Software Engineering for Students A Programming Approach

142 Chapter 11 ■ Object-oriented design
Game
Defender
Alien
Laser
Figure 11.2 The non-library classes involved in the game program
are displayed. Another object is a random number generator, created from the library
class Random, to control when the alien launches bombs.
We have now identified the classes that make up the game program.
We now scan the specification again, this time looking for verbs that we can attach
to the above list of objects. We see:
display, move, hit, launch, click, win, lose
Again, some of these words are mentioned more than once. For example, both the
alien and the defender move. Also all the objects in the game need to be displayed.
We now allocate methods to classes, with the help of the specification. This concludes
the design process.
Now although we used the specification to arrive at classes and methods, we could
have used an alternative formulation of the specification, use cases. Use cases were
explained in Chapter 4 on requirements specification. A use case is a simple, natural
language statement of a complete and useful function that a system carries out. For the
game, some use cases are:
■ defender move – when the user moves the mouse left and right, the defender
object moves left and right on the screen
■ fire laser – when the player clicks the mouse, a laser is launched upwards from the
defender object
■ laser hits alien – when a laser hits the alien, the player wins and the game is over
Notice that some of the use cases are not initiated by the user of the system. Instead
they are initiated by objects within the game, such as when an alien launches a bomb.
A complete set of use cases constitutes a specification of what a system should do.
We can then use them to derive classes and their methods. Again, we seek verbs (which
are the classes) and verbs (which are the methods). Fortunately use cases are very suited
to this process, because they emphasize actions acting on objects. So, for example,
the use case defender move implies that there is an object defender that embodies
methods moveLeft and moveRight.
Bomb