Section 2.1. Classes

2.1. Classes

A class represents a group of things that have common state and behavior. You can think of a class as a blueprint for an object in an object-oriented system. In UML speak, a class is a kind of classifier. For example, Volkswagen, Toyota, and Ford are all cars, so you can represent them using a class named Car. Each specific type of car is an instance of that class, or an object. A class may represent a tangible and concrete concept, such as an invoice; it may be abstract, such as a document or a vehicle (as opposed to an invoice, or a motorcycle greater than 1000 cc), or it may represent an intangible concept such as a high-risk investment strategy.

You represent a class with a rectangular box divided into compartments. A compartment is simply an area in the rectangle to write information. The first compartment holds the name of the class, the second holds attributes (see "Attributes"), and the third is used for operations (see "Operations"). You can hide any compartment of the class if that increases the readability of your diagram. When reading a diagram, you can make no assumptions about a missing compartment; it doesn't mean it is empty. You may add compartments to a class to show additional information, such as exceptions or events, though this is outside of the typical notation.

UML suggests that the class name:

• Start with a capital letter

• Be centered in the top compartment

• Be written in a boldface font

• Be written in italics if the class is abstract (see "Abstract Classes")

Figure 2-1 shows a simple class.

Figure 2-1. Simple class representation

2.1.1. Objects

An object is an instance of a class. For example, you may have several instances of a class named Car: one two-door red car, one four-door blue car, and one hatchback green car. Each instance of Car is an object and may be given its own name, though it is common to see unnamed, or anonymous, objects on object diagrams. Typically you show the name of the object followed by a colon followed by its type (i.e., class). You show that this is an instance of a class by underlining the name and type. Figure 2-2 shows an instance of a class Car named Toyota. Note that in this figure, we have hidden the empty compartments.

Figure 2-2. An instance of Car