Smalltalk and Object Orientation: an Introduction - Free

Car
name
hasEngine
type
Engine
Golf : Car
Petrol : Engine
Figure 17.6: Association between classes / link between objects
Each role (i.e. each end of the association) indicates the multiplicity of its class (i.e. how many
instances of the class can be associated with one instance of the other class). This is indicated by a text
expression on the role. The expression can either be a * (indicating zero or more), a number (e.g. 1), a
range (e.g. 0..3) or nothing (indicating exactly 1). These expressions are illustrated in Figure 17.7. It is
also possible to specify that the multiple objects should be ordered using the text {Ordered}. It is also
possible to annotate the association with additional text (such as {Sorted}) but this is primarily for the
readers benefit and has no semantic meaning in UML.
In some situations it is sensible for associations to have attributes of their own. In these situations
associations need to be treated as a class. Such a situation is illustrated in Figure 17.8. These
associations have a dashed line from the association line to the association class. This class is just like
any other class and can have a name, attributes and operations.
1..
*
Wheels
Wheels
1
1 or more
0..1
*
Wheels
Wheels
Zero or 1
Zero or more
3..
*
{Ordered}
Wheels
Specifies ordering
Figure 17.7: Types of association annotations
In Figure 17.8 the associations possess an access permissions attribute which indicates the type of
access allowed for each user for each file.
File
*
AuthorizedFor
*
User
Authorization
Access Permissions
/etc/termcap
/usr/home/pah/.login
(read)
(read/write)
Pheobe
Pheobe
Figure 17.8: Associations with attributes
Aggregation represents part-whole relationships. That is, it indicates that one or more objects are
dependent on another object for their existence. For example, in Figure 17.9 the Micro Computer is
formed from the Monitor, the System box, the Mouse and the Keyboard. All of these objects together
are needed for the fully functioning Computer. Aggregation is actually represented as an empty
diamond on the role attached to the whole object.
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