Software Engineering for Students A Programming Approach

144 Chapter 11 ■ Object-oriented design
Next we design and document the Alien class. The alien has a position and a size.
Whenever the clock ticks, it moves. Its direction and speed is controlled by the step size
that is used when it moves. It can be created and displayed.
Class Alien
Instance variables
X
Y
height
width
xStep
Methods
Alien
move
display
getX
getY
getHeight
getWidth
SELF-TEST QUESTION
11.2 Write the class diagram for the Bomb class.
We now have the full list of classes, and the methods and instance variables associated
with each class – we have modeled the game and designed a structure for the program.
11.3 ● Looking for reuse
The next act of design is to check to make sure that we are not reinventing the wheel.
One of the main benefits of OOP is reuse of software components. At this stage we
should check whether:
■ what we need might be in one of the libraries
■ we may have written a class last month that is what we need
■ we may be able to use inheritance.
We see in the cyberspace invaders software can make good use of GUI components,
such as the panel, available in the library. Other library components that are useful are
a timer and a random number generator.
If we find classes that are similar, we should think about using inheritance. We look
at how to write the code to achieve inheritance in Chapter 15 on OOP. In Chapter 13
on refactoring, we look at identifying inheritance using the “is-a” and “has-a” tests.