Software Engineering for Students A Programming Approach

146 Chapter 11 ■ Object-oriented design
Class name: Alien
Responsibilities Collaborators
moves Graphics
displays itself
provides its x coordinate
provides its y coordinate
provides its height
provides its width
Figure 11.3 A Sample CRC card – the class Alien in the cyberspace invaders game
table. It seems that the act of physically handling the cards contributes to an improved
understanding of the structure of the software.
11.6 ● Iteration
Iteration is a crucial ingredient of OOD. This is because there is no guaranteed formula
for finding the right set of objects to model a problem. Therefore the process is
exploratory and iterative; it is common for classes to be initially proposed during a
design but subsequently be discarded as the design progresses. Similarly, the need for
some classes will not emerge until the implementation (coding) stage.
During each iteration, the design is refined and reanalyzed continuously. Indeed,
whereas design and implementation are considered largely distinct activities in traditional
software development, using the object-oriented paradigm the two activities are often
indistinguishable. There are a number of reasons for this, which we will now discuss:
Prototyping is seen as an integral part of the object-oriented software design and implementation
process. Prototyping recognizes that in most cases the requirements for a system
are at best vague or not well understood. It is an exploratory process providing early
validation (or otherwise) of analysis, design and user interface alternatives. Prototyping dictates
that design and implementation proceed in at least large-grain iterative steps.
The activities which take place and the concerns which are addressed during the
refinement of an OOD are identical whether it is the design or a working prototype that
is being refined. Moreover, such activities as reorganizing the class to reflect some newly
recognized abstraction (see Chapter 13 on refactoring) are just as likely to take place
during the implementation of a prototype as during the design. The design must now
be updated to reflect this change – design and implementation are now proceeding in
small-grain iterative steps.
Designers must be far more aware of the implementation environment because of the
impact that a large reusable class library can have on a design. Designers must not only
be aware of the library classes but also design patterns (see Chapter 12).
To sum up, as noted by Meyer, one of the gurus of object-oriented development,
designing is programming and programming is designing.