Class diagrams

The class diagram is the most commonly used UML diagram, as it provides a visual description of a system's objects. Consider that, in Java, everything is an object, so you can see the relevance and reason as to why this particular diagram is so widely used. Class diagrams do more than just display objects—they visually depict their construction and relationships with other classes. 

As you can see here, the basic component of the class diagram is a rectangle, divided into three sections. Each overall rectangle represents a class, and the class name appears using a bold typeface in the top section. The middle section contains attributes that correlate to variable fields. The third section contains operation data which, in Java, means functions and methods:

A simple example of a class diagram for a Kennel is displayed in the following diagram. The class name is Kennel, and there are three attributes (animal, breed, and name) and two operations (intake and discharge):

We will further explore class diagrams using our Kennel class example later in this chapter.