UML Weekend Crash Course™ - To Parent Directory

Session 10—The Class Diagram: Associations 107
Person Owns Car
Person Drives Car
Car Rents Person
Figure 10-1
Directional notation for association names
Tip
Remember the direction indicator when you’re making a lot of changes to a
diagram where you have to rearrange the classes. It is easy for classes to
reverse position on the diagram, resulting in nonsensical association names.
The indicators can prevent unnecessary confusion.
Association multiplicity
The UML allows you to handle some other important questions about associations: “How
many Cars may a Person own” “How many can they rent” “How many people can drive a
given Car” Associations define the rules for how objects in each class may be related. So
how do you specify exactly how many objects may participate in the relationship
Multiplicity is the UML term for the rule that defines the number of participating objects.
A multiplicity value must be assigned to each of the participating classes in an association.
As illustrated in Figure 10-2, you need to ask two separate questions.
Person
Owns
Car
How many
People own
each Car
How many Cars
are owned
by each Person
Figure 10-2
Assigning multiplicity to each end of an association