our software so that we can extend the capabilities of our
software without changing it.
As contradictory as this may sound at first, you have already seen
examples of it. In the Bridge pattern, for instance, it is quite possible to
add new implementations (that is, to extend the software) without
changing any of the existing classes.
The Principle of Designing from Context
Alexander tells us to design from context, to create the big picture
before designing the details in which our pieces appear. Most design
patterns follow this approach, some to a greater extent than others.
Of the four patterns I have described so far, the Bridge pattern is the
best example of this.
Refer to the Bridge pattern diagram in Chapter 9, "The Bridge Pattern,"
(see Figure 9-13). When deciding how to design the Imple -
mentation classes, think about their context: the way that the
classes derived from the Abstraction class will use them.
For example, if I were writing a system that needed to draw shapes
on different types of hardware and that therefore required different
implementations, I would use a Bridge pattern. The Bridge tells me
that the shapes will use my implementations (that is, the drawing
1. Meyer, B., Object-Oriented Software Construction, Upper Saddle River, N.J.: Prentice
Hall, 1997, p. 57.
2. See this book's Web site for a link to "The Open-Closed Principle," an excellent
article by Robert C. Martin. Go to http://www.netobjectives.com/dpexplained.