Class diagram

The class diagram show the system's classes, attributes, methods and relationships. It's mostly used in OOP.

Notation

Defining class

You define your class with three sections:

1. Name: Stereotype (e.g «enum», «interface»), class name
2. Properties: Visibility, property name, data type, cardinality
3. Methods: Visibility, method name, parameters, return data type

classDiagram
	class Animal {
		<<interface>>
		+name: string
		-dateBirth: date
		#owner: Human[0..1]
		+getAge(): int
		+eat(food: Food): void
	}

Visibility

• +public – Access from any other classes
• -private – Access from inside its own classes
• #protected – Access from inside its own classes and sub-classes
• ~package – Access from classes from the same package (default)
• _static_ (underlined) – Access without instantiating class

Relationship

Name	Example	Explanations
Association	A--B	A is associated to B
Generalization	A-->B	A inherits from B
Aggregation	A--oB	A is a dependent piece of B
Composition	A--*B	A is an independent piece of B
Realization	A..>B	A inherits from interface B

Read more: Relations in OOP

Cardinality

Also called multiplicity, cardinality indicates how many instances of a class can be present in another class.

• One-to-one: A 1--1 B
• One-to-many: A *--1 B
• Many-to-many: A *--* B

We can also specified more precises numbers or define ranges: 0..1, 0..*, 1..n, etc.

Example

Resources

• Class and Object Diagrams Overview – UML diagrams
• Class diagrams syntax – Mermaid modeling language