The class diagram is a static representation used in the programming area to describe the structure of a system, showing its classes, attributes, operations and the relationships between objects.

This type of representation is very useful in the development of computer systems and software , since it defines all the classes that the system needs to have and serves as the basis for the construction of other diagrams that define the type of communication, sequence and states of the systems. .

The class diagram is the core of the Unified Modeling Language (UML ). It represents the main purposes of the UML, having the function of separating the design elements from the coding of the system.

This language helps model various subsets of diagrams, including behavior, interaction, and structure diagrams. Typically used by engineers to document software architecture .

Class Diagram Components

The diagramming elements that make up a class diagram can represent the classes that will actually be programmed, as well as the main objects or the interaction between classes and objects in the system. These elements are:

Class

Abstract element that represents a set of objects. It contains the specification of the object, its characteristics, attributes and methods.

Attribute

Define class characteristics such as visibility, name, data type, multiplicity, initial value, and property.

Visibility can be public (represented by the + symbol) or private (represented by the – symbol). When it’s public, other classes can access the attribute and when it’s private, only the class itself has access.

It can also be protected (represented by the # symbol) or it can be packed (represented by the ~ symbol), where the attribute is accessed by the class’s relationship to the outer class.

The name is the identification of the attribute and the data type indicates the specification of the attribute’s data, as well as its initial value and its properties , which depend on the language used in programming. Multiplicity indicates the possibility of establishing attribute relationships with other classes.

Operation

This is the required function of an abstract object. This component also contains features such as name, visibility, and parameters.

Association

This component deals with the ability of classes to interact. It can also contain the name, multiplicity, and navigation type, which indicate where the class information comes from and where it will go.

In terms of structure, the class diagram is laid out in such a way that the shape of the class itself consists of a rectangle with three lines.

The top line contains the name of the class, the middle line the attributes of the class, and the bottom line expresses the methods or operations that the class can use. In a diagram, classes and subclasses are grouped together to show the static relationship between each object.

The following figure shows an example of a simple class diagram :

See also the meaning of Software and Diagram.