Object-oriented Software in Ada 95

Software design 23
Inheritance diagram
Responsibilities:
Telephone
Establish contact with another phone point.
Convert sound to/from electrical signals.
Fax machine
All the responsibilities of a telephone plus:
Convert images to/from electrical signals.
Figure 2.3 Relationship between a telephone and a fax machine.
Note:
The superclass (telephone) is the class from which a subclass (fax machine) is inherited.
Inheritance requires you to take all the responsibilities from the superclass; you cannot selectively
choose to take only some. However, even though you inherit the responsibilities you do not need to use
them.
The inheritance relationship is an important concept in object-oriented programming as it enables new objects
to be created by specializing an existing object. In creating the new object, only the addition, responsibilities have
to be constructed. The development time for a new software object is reduced as the task of creating the inherited
responsibilities has already been done. This process leads to a dramatic reduction in the time and effort required to
create new software objects.
2.8 Polymorphism
In a collection of different objects if all the objects are capable of receiving a specific message then this message
may be sent to any object in the collection. The method executed when this message is received by an object will
depend on the type of the object that receives the message.
For example, in a group of individuals if you ask a person how to take part in their favourite sport, you will
probably get many different answers. In effect the message ‘How to take part in your favourite sport’ is
polymorphic in that the answer you get depends on the individual person you select to ask. A tennis player for
example, would give a different answer than a golfer.
2.9 Self-assessment
• Explain why the solution to a small problem may not always scale up to solve a much larger and
complex problem.
• What is a “Responsibility”?
• What are the responsibilities of:
• A video camera.
• An alarm clock.
• A traffic light.
• An actress playing the role of Olgar in the Three sisters by Chekov.
• What is the relationship between an object, message and a method?
© M A Smith - May not be reproduced without permission