Chapter 3. Class Diagrams: The Essentials

If someone were to come up to you in a dark alley and say, "Psst, wanna see a UML diagram?" that diagram would probably be a class diagram. The majority of UML diagrams I see are class diagrams.

The class diagram is not only widely used but also subject to the greatest range of modeling concepts. Although the basic elements are needed by everyone, the advanced concepts are used less often. Therefore, I've broken my discussion of class diagrams into two parts: the essentials (this chapter) and the advanced (Chapter 5).

A class diagram describes the types of objects in the system and the various kinds of static relationships that exist among them. Class diagrams also show the properties and operations of a class and the constraints that apply to the way objects are connected. The UML uses the term feature as a general term that covers properties and operations of a class.

Figure 3.1 shows a simple class model that would not surprise anyone who has worked with order processing. The boxes in the diagram are classes, which are divided into three compartments: the name of the class (in bold), its attributes, and its operations. Figure 3.1 also shows two kinds of relationships between classes: associations and generalizations.