Software Engineering for Students A Programming Approach

C.2 ● Class diagrams
C.2 Class diagrams 413
These describe classes and their interrelationships. Classes are shown as rectangles containing
the class name. The simplest relationship is where a class uses another. For
example, in Figure C.2, class Game uses classes Defender, Alien, Laser and Bomb.
This means that Game creates objects from these classes and/or calls methods in objects
created from these classes.
A class diagram can also show the inheritance relationships between classes – the subclasses
and superclasses. As illustrated in Figure C.3, to show that a class extends another,
a line is drawn from the subclass to the superclass, with the arrowhead pointing to the
superclass. Thus Sprite is the superclass of both Alien and Bomb.
If a class is an abstract class, the name of the class is written in italics. This can be difficult
to see, particularly when hand-written. So the name of an abstract class can be followed
by the text {abstract} to clarify the meaning.
An interface is described in the same way as a class – as a box. The difference is that
the text precedes the name. A class that implements an interface has
a dashed line with an arrow leading to the interface box (see Figure C.4).
A class can be described in more detail, as illustrated in Figure C.5. There are three
compartments in this type of class diagram. The first compartment holds the class
Figure C.2 Class diagram
Game
Figure C.3 Class diagram showing inheritance
Sprite
Defender
Alien
Laser
Bomb
Alien Bomb