UML Class Diagrams

UML (Unified Modelling Language) diagrams are the standard, language-agnostic way of visually describing the classes in a system and the relationships between them — useful for planning a design before writing code, and for communicating that design to others.

Class boxes

A class box has three parts:

ClassName
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attributes
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methods()

Each attribute/method is prefixed with a visibility marker:

Symbol	Visibility
-	private
+	public
#	protected

Relationship types

Relationship	Line style	Meaning
Inheritance	solid line, hollow (open) triangle arrowhead pointing to the parent	“is-a” — subclass inherits from superclass
Association	plain solid line	one class uses/references another, without ownership
Aggregation	solid line, hollow (open) diamond at the “whole” end	weak “has-a” — the part can exist independently of the whole
Composition	solid line, filled (solid) diamond at the “whole” end	strong “has-a” — the part cannot exist independently of the whole

Multiplicity

Multiplicity annotations on association/aggregation/composition lines describe how many objects on each end participate in the relationship:

Notation	Meaning
0..1	zero to one
n	an exact number
0..*	zero to many
1..*	one to many
m..n	a specific range

Why use UML?

• Plan a class hierarchy/object graph before writing any code.
• Communicate a design to teammates without needing to read source code.
• Document the intended structure and relationships for future maintenance.
• Class diagrams are the most commonly used UML diagram type in object-oriented design, but UML also includes other diagram types (e.g. sequence diagrams, use-case diagrams) not covered here.