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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OrderedCollection class (respectively). Section seven then presents how the<br />
OrderedCollection class c<strong>an</strong> be used to construct Stack <strong><strong>an</strong>d</strong> Queue classes.<br />
10.2 The Collection class hierarchy<br />
A collection is a group of objects (these objects are called the elements of the collection). Collections<br />
are the <strong>Smalltalk</strong> mech<strong>an</strong>ism for building data structures of various sorts; it is therefore import<strong>an</strong>t to<br />
become familiar with the collection hierarchy <strong><strong>an</strong>d</strong> its functionality.<br />
Class Collection is the abstract superclass of all collections in the system. Figure 10.1<br />
summarizes the collection classes in VisualWorks. Some of the classes illustrated are abstract classes on<br />
which others build. In fact the collection class hierarchy is a classic example of the use of abstract<br />
classes <strong><strong>an</strong>d</strong> how they c<strong>an</strong> be used to group together functionality as well as indicate what is expected of<br />
subclasses. Now might be a go od time stop <strong><strong>an</strong>d</strong> examine the collection class itself. Abstract classes in<br />
this hierarchy include:<br />
• Collection. This is the abstract root class for the whole hierarchy.<br />
• SequenceableCollection. This is the abstract class for collections which have a defined<br />
sequence. That is, the subclasses of this class support the notion of having <strong>an</strong> order associated<br />
with the elements.<br />
• ArrayedCollection. This is the abstract class for those classes which have <strong>an</strong> array like<br />
behaviour. That is, subclasses of this class support the notion of <strong>an</strong> externally defined ordering.<br />
Figure 10.1: Part of the collection hierarchy in <strong>Smalltalk</strong><br />
The concrete classes in the collection class c<strong>an</strong> either be ordered or unordered <strong><strong>an</strong>d</strong> may or may not<br />
possess duplicates. Concrete classes which are unordered include:<br />
• Bag. A Bag contains all the objects put in it, in no particular order. Duplicate objects are<br />
permitted.<br />
• Set. Sets contains all the elements put into it, in <strong>an</strong>y order. Duplicates are not kept; adding equal<br />
objects m<strong>an</strong>y times results in only one such object in the Set.<br />
• Dictionary. Class Dictionary is a subclass of Set <strong><strong>an</strong>d</strong> represents a set of associations<br />
between pairs of objects.<br />
Concrete classes which are ordered include:<br />
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