Object-oriented Software in Ada 95

20 Software design
2.3.1 The car as an object
A car is made up of many components or objects. From a user’s perspective some of the major objects that make
up a car are:
• The shell or body of the car.
• The engine.
• The gearbox.
• The clutch.
• The battery that provides electric power.
We can think of the body or shell of the car as a container for all the other objects, that when combined, form a
working car. These other objects are hidden from the driver of the car. The driver can, however, interact with
these objects by using the external interfaces that form part of the car shell. This arrangement of objects is
expressed diagrammatically using the UML notation in Figure 2.1.
Car body
1
1
1
1 1
1 1 1
Engine
Gearbox
Clutch
Battery
Figure 2.1 Objects that make up a car.
In Figure 2.1 the following style of notation is used:
Engine
Denotes an object. In this specific case the car engine.
Engine
1
4
Piston
Denotes aggregation. The engine contains 4 pistons.
Note:
A
A
B
Denotes aggregation, the component B is
contained in the container A.
B
Denotes composition, in addition the
component B is created and destroyed by the
container A.
By using this notation, we can express the ‘part of’ relationship between objects. The engine, gearbox, clutch
and battery are ‘part of’ a car.
© M A Smith - May not be reproduced without permission