Smalltalk and Object Orientation: an Introduction - Free
Smalltalk and Object Orientation: an Introduction - Free
Smalltalk and Object Orientation: an Introduction - Free
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services working? working? working? working?<br />
Switch<br />
Water<br />
bottle<br />
Motor Fuse Pump Relay<br />
(a)<br />
Component<br />
Switch<br />
Water<br />
bottle<br />
(aPump)<br />
(aFuse) (aRelay) (aMotor) (aSwitch) (aWaterbottle)<br />
(b)<br />
Switch<br />
Component<br />
Waterbottle<br />
(aSwitch) (aPump) (aFuse)<br />
(aRelay)<br />
Motor<br />
(aWaterbottle)<br />
(aMotor)<br />
(c)<br />
Figure 3.6: Possible class inherit<strong>an</strong>ce relationships<br />
From this table it appears that the only way in which they differ is th at they have different names.<br />
This would suggest that they are all inst<strong>an</strong>ces of a common class such as Component (see Figure 3.6.b).<br />
This class would possess <strong>an</strong> additional inst<strong>an</strong>ce variable name to simplify object ident ification.<br />
However, the problem with making them all inst<strong>an</strong>ces of a common class is that they must all behave in<br />
exactly the same way. This is not the case. We w<strong>an</strong>t the pump to start the <strong>an</strong>alysis process off when it<br />
receives the message working?. Thus the definition of working? that it possesses must be different from<br />
fuse <strong><strong>an</strong>d</strong> relay. However, in other ways it is very similar to fuse <strong><strong>an</strong>d</strong> relay. Therefore, what we w<strong>an</strong>t is<br />
for fuse <strong><strong>an</strong>d</strong> relay to be inst<strong>an</strong>ces of a class (say Component) <strong><strong>an</strong>d</strong> for pump to be <strong>an</strong> inst <strong>an</strong>ce of a class<br />
which inherits from Component (but which redefines working?). This is illustrated in Figure 3.6.c.<br />
SimulationClass<br />
name: ''<br />
traceMessages()<br />
Switches<br />
Component<br />
Waterbottle<br />
state<br />
moveUp()<br />
moveDown()<br />
state?<br />
state<br />
working?<br />
level<br />
empty()<br />
fill()<br />
extract()<br />
(fuse)<br />
(relay)<br />
(switches)<br />
Motor<br />
Pump<br />
(water bottle)<br />
working?<br />
working?<br />
(motor)<br />
(pump)<br />
class<br />
inst<strong>an</strong>ce<br />
inherit<strong>an</strong>ce<br />
inst<strong>an</strong>ce<br />
of<br />
Figure 3.7: The final class hierarchy <strong><strong>an</strong>d</strong> inst<strong>an</strong>ce diagram<br />
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