Object-oriented Software in Ada 95

Software design 19
In software the same problems of scale exist; the techniques that we use to implement a small program cannot
usually be successfully used on a large programming project. The computer literature is full of examples of
software disasters that have occurred when a computer system has been started without a full understanding of the
problem that is to be solved.
2.2.1 Responsibilities
Since our earliest days we have all been told that we have responsibilities. Initially, these responsibilities are very
simple, but as we grow older so they increase. A responsibility is a charge, trust or duty of care to look after
something. At an early age this can be as simple as keeping our room neat and tidy. In later life, the range and
complexity of items that we have responsibility for, increases dramatically.
A student for example, has the responsibility to follow a course of study. The lecturer has the responsibility of
delivering the course to the students in a clear and intelligible manner. The responsibilities of the student and
lecturer are summarized in tabular form below:
Responsibilities of a student
Follow the course of study.
Perform to the best of their ability
in the exam/assessment for the
course.
Responsibilities of a lecturer
Deliver the course.
Set and mark the assessment for
the course.
Attend the exam board for the
delivered course.
Software too has responsibilities. For example, a text editor has the responsibility of entering the user’s typed
text correctly into a document. However, if the text that is entered into the text editor is incorrect or meaningless,
then the resultant document will also be incorrect. It is not the role of the text editor to make overall decisions
about the validity of the entered text.
In early computing literature, a common saying was “Garbage in, garbage out”. Even though the software
package implements its responsibilities correctly, the results produced may be at least meaningless, at worse
damaging if used.
2.3 Objects
The world we live in is composed of many different objects. For example, a person usually has access to at least
some of the following objects:
• A telephone.
• A computer.
• A car.
Each object has its own individual responsibilities. For example, some of the responsibilities associated with
the above objects are:
Object
Responsibilities
Telephone • Establish contact with another phone point.
• Convert sound to/from electrical signals.
Computer • Execute programs.
• Provide a tcp/ip connection to the internet.
Car • Move
• Go faster/slower
• Turn left/right
• Stop.
A responsibility here, is a process that the object performs. For example, a car can move forwards or
backwards. However, the car has to be instructed by the driver to perform this task. The object is passive, and only
performs an action when instructed to do so.
© M A Smith - May not be reproduced without permission