Smalltalk and Object Orientation: an Introduction - Free

| temp |
temp := Set new.
temp add: 'John'.
temp add: 'Paul'.
temp add: 'Peter'.
temp add: 'John'.
temp inspect.
You will find that only one occurrence of the string ‘ John’ was added to the set. The second
addition was ignored.
There is also a version of the collection class Set called IdentitySet. This type of set performs
an identity test to determine if an object is a duplicate or not. That is, it uses the == test rather than the =
test. This means that rather than an object having the same value as an object already in a se t, it must
actually be that object. If it is only equivalent then the object will be added to the set. In this way it may
appear that duplicates have been added to the set. However, they are not the same object and are
therefore allowed.
The IdentitySet can be useful if you know that duplicate elements to be added to the set will be
the same object. In such cases the Identity set can be used as it is more efficient than the standard set.
10.7 OrderedCollection
The OrderedCollection cla ss can hold any type of object. It does so in a specified order. That
order is determined by the order in which objects are added to the OrderedCollection instance. An
instance of OrderedCollection is created by using either new or new:. The second version of new
allows an integer to be provided which indicates the maximum size of the collection required. This can
be useful in situations where this number is known and time is optimal. That is, it is not necessary to
waste time incrementally growing the collection.
OrderedCollection can be used in situations where the order in which the objects were added
to the instance must be preserved. For example, try out the following example of using an
OrderedCollection:
| x |
x := OrderedCollection new.
x add: 'one'.
x add: 'two'.
x add: 'three'.
x inspect.
You will find that the strings ‘one’, ‘two’ and ‘three’ remain in the order in which they were added
(unlike instances of Bag and Set).
There is a wide range of order related messages which allow obj ects to be added and accessed with
reference to the order in the OrderedCollection instance. For example, it is possible to access an
object “before” or “after” another object by using the before: or after: messages, e.g.:
x after: 'two'.
It is also possible to add an object either before or after another object. For example:
x add: 'five' after: 'one'.
x add: 'six' before: 'three'.
To remove an object from the collection use remove:ifAbsent:, this removes t he first instance
of the object specified in the collection. If the object is not present, then the absent block will be
executed. For example:
a remove: 'John' ifAbsent: [Dialog warn: 'John not present'].
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