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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Next we will define a message protocol called “operations”. It is within this protocol that we will<br />
define all the methods which will provide the functionality of the Stack. These methods will be push:,<br />
pop <strong><strong>an</strong>d</strong> top.<br />
The push: method is used to push something onto the top of the stack. It takes one argument, (<strong>an</strong><br />
object) <strong><strong>an</strong>d</strong> adds it to the top of the stack. To do this it uses the addLast: method inherited from<br />
OrderedCollection.<br />
push: new<strong>Object</strong><br />
self addLast: new<strong>Object</strong><br />
Next we will define the pop method. This method removes the top element from the stack <strong><strong>an</strong>d</strong><br />
returns it as the result of evaluating pop. Again it uses methods inherited from<br />
OrderedCollection to do this.<br />
pop<br />
self isEmpty<br />
ifTrue: [^nil]<br />
ifFalse: [^self removeLast].<br />
Finally, we will define the method top. This method is like pop, except that it does not remove the<br />
top most element from the stack. To do this we use the message<br />
last inherited from<br />
OrderedCollection.<br />
top<br />
self isEmpty<br />
ifTrue: [^nil]<br />
ifFalse: [^self last].<br />
This class is <strong>an</strong> excellent example of w hy <strong>Smalltalk</strong> is so powerful. We have defined a class which<br />
provides the functionality of a Stack (no matter what the contents of that stack will be) in just a few<br />
lines of code. Consider the amount of work required to achieve the same result in a l<strong>an</strong>guage such as<br />
Pascal or C (without purchasing additional libraries).<br />
10.8.2 The Queue class<br />
This class definition is very similar to the Stack example, it is therefore presented here with little<br />
additional comment.<br />
OrderedCollection variableSubclass: #Queue<br />
inst<strong>an</strong>ceVariableNames: ''<br />
classVariableNames: ''<br />
poolDictionaries: ''<br />
category: 'Additional-Collections'<br />
Again a class comment should be defined. This is left <strong>an</strong> <strong>an</strong> excersie for the reader. The methods<br />
should again be placed in a protocol called “operations”. Each of the methods are listed below.<br />
add: new<strong>Object</strong><br />
self addLast: new<strong>Object</strong>.<br />
delete<br />
self isEmpty<br />
ifTrue: [^nil]<br />
ifFalse: [self removeFirst].<br />
next<br />
self isEmpty<br />
ifTrue: [^nil]<br />
ifFalse: [^self first].<br />
The state of the class definition at this point is illustrated in Figure 10.5.<br />
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