An association class is a cross between an association (see the section “Associations,” earlier in this chapter) and a class (see Chapter 1).

You use an association class to model an association that has interesting characteristics of its own outside of the classes it connects. This construct also comes in handy when you have a many-to-many relationship that you’d like to break into a set of one-to-many relationships (as discussed in the section “Discovering Classes” in Chapter 1).

An association class itself appears as a regular class box. You indicate that it’s an association class by connecting it to the association between the other two classes using a dashed line.

Figure 2-27 shows an example of an association class that was introduced in Chapter 1.

Figure 2-27.  Association class

There would normally be a many-to-many relationship between Author and Book, because an Author may have written more than one Book and a Book may have more than one Author. The presence of the BookAndAuthor association class allows the pairing of one Author with one Book; the role attribute provides the option of stating whether the Author was the primary author, supporting author, or editor, for example.

Refer to the «enumeration» stereotype in the section “Other Stereotypes on Classes” in Chapter 1 for a discussion of an alternate way to model the role attribute. One of the most useful things about the UML is that it often offers various options for modeling a particular idea. Of course, this can also be a curse, but in general, it’s the opinion of this author that having options is a good thing.

In the next chapter, you look at the UML diagrams that show combinations of classes and the relationships among them.

1 Ed Roman, Mastering Enterprise JavaBeans (Second Edition) (New York, NY: John Wiley & Sons, 2001).