UML Class Diagram Explanation

This section explains the key concepts of Class Diagrams and UML.

Class Diagram Symbols

The main symbols shown on class diagrams are:

Classes: Represent the types of data themselves.
Associations: Represent linkages between instances of classes.
Attributes: Simple data found in classes and their instances.
Operations: Functions performed by the classes and their instances.
Generalizations: Group classes into inheritance hierarchies.

Diagram: Class Diagram Symbols

Hint: A rectangle represents a class. Look for examples below for the various symbols.

Classes

A class represents a concept and encapsulates state (attributes) and behavior (operations). Each attribute has a type, and each operation has a signature. The class name is the only mandatory information.

Diagram: Class Example

Hint: A class shows attributes and operations. Attributes are usually shown with a hyphen prefix, operations with a plus.

Associations and Multiplicity

Associations show how two classes are related to each other. Multiplicity symbols indicate how many instances of one class are related to another. For example:

One-to-One: A company has exactly one board of directors.
Many-to-One: A company has many employees, but an employee works for only one company.
Many-to-Many: An assistant can work for many managers, and a manager can have many assistants.

Diagram: Association Example

Hint: A line connects classes to show an association. Multiplicity (how many) is often shown.