UML Diagram

UML Relationships

Visual Notation

Relationship	Notation	Mermaid
Association	Simple arrow	A --> B
Dependency	Simple arrow, dashed line	A ..> B
Aggregation	Simple arrow, empty diamond back	A o-- B
Composition	Simple arrow, filled diamond back	A *-- B
Inheritance	Empty triangle head	A —|> B
Realization	Empty triangle head, dashed line	A ..|> B

Relationship Reference

Relationship	Question	Example	Key Property
Association	”Does A use/know B?”	Customer → Order	A holds a reference to B
Dependency	”Does A temporarily use B?”	Order → TaxCalculator	B appears only in method param/return, not stored
Aggregation	”Does A have B, but B can exist alone?”	Team → Player	A contains B; B has independent lifecycle
Composition	”Does A own B, and B cannot exist alone?”	Car → Engine	B’s lifecycle is bound to A
Inheritance	”Is A a B?”	Dog → Animal	A is a subtype of B
Realization	”Does A implement interface B?”	Dog → IAnimal	A fulfills the contract defined by B

Notes

• Composition vs Aggregation: Both use diamond notation. The difference is lifecycle — in composition, the contained object cannot exist without the container. In aggregation, it can.
• Association vs Dependency: Association implies a persistent reference (field). Dependency implies a transient use (local variable, parameter).
• Inheritance vs Realization: Inheritance is class→class. Realization is class→interface.
• Abstract class: Name written in italic in diagrams.
• <<extend>> / <<include>>: These are use case diagram relationships, not class diagram relationships. Do not confuse with Inheritance/Realization.

Mermaid Example

classDiagram
    Animal <|-- Dog
    IAnimal <|.. Dog
    Car *-- Engine
    Team o-- Player
    Customer --> Order
    Order ..> TaxCalculator