Smalltalk and Object Orientation: an Introduction - Free

holding other objects. Typically, collections are used as data structures in Smalltalk systems. They can
also be used as temporary holding places for groups of calculations, results, operations etc.
The Collection class assumes that a new subclass will provide at least the following:
• add: This adds an object to the collection
• remove:ifAbsent: This removes an object from the collection. If it is not present in the
collection, the block provided as the parameter to ifAbsent: is evaluated.
• do: This applies the associated block to each element in the collection
• copyEmpty: (if the subclass possesses named instance variables). This creates a copy of the
receiving collection which contains no elements.
The Collection class defines the common protocols to which all types of collection are expected to
respond. For example, objects may be added to collections using:
• aCollection add: anObject Subclasses redefine this message such that it will add a
single instance of a nObject to the collection. In some classes this is dependent on the current
contents of the collection (e.g. class Set).
• aCollection addAll: oldCollection Add each element in oldCollection to
aCollection. This method uses add: and therefore does not need to be redefined in a subclass.
• size. The current size of the collection (i.e. the number of objects it contains) may be
obtained using the size message: aCollection size.
Objects may be removed using one of the following:
• aCollection remove: oldObject. Remove the first element which equals
oldObject
• aCollection remove: oldObject ifAbsent: aBlock. Subclasses redefine this
method. The intention is that the method will remove the first instance of oldObject. They
will run aBlock if that object is not present in the collection.
• removeAll: aCollection. This method removes all of the element contained in
aCollection from the receiving collection.
The Collection class also define s a range of test methods which can be used to identify the current
state of a collection or determine if an object is a member of a collection. The test operations supported
include:
• aCollection includes: anObject. This test s whether an element is currently a
member of a collection. This method uses the = test.
• aCollection isEmpty. This message returns true or false depending on whether the
collection contains any objects or not.
• aCollection occurrencesOf: anObject. This message returns the number of times
anObject is present in the collection. Again this uses the #= test.
Additional protocols are implemented by subclasses to support operations suitable only for that
class. For example, OrderedCollection supports first, atFirst:, after: and
add:before: while arrays understand at: and at:put:.
10.5 Bag
Abstractly a Bag can be considered to be any collection of objects, which can be of any class; these
objects are the elements of the Bag. It is a general place holder for collections of objects. There is no
order assumed. It is the most general form of collection available in Smalltalk. (In many
implementations each entry in the bag is actually an association which indicates the object and the
number of times it has been placed in the bag. This is intended as a performance enhancement and is
normally transparent to the user.)
89