Class Diagram

Source: PlantUML: Class Diagram Digest Date: January 15, 2023

Note: The UML preview of this documentation is powered by mermaid, some PlantUML symbols are not supported by mermaid(e.g. #--), so I didn't record these special symbols.

• 1. Declaring Element

• 2. Relations between Classes

• 3. Label on Relations

• 4. Using non-letters in element names and relation labels

• 5. Adding methods

• 6. Defining visibility

1. Declaring Element

declaring-elements

Note: protocol and struct was added in PR-1028.

2. Relations between Classes

Relations between classes are defined using the following symbols :

Type	Symbol	Drawing
Extension	`<	--`
Composition	*--
Aggregation	o--

It is possible to replace -- by .. to have a dotted line.

Knowing those rules, it is possible to draw the following drawings:

Source:

@startuml
Class01 <|-- Class02
Class03 *-- Class04
Class05 o-- Class06
Class07 .. Class08
Class09 -- Class10
@enduml

Preview by using mermaid:

classDiagram
    Class01 <|-- Class02
    Class03 *-- Class04
    Class05 o-- Class06
    Class07 .. Class08
    Class09 -- Class10

---

Source:

@startuml
Class11 <|.. Class12
Class13 --> Class14
Class15 ..> Class16
Class17 ..|> Class18
Class19 <--* Class20
@enduml

Preview by using mermaid:

classDiagram
    Class11 <|.. Class12
    Class13 --> Class14
    Class15 ..> Class16
    Class17 ..|> Class18
    Class19 <--* Class20

3. Label on Relations

It is possible to add a label on the relation, using : , followed by the text of the label.

For cardinality, you can use double-quotes "" on each side of the relation.

Source:

@startuml
Class01 "1" *-- "many" Class02 : contains
Class03 o-- Class04 : aggregation
Class05 --> "1" Class06
@enduml

Preview by using mermaid:

classDiagram
    Class01 "1" *-- "n" Class02 : contains
    Class03 o-- Class04 : aggregation
    Class05 --> "1" Class06

You can add an extra arrow pointing at one object showing which object acts on the other object, using < or > at the begin or at the end of the label.

Source:

@startuml
class Car
Driver - Car : drives >
Car *- Wheel : have 4 >
Car -- Person : < owns
@enduml

Preview by using mermaid:

classDiagram
    class Car
    Driver -- Car : drives >
    Car *-- Wheel : have 4 >
    Car -- Person : < owns

Warning: This feature is not fully supported by mermaid.

4. Using non-letters in element names and relation labels

Seems not so useful, omit for now.

5. Adding methods

To declare fields and methods, you can use the symbol : followed by the field's or method's name.

The system checks for parenthesis to choose between methods and fields.

Source:

@startuml
Object <|-- ArrayList

Object : equals()
ArrayList : Object[] elementData
ArrayList : size()
@enduml

Preview by using mermaid:

classDiagram
    Object <|-- ArrayList

    Object : equals()
    ArrayList : Object[] elementData
    ArrayList : size()

It is also possible to group between brackets {} all fields and methods.

Note that the syntax is highly flexible about type/name order.

Source:

@startuml
class Dummy {
  String data
  void methods()
}

class Flight {
   flightNumber : Integer
   departureTime : Date
}
@enduml

Preview by using mermaid:

classDiagram

    class Dummy {
        String data
        void methods()
    }

    class Flight {
        flightNumber : Integer
        departureTime : Date
    }

You can use {field} and {method} modifiers to override default behaviour of the parser about fields and methods.

Source:

@startuml

class Dummy {
    {field} A field (despite parentheses)
    {method} Some method
}

@enduml

Preview by using mermaid:

classDiagram

    class Dummy {
        {field} A field (despite parentheses)
        {method} Some method
    }

Warning: This feature is not supported by mermaid.

6. Defining visibility