Smalltalk and Object Orientation: an Introduction - Free

• OrderedCollection. A collection where the ordering is given by the order in which
elements were added.
• Array. A collection which provides array like behaviour.
• String. An indexed collection of Characters.
• SortedCollection. A collection of objects, which are sorted accordi ng to an ordering
defined in the instance of SortedCollection.
• Symbol. A subclass of String. All symbols are unique, while there can be two different
Strings containing the same characters.
• Text. A class which understands a notion of a collection of fonts associated with a string.
A relatively new class which incorporates some of the more useful features of the Array,
OrderedCollection with SortedCollection classes is the List class.
The basic operations that are performed by the collection data structures include adding and
removing elements, determining the size of the collection, querying the presence or absence of elements
and iterating over the elements.
10.3 Choosing a Collection class
It can sometimes be confusing for those new to Smalltalk to decide whi ch collection class to use. Some
make the mistake of always using an array (because it is similar to the constructs that they are used to).
However, this is failing to understand the way one should work with collections (data structures) in
Smalltalk. To this end the following decision tree may be of use.
Collection
no
Are elements ordered?
yes
Accessible by key?
SequenceableCollection
yes
no
order determined
Access test
Duplicate allowed?
externally
internally
= = = yes no Accessible by key?
class of elements
Dictionary IdentityDictionary Bag Set
yes
no
Number
any
ArrayedCollection class of elements Interval SortedCollection
class of elements
SmallInteger Character any
any
Link
ByteArray String Text Array RunArray List OrderedCollection LinkedList
Figure 10.2: Selecting which Collection class to use
The most commonly used collection classes are, Array, SortedCollection,
OrderedCollection, List, Dictionary and Set.
10.4 The Collection class
The collection class is the abstract superclass which acts as the root of all collection classes. It provides
facilities for creating instances of a collection class, adding an element to a collection, accessing
elements in a collection, removing an element from a collection and indicating the size of the collection.
It also provides facilities which allow an operation to be applied to all elements of the collect ion (in a
similar fashion to the mapcar function in Lisp). It also provides a number of conversion routines which
allow one collection to be converted into another.
With one or two exceptions (e.g. ByteArray) any object can be stored into any collection. The
only message that these operations must be able to respond to is = (although objects to be placed in a
Sorted collection must also respond to