Object-oriented Software in Ada 95

Software design 21
2.4 The class
In object-oriented terminology a class is used to describe all objects that share the same responsibilities and
internal structure. A class is essentially the collective name for a group of like objects. For example, the following
objects all belong to the class car:
Corinna’s red car Mike’s silver car Paul’s blue car
Although the objects differ in detail, they all have the same internal structure and responsibilities. Each object
is an instance of the class Car. The notation for a class is slightly different from that of an object. The UML
notation for a class and an object are illustrated below:
A class
An object (an instance of a class)
Car
Corinna’s car
Note:
The name of the object is underlined.
It is important to distinguish between a class and an object. A very simple rule is that objects usually have a
physical representation, whereas classes are an abstract concept.
2.5 Methods and messages
A method implements a responsibility for a class. For example, some of the responsibilities for the class Car are
as follows.
Responsibilities of the class Car
• Start/stop engine
• Go faster/slower
• Turn left/right
• Stop.
An instance of the class Car is an object. By sending a message to the object a hidden method inside the
object (a responsibility of the class Car) is invoked to process the message. For example, the driver of the car by
pressing down on the accelerator, sends the message ‘go faster’. The implementation of this is for the engine
control system to feed more petrol to the engine. Normally however, the details of this operation are not of
concern to the driver of the car.
2.6 Class objects
We have looked at a car's shell as a container for objects and can look at a laptop computer as a container for
several computing devices or objects. A laptop computer is composed of:
• The shell of the laptop, that has external interfaces of a keyboard, touch pad and display
screen.
• The local disk drive.
• The network file system.
• The CPU.
• The sound and graphics chipset.
© M A Smith - May not be reproduced without permission