Class Diagram

A class diagram is a static diagram. It represents the static view of an application. The diagram is used not only to visualize, describe and document various aspects of a system but also to build the executable code of a software application. A class diagram describes the attributes, operations and also the constraints that occur in the system. Class diagrams are widely used in modeling OO systems because they are the only UML diagrams that can be mapped directly to object-oriented languages. A class diagram shows a collection of classes, interfaces, associations, collaborations and constraints. Class diagrams are also known as structural diagrams.

Implementing a Class Diagram

The class diagram is the most popular UML diagram used to build software applications. Therefore, it is very important to learn the procedure for drawing a class diagram. There are many properties to consider when drawing a class diagram. In this context, however, the class diagram is only viewed at a top level. A class diagram is basically a graphical representation of the static view of the system and represents various aspects of the application. A collection of class diagrams can be seen as a representation of the entire system. The following should be kept in mind when drawing a class diagram:

• The name of the class diagram should be meaningful in describing an aspect of the system.
• Each element and their relationships should be identified beforehand.
• The attributes and operations of each class should be clearly identified.
• For each class, the minimum number of properties should be specified, because unnecessary properties will make the diagram complicated.
• Use any notes needed to explain some aspects of the diagram, but they must still be understandable to developers/programmers.
• Finally, think repeatedly before producing the final version.

Make sure that the class diagram you draw truly represents the real state of the system.
To give a more detailed understanding of how class diagrams are used, here is an example of their use in an ordering system of an application:

• First, Order and Customer are identified as two elements of the system. They have a one-to-many relationship because a customer can have several orders.
• The Order class is an abstract class and has two concrete classes (inheritance relationship): Special Order and Normal Order.
• The two inherited classes have all the properties of their parent class, the Order class. In addition, they have additional functions such as dispatch() and receive().

The description of the above conditions can be depicted in a class diagram as shown in the Figure. Use the explanations in the previous chapters to get a better understanding of this class diagram figure.