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Class Diagrams in UML

Posted Oct 17, 2023 Updated Dec 10, 2023
In (CS) Learning Note
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What is a Class Diagram?

A class diagram describes the structure of a system by showing the system’s classes, their attributes, and the relationships between the classes. It serves as a blueprint for designing and understanding software projects.

  • We use the class diagram to model the static structure of a system, thus describing the elements of the system and the relationships between them.
    • These elements and the relationships between them do not change over time.
    • A class diagram partitions the system into areas of responsibility (classes) and shows “associations”(dependencies) between them.
  • The class diagram is without doubt the most widely used UML diagram.
  • It is applied in various phases of the software development process.
    • The level of detail or abstraction of the class diagram is different in each phase
  • Class diagrams can be presented at three distinct levels, or perspectives:
    • Conceptual: the diagram represents the concepts in the project/business domain.
    • Specification: shows interfaces between components in the high-level design stage.
    • Implementation: shows classes that correspond directly to computer code/implemented system (often Java or C++ classes).

Main Components of a Class Diagram

  1. Class: Represents a blueprint for an object. It’s depicted as a rectangle with the class name at the top, followed by attributes and methods. A class describes a group of objects with:
    • similar properties (attributes),
    • common behaviour (operations),
    • common relationships to other objects,
    • and common meaning (“semantics”).
  2. Attributes: Variables that hold data.
  3. Methods: Functions associated with the class.
  4. Modifiers: They are used to indicate visibility of attributes and operations.
    • + is used to denote Public visibility (Visibility: everyone)
    • # is used to denote Protected visibility (Visibility: friends and derived)
    • - is used to denote Private visibility (Visibility: no one)
  5. Relationships: Including associations, generalizations (inheritance), and dependencies.

The full notation of Class

Relationships in Detail of a Class Diagram

  1. Association: Represents a “has-a” or “using” relationship between two or more classes.
    • There is an association between two classes if an instance of one class must know about the other in order to perform its work.
    • An association has two ends. An end may have a role name to clarify the nature of the association. For example, an OrderDetail is a line item of each Order.
    • A navigability arrow on an association shows which direction the association can be traversed or queried. An OrderDetail can be queried about its Item, but not the other way around.
    • The arrow also lets you know who “owns” the association’s implementation; in this case, OrderDetail has an Item. Associations with no navigability arrows are bi-directional.
  2. Aggregation: Represents a “whole-part” relationship. The part can exist independently of the whole.
    • Class2 is part of Class1.
    • Many instances (denoted by the *) of Class2 can be associated with Class1.
    • Objects of Class1 and Class2 have separate lifetimes.
  3. Composition: A “strong” form of aggregation. The part cannot exist without the whole. It is binary association, part could be included in at most one composite (whole) at a time, and if a composite (whole) is deleted, all of its composite parts are “normally” deleted with it.
    • Objects of Class2 live and die with Class1.
    • Class2 cannot stand by itself.

  4. Generalization (Inheritance): Represents an “is-a” relationship. An inheritance link indicating one class is a superclass of the other.

  5. Realization: This relationship is used to depict the realization of UML interfaces by UML classes. It means that one entity (usually a class) implements the behavior specified by another entity (typically an interface). It’s represented by a dashed line with a hollow arrowhead pointing from the implementing class to the interface.

  6. Dependency: Indicates that one class depends on another.

The difference between Association and Dependency in UML:

  • Association:
    1. Nature: Association represents a bi-directional, structural link between two classes, suggesting that there’s a long-term relationship between them.
    2. Lifetime: Objects at both ends of an association may have similar lifetimes, or one object’s lifecycle might be independent of the other. For instance, if you have a Teacher class and a Student class, an association might suggest that a Teacher can be associated with multiple Students, and vice versa.
    3. Usage: Often used to depict relationships like “has-a”, suggesting one class contains or is linked to another.
  • Dependency:
    1. Nature: Dependency represents a temporary, unidirectional relationship. It indicates that one class (the dependent or client) relies on another (the supplier) for some functionality or behavior, but it’s not a long-term relationship.
    2. Lifetime: In a dependency relationship, the lifetime of the dependent is often independent of the supplier. The relationship only suggests that at some point, the dependent class may use the supplier.
    3. Usage: Common in cases where a class uses another transiently, such as a method parameter, local variable, or in a method invocation. Dependency suggests a weaker relationship compared to association and hints at the idea that a change in the supplier might affect the dependent class.

How to Draw a Class Diagram?

  1. Identify Classes: Start by listing down the entities.
  2. Add Attributes and Methods: For each class, define its attributes and methods.
  3. Define Relationships: Determine how each class interacts with others.
  4. Finalize the Diagram: Ensure that all relationships and elements are correctly captured.

Example: How to Draw a Class Diagram of “Information system of a university”

  • A university consists of multiple faculties which are composed of various institutes. Each faculty and each institute has a name. An address is known for each institute.
  • Each faculty is led by a dean, who is an employee of the university.
  • The total number of employees is known. Employees have a social security number, a name, and an e-mail address. There is a distinction between research and administrative personnel.
  • Research associates are assigned to at least one institute. The field of study of each research associate is known. Furthermore, research associates can be involved in projects for a certain number of hours, and the name, starting date, and end date of the projects are known. Some research associates teach courses. They are called lecturers.
  • Courses have a unique number (ID), a name, and a weekly duration in hours.

Identifying the classes

Identifying the attributes

Identifying the relationships between classes

Full Example

(CS) Learning Note, Software Modelling and Design
software engineering UML OOP
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