UML Weekend Crash Course™ - To Parent Directory

Session 10—The Class Diagram: Associations 109
Association roles
Sometimes the association name is a bit hard to determine. For example, what English word
could you use for the association name between parents and children The UML provides an
alternative that may be used in place of the name or along with it to help make the reason
for the association as clear as possible. This alternative is called a role because it describes
how an object participates in the association.
For example, many employees contribute to a project. But you know from experience that
they participate in different ways. Figure 10-3 shows how you can draw multiple associations
and label them to differentiate the types of participation. Each role is placed at the
end of the association next to the type of object that plays the role. You may use them on
one, both, or neither end of each association.
projectlead
participates in
Employee
programmer
participates in
Project
uidesigner
participates in
Figure 10-3 Role names on an association
There is one other thing worth noting about roles and names. Role names generate code.
Association names do not generate code. The role name can be used to name the attribute
that holds the reference to the object that plays the role. In Figure 10-3, the Project
object could have an attribute named programmer that holds a reference to an Employee
object that plays the role of programmer, and another attribute called projectlead that
holds reference to another Employee object that plays the role of project lead.
Association constraints
Constraints appear throughout the UML notation. You used them in Session 9 when you
declared attributes and operations. Constraints fulfill much the same function for associations.
First take a look at Figure 10-4, in which no constraints are specified.
Person
1..1
Drives
0..* Car
Figure 10-4
An association without constraints