Class diagrams: 6 UML relationship arrows

简介 / Introduction

Class diagrams are a core part of UML, describing the static relationships between classes in a system. Both PlantUML and Mermaid make it easy to draw the six most common relationship types: Association, Aggregation, Composition, Generalization, Dependency, and Realization.

PlantUML 版

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@startuml
hide circle
skinparam classAttributeIconSize 0

class Driver
class Car
class Engine

' Association
Driver "1" -- "1" Car : drives >

' Aggregation
Car o-- "1" Engine : has >

' Composition
Car *-- "1..4" Wheel : has >

' Generalization
Sedan --|> Car

' Dependency
Car ..> Driver : creates

' Realization
ICruiseControl <|.. CruiseControl

class ICruiseControl
class CruiseControl
@enduml

Mermaid 版

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classDiagram
    class Driver
    class Car
    class Engine
    class ICruiseControl
    class CruiseControl

    Driver "1" -- "1" Car : drives
    Car o-- "1" Engine : has
    Car *-- "1..4" Wheel : has
    Sedan --|> Car
    Car ..> Driver : creates
    ICruiseControl <|.. CruiseControl

对比 / Side-by-side

小结 / Wrap-up

PlantUML and Mermaid use nearly identical arrows for class-diagram relationships, making it easy to switch between them. Solid lines (--) denote structural relationships while dotted lines (..) denote dynamic dependencies. Arrow direction points toward the depended-upon element: composition beats aggregation beats plain association.