UML Weekend Crash Course™ - To Parent Directory

106
Saturday Morning
Modeling Basic Association Notations
The following notations appear on almost every association you will model. Most of these
elements are similar to those you find in data modeling or database design. In fact, most of
the concepts come from these fields. The concepts worked well in data modeling, so they
were simply brought forward into object modeling as a form of “best practices.” I suggest
you memorize them because you will spend a lot of time working with them.
Association name
The purpose of the association can be expressed in a name, a verb or verb phrase that
describes how objects of one type (class) relate to objects of another type (class). For example,
a person owns a car, a person drives a car, and a person rents a car. Even though the
participants are the same in each association, the purpose of each association is unique,
and as such they imply different rules and interactions.
To draw the UML associations for these three examples, you need to start with four basic
elements.
The participating classes, Person and Car. In this session I show only the name compartment
so that your attention remains focused on the classes and their associations.
The association, represented by a line between the two classes (pretty technical
huh).
The name of the association, represented by a verb or verb phrase on the association
line. Don’t worry about the exact position of the name. As long as the name
appears somewhere in the middle of the line, you’re okay. Just leave room at both
ends of the association for all the other things you’ll learn about later in this
session.
The direction to read the name (indicating the direction is optional).
The first two examples in Figure 10-1 read pretty much the way I described them in the
text — Person owns Car and Person drives Car. Note that if these two statements are true,
then the reverse would be equally true — Car is owned by Person and Car is driven by
Person. Associations may be read in both directions as long as you remember to reverse the
meaning of the association name from active to passive.
But in the third example in Figure 10-1, the association name would not make sense if you
read it in the typical left to right fashion — Car rents Person. This is a case where the direction
indicator is particularly appropriate, even required, to make sense of the association by
reversing the normal reading order so that it reads from right to left — Person rents Car.