UML Class Diagrams

What Is UML?

UML, Unified Modelling Language, is an international standard for visual modelling of software systems. It includes specifications for thirteen different diagrams that support all stages of the software development process.

One of the most important and widely used of the UML diagrams is the class diagram.

Simple Classes

The simplest possible representation of a class is a rectangle, containing the class name:

---
config:
  theme: neutral
  look: handDrawn
  class:
    hideEmptyMembersBox: true
---
classDiagram
  class Student

This simple representation is typically adopted in the early stages of analysis, when we are focused on which classes might be needed to model a system, and how they might relate to each other:

---
config:
  theme: neutral
  look: handDrawn
  class:
    hideEmptyMembersBox: true
---
classDiagram
  Lecturer -- Module : teaches
  Student -- Module : enrols on

Notice how the relationships between these classes are drawn as solid lines, labelled to describe the nature of the relationship. This is the syntax for associations. We consider associations and other types of class relationship in more detail later. We will defer further discussion of UML syntax for relationships until then.

Adding Detail

Classes can include details of their attributes, their operations or both. In this representation, we divide the rectangle into three compartments. The class name occupies the top compartment, the attributes are listed in the middle compartment, and the operations are listed in the bottom compartment:

---
config:
  theme: neutral
  look: handDrawn
---
classDiagram
  direction LR
  Student -- Module : enrols on
  class Module {
    moduleCode
    title
    numCredits
    enrolStudent()
  }
  class Student {
    name
    degreeProgramme
    levelOfStudy
  }

In the example above, neither the attribute lists nor the operations lists are complete. Also, we have not specified types for the attributes, nor have we provided information on parameters and types for the enrolStudent() operation.

This is entirely normal. The amount of detail shown for a class is a choice made by the analyst, based on how much is currently known about the class and what information the class diagram is intended to convey.

Design-Level Diagrams

Class diagrams differ in appearance, depending on whether they are drawn at the analysis level or the design/implementation level.

A design-level class diagram contains more detailed information about classes. On such a diagram, we would specify some or all of the following:

• Attribute types
• Attribute visibility
• Type of value returned by an operation
• Parameters needed by an operation (names and types)

This information provides further guidance to the programmer tasked with implementing the classes on the diagram.

For example, a design-level version of the class diagram above might look like this:

---
config:
  theme: neutral
  look: handDrawn
---
classDiagram
  Student -- Module : enrols on
  class Module {
    moduleCode: String
    title: String
    numCredits: Int
    enrolStudent(student: Student)
    generateTranscript(student: Student)
  }
  class Student {
    id: Int
    name: String
    degreeProgramme: String
    levelOfStudy: Int
  }