Smalltalk and Object Orientation: an Introduction - Free

The OrderedCollection class can be used to construct a Stack or Queue class. An example
of how to do this is presented in the next section. Notice how little code has to be written and how much
is inherited.
10.8 Stack and queue classes in Smalltalk
In this section we will work t hrough an example illustrating how we can define new collection classes
(data structures). We shall put both classes we will define into a class category called “Additional -
Collections”.
10.8.1 The Stack class
The first class we will define is the Stack class. Th e class definition of the Stack is illustrated
below 9 . Note that we are making Stack a subclass of OrderedCollection. This is because the
elements of a stack are ordered, and may be duplicated within the stack. Also note that we are not
defining any new in stance variables. The functionality of the Stack will come solely from extending
the interface, the structure of the class will be completely inherited from the OrderedCollection
class.
OrderedCollection variableSubclass: #Stack
instanceVariableNames: ''
classVariableNames: ''
poolDictionaries: ''
category: 'Additional-Collections'
As before I am also providing a class comment (see Figure 10.4). Again it is up to you to decide
whether you wish to provide one as well (although I strongly recommend doing so - it is a good habit to
get into).
Figure 10.4: The Stack class comment
9
As OrderedCollections are variable in size the variableSub class: class creation message is used. Don't worry too much
about this as the tools provided with environments such as VisualWorks ensure that the appropriate message is always used.
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