Software Engineering for Students A Programming Approach

152 Chapter 12 ■ Design patterns
period of time, experienced programmers build up a large repertoire of memories of
programming patterns such as this one.
A number of patterns have been identified, given names, cataloged and documented
in catalogs or books. These patterns are available for off-the-shelf use, just as classes
are available in class libraries. Software engineering patterns are patterns on a larger
scale than the simple program seen above. The established patterns specify the structure
of useful software at the architectural level. In object-oriented development, this means
structures expressed in terms of classes and their interrelationships.
The strength of design patterns is that good design ideas are recorded, given a name,
and explained with the aid of examples. This extends the vocabulary of software developers.
It also extends the repertoire of ideas that they can use without reinventing the
wheel.
To make use of patterns, the software engineer needs some recollection of the standard
patterns. This is obtained by browsing a patterns catalog prior to a project. The
engineer thereby retains some memory (perhaps only a partial recollection) of the patterns,
then, during the early phase of software architectural design, the engineer realizes
that one or more patterns may be useful. They then consult the catalog to confirm
the appropriateness of the pattern and see exactly how to use it. The next step is to use
the pattern as part of the design.
In summary the stages are:
1. browse a design pattern catalog, to obtain some feel for the available patterns
2. embark on a new design with an awareness of the patterns
3. recognize the need for one of the established patterns
4. consult the catalog to check the applicability of the pattern
5. use the catalog for information on the how to use the pattern
6. use the pattern as part of the design.
As well as architectural structure, patterns are available for such domains as user
interfaces, file systems and multithreading. Patterns are also provided for activities such
as testing and project management.
In order to use design patterns the programmer needs considerable experience and
understanding of OOD and OOP.
Just as there are patterns (which are valuable structures) so there are anti-patterns,
which are undesirable structures. The reason for identifying and cataloguing anti-patterns
is to avoid them. We look at one such pattern.
In this chapter we present a number of useful patterns and use the cyberspace
invaders game as an example in explaining some of the patterns.
12.2 ● Inheritance
It bears repeating that one of the major goals of the object-oriented paradigm is to
produce reusable software components – components which can be reused both
within the application in which they were generated but also in future applications.