Smalltalk and Object Orientation: an Introduction - Free

3. Constructing an Object Oriented System
3.1 Introduction
This chapter takes you th rough the design of a simple object oriented system. It does not concern itself
with implementation issues nor with the details of any particular language. Instead, the aim of this
chapter is to illustrate how the concepts described in the last two chapter s can be used to construct a
software system. In the remainder of the chapter we describe the application to be constructed. We then
consider where to start looking for objects and from there, what the objects should do and how they
should do it. We conclu de by discussing issues such as class inheritance and answer questions such as
“where is the structure of the program?”.
3.2 The application: windscreen wipe simulation
Wash wipe
switch
Wiper
motor
-
Relay
+
Fuse
Pump
Water
bottle
Figure 3.1: The windscreen wash wipe system structure
The aim of this system is to provide a system diagnosis tutor for the equipment illustrated in Figure 3.1.
That is, rather than use the wash wipe system from a real car, students on a car mechanics diagnosis
course will use this software simulation. The software system will provide a simulation whose behavior
will mimic that of the actual system, thus the behavior of the pump will depend on information provided
by the relay and the water bottle.
The operation of the wash wipe system is controlled by the wash wipe switch which can be in one of
5 different positions. These are: off, intermittent, slow, fast and wash. Each of these settings places the
system into a different state:
OFF. The system is inactive.
INTERMITTENT. The wiper motor wipes the wiper blades across the windscreen (or windshield
if you are American) every few second.
SLOW. The wiper motor continuously wipes the wiper blades across the windscreen.
FAST. The wiper motor continuously wipes the wiper blades quickly across the windscreen.
WASH. The wash setting is a power wash in which the water pump sprays water onto the
windscreen. This water is drawn from the water bottle.
For the pump or the wiper motor to work corr ectly, the relay must function correctly. In turn the relay
must be supplied with an electrical circuit. This electrical circuit is negatively fused and thus the fuse
must be intact for the circuit to be made. Note cars (automobiles) are negatively switche d as this
reduces the chances of short circuits which lead to unintentional switching of circuits.
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