Software Engineering for Students A Programming Approach

140 Chapter 11 ■ Object-oriented design
and testing time. It is common experience for someone setting out to write OO software
to find that nearly all of the software already exists within the library. OO development
is therefore often regarded as (merely) extending library classes. This, of
course, requires a discipline – the initial investment in the time to explore and learn
about what might be a large library, set against the benefits that accrue later.
11.2 ● Design
One of the principles used in the design of object-oriented software is to simulate real
world situations as objects. You build a software model of things in the real world. Here
are some examples:
■ if we are developing an office information system, we set out to simulate users, mail,
shared documents and files
■ in a factory automation system, we set out to simulate the different machines,
queues of work, orders, and deliveries.
The approach is to identify the objects in the problem to be addressed and to model
them as objects in the software.
One way to carry out OOD is to examine the software specification in order to
extract information about the objects and methods. The approach to identifying objects
and methods is:
1. look for nouns (things) in the specification – these are the objects
2. look for verbs (doing words) in the specification – these are the methods
Here is the specification for the cyberspace invaders program:
A panel (Figure 11.1) displays a defender and an alien. The alien moves sideways.
When it hits a wall, it reverses its direction. The alien randomly launches a bomb that
moves vertically downwards. If a bomb hits the defender, the user loses and the game is
over. The defender moves left or right according to mouse movements. When the mouse
is clicked, the defender launches a laser that moves upwards. If a laser hits the alien, the
user wins and the game is over.
A button is provided to start a new game.
Scanning through the specification, we find the following nouns. As we might
expect, some of these nouns are mentioned more than once, but the repetition does not
matter for the purpose of design.
panel, defender, alien, wall, bomb, mouse, laser
These nouns correspond to potential objects, and therefore classes within the program.
So we translate these nouns into the names of classes in the model. The noun
panel translates into the Panel class, available in the library. The nouns defender and
alien translate into the classes Defender and Alien respectively. The noun wall