Class Diagrams Syntax

"In software engineering, a class diagram in the Unified Modeling Language (UML) is a type of static structure diagram that describes the structure of a system by showing the system's classes, their attributes, operations (or methods), and the relationships among objects."

-Wikipedia

The class diagram is the main building block of object-oriented modeling. It is used for general conceptual modeling of the structure of the application, and for detailed modeling to translate the models into programming code. Class diagrams can also be used for data modeling. The classes in a class diagram represent both the main elements, interactions in the application, and the classes to be programmed.

Mermaid can render class diagrams.

---
title: Animal example
---
classDiagram
    note "From Duck till Zebra"
    Animal <|-- Duck
    note for Duck "can fly\ncan swim\ncan dive\ncan help in debugging"
    Animal <|-- Fish
    Animal <|-- Zebra
    Animal : +int age
    Animal : +String gender
    Animal: +isMammal()
    Animal: +mate()
    class Duck{
        +String beakColor
        +swim()
        +quack()
    }
    class Fish{
        -int sizeInFeet
        -canEat()
    }
    class Zebra{
        +bool is_wild
        +run()
    }

---
title: Animal example
---
classDiagram
    note "From Duck till Zebra"
    Animal <|-- Duck
    note for Duck "can fly
can swim
can dive
can help in debugging"
    Animal <|-- Fish
    Animal <|-- Zebra
    Animal : +int age
    Animal : +String gender
    Animal: +isMammal()
    Animal: +mate()
    class Duck{
        +String beakColor
        +swim()
        +quack()
    }
    class Fish{
        -int sizeInFeet
        -canEat()
    }
    class Zebra{
        +bool is_wild
        +run()
    }

Syntax

Class

UML provides mechanisms to represent class members, such as attributes and methods, and additional information about them. A single instance of a class in the diagram contains three compartments:

• The top compartment contains the name of the class. It is printed in bold and centered, and the first letter is capitalized. It may also contain optional annotation text describing the nature of the class.
• The middle compartment contains the attributes of the class. They are left-aligned and the first letter is lowercase.
• The bottom compartment contains the operations the class can execute. They are also left-aligned and the first letter is lowercase.

---
title: Bank example
---
classDiagram
    class BankAccount
    BankAccount : +String owner
    BankAccount : +Bigdecimal balance
    BankAccount : +deposit(amount)
    BankAccount : +withdrawal(amount)
 

---
title: Bank example
---
classDiagram
    class BankAccount
    BankAccount : +String owner
    BankAccount : +Bigdecimal balance
    BankAccount : +deposit(amount)
    BankAccount : +withdrawal(amount)

Define a class

There are two ways to define a class:

• Explicitly using keyword class like class Animal which would define the Animal class.
• Via a relationship which defines two classes at a time along with their relationship. For instance, Vehicle <|-- Car.

classDiagram
    class Animal
    Vehicle <|-- Car

classDiagram
    class Animal
    Vehicle <|-- Car

Naming convention: a class name should be composed only of alphanumeric characters (including unicode), underscores, and dashes (-).

Class labels

In case you need to provide a label for a class, you can use the following syntax:

classDiagram
    class Animal["Animal with a label"]
    class Car["Car with *! symbols"]
    Animal --> Car

classDiagram
    class Animal["Animal with a label"]
    class Car["Car with *! symbols"]
    Animal --> Car

Defining Members of a class

UML provides mechanisms to represent class members such as attributes and methods, as well as additional information about them.

Mermaid distinguishes between attributes and functions/methods based on if the parenthesis () are present or not. The ones with () are treated as functions/methods, and all others as attributes.

There are two ways to define the members of a class, and regardless of whichever syntax is used to define the members, the output will still be same. The two different ways are :

• Associate a member of a class using : (colon) followed by member name, useful to define one member at a time. For example:

classDiagram
class BankAccount
BankAccount : +String owner
BankAccount : +BigDecimal balance
BankAccount : +deposit(amount)
BankAccount : +withdrawal(amount)

classDiagram
class BankAccount
BankAccount : +String owner
BankAccount : +BigDecimal balance
BankAccount : +deposit(amount)
BankAccount : +withdrawal(amount)

• Associate members of a class using brackets, where members are grouped within curly brackets. Suitable for defining multiple members at once. For example:

classDiagram
class BankAccount{
    +String owner
    +BigDecimal balance
    +deposit(amount)
    +withdrawal(amount)
}

classDiagram
class BankAccount{
    +String owner
    +BigDecimal balance
    +deposit(amount)
    +withdrawal(amount)
}

Return Type

Optionally you can end a method/function definition with the data type that will be returned (note: there must be a space between the final ) and the return type). An example:

classDiagram
class BankAccount{
    +String owner
    +BigDecimal balance
    +deposit(amount) bool
    +withdrawal(amount) int
}

classDiagram
class BankAccount{
    +String owner
    +BigDecimal balance
    +deposit(amount) bool
    +withdrawal(amount) int
}

Generic Types

Generics can be represented as part of a class definition, and for class members/return types. In order to denote an item as generic, you enclose that type within ~ (tilde). Nested type declarations such as List<List<int>> are supported, though generics that include a comma are currently not supported. (such as List<List<K, V>>)

note when a generic is used within a class definition, the generic type is NOT considered part of the class name. i.e.: for any syntax which required you to reference the class name, you need to drop the type part of the definition. This also means that mermaid does not currently support having two classes with the same name, but different generic types.

classDiagram
class Square~Shape~{
    int id
    List~int~ position
    setPoints(List~int~ points)
    getPoints() List~int~
}
 
Square : -List~string~ messages
Square : +setMessages(List~string~ messages)
Square : +getMessages() List~string~
Square : +getDistanceMatrix() List~List~int~~

classDiagram
class Square~Shape~{
    int id
    List~int~ position
    setPoints(List~int~ points)
    getPoints() List~int~
}

Square : -List~string~ messages
Square : +setMessages(List~string~ messages)
Square : +getMessages() List~string~
Square : +getDistanceMatrix() List~List~int~~

Visibility

To describe the visibility (or encapsulation) of an attribute or method/function that is a part of a class (i.e. a class member), optional notation may be placed before that members' name:

• + Public
• - Private
• # Protected
• ~ Package/Internal

note you can also include additional classifiers to a method definition by adding the following notation to the end of the method, i.e.: after the () or after the return type:

• * Abstract e.g.: someAbstractMethod()* or someAbstractMethod() int*
• $ Static e.g.: someStaticMethod()$ or someStaticMethod() String$

note you can also include additional classifiers to a field definition by adding the following notation to the very end:

• $ Static e.g.: String someField$

Defining Relationship

A relationship is a general term covering the specific types of logical connections found on class and object diagrams.

[classA][Arrow][ClassB]

There are eight different types of relations defined for classes under UML which are currently supported:

classDiagram
classA <|-- classB
classC *-- classD
classE o-- classF
classG <-- classH
classI -- classJ
classK <.. classL
classM <|.. classN
classO .. classP
 

classDiagram
classA <|-- classB
classC *-- classD
classE o-- classF
classG --> classH
classI -- classJ
classK ..> classL
classM ..|> classN
classO .. classP

We can use the labels to describe the nature of the relation between two classes. Also, arrowheads can be used in the opposite direction as well:

classDiagram
classA --|> classB : Inheritance
classC --* classD : Composition
classE --o classF : Aggregation
classG --> classH : Association
classI -- classJ : Link(Solid)
classK ..> classL : Dependency
classM ..|> classN : Realization
classO .. classP : Link(Dashed)
 

classDiagram
classA --|> classB : Inheritance
classC --* classD : Composition
classE --o classF : Aggregation
classG --> classH : Association
classI -- classJ : Link(Solid)
classK ..> classL : Dependency
classM ..|> classN : Realization
classO .. classP : Link(Dashed)

Labels on Relations

It is possible to add label text to a relation:

[classA][Arrow][ClassB]:LabelText

classDiagram
classA <|-- classB : implements
classC *-- classD : composition
classE o-- classF : aggregation

classDiagram
classA <|-- classB : implements
classC *-- classD : composition
classE o-- classF : aggregation

Two-way relations

Relations can logically represent an N:M association:

classDiagram
    Animal <|--|> Zebra

classDiagram
    Animal <|--|> Zebra

Here is the syntax:

[Relation Type][Link][Relation Type]

Where Relation Type can be one of:

And Link can be one of:

Define Namespace

A namespace groups classes.

classDiagram
namespace BaseShapes {
    class Triangle
    class Rectangle {
      double width
      double height
    }
}

classDiagram
namespace BaseShapes {
    class Triangle
    class Rectangle {
      double width
      double height
    }
}

Cardinality / Multiplicity on relations

Multiplicity or cardinality in class diagrams indicates the number of instances of one class that can be linked to an instance of the other class. For example, each company will have one or more employees (not zero), and each employee currently works for zero or one companies.

Multiplicity notations are placed near the end of an association.

The different cardinality options are :

• 1 Only 1
• 0..1 Zero or One
• 1..* One or more
• * Many
• n n (where n>1)
• 0..n zero to n (where n>1)
• 1..n one to n (where n>1)

Cardinality can be easily defined by placing the text option within quotes " before or after a given arrow. For example:

[classA] "cardinality1" [Arrow] "cardinality2" [ClassB]:LabelText

classDiagram
    Customer "1" --> "*" Ticket
    Student "1" --> "1..*" Course
    Galaxy --> "many" Star : Contains

classDiagram
    Customer "1" --> "*" Ticket
    Student "1" --> "1..*" Course
    Galaxy --> "many" Star : Contains

Annotations on classes

It is possible to annotate classes with markers to provide additional metadata about the class. This can give a clearer indication about its nature. Some common annotations include:

• <<Interface>> To represent an Interface class
• <<Abstract>> To represent an abstract class
• <<Service>> To represent a service class
• <<Enumeration>> To represent an enum

Annotations are defined within the opening << and closing >>. There are two ways to add an annotation to a class, and either way the output will be same:

• In a separate line after a class is defined:

classDiagram
class Shape
<<interface>> Shape
Shape : noOfVertices
Shape : draw()

classDiagram
class Shape
<<interface>> Shape
Shape : noOfVertices
Shape : draw()

• In a nested structure along with the class definition:

classDiagram
class Shape{
    <<interface>>
    noOfVertices
    draw()
}
class Color{
    <<enumeration>>
    RED
    BLUE
    GREEN
    WHITE
    BLACK
}
 

classDiagram
class Shape{
    <<interface>>
    noOfVertices
    draw()
}
class Color{
    <<enumeration>>
    RED
    BLUE
    GREEN
    WHITE
    BLACK
}

Comments

Comments can be entered within a class diagram, which will be ignored by the parser. Comments need to be on their own line, and must be prefaced with %% (double percent signs). Any text until the next newline will be treated as a comment, including any class diagram syntax.

classDiagram
%% This whole line is a comment classDiagram class Shape <<interface>>
class Shape{
    <<interface>>
    noOfVertices
    draw()
}

classDiagram
%% This whole line is a comment classDiagram class Shape <<interface>>
class Shape{
    <<interface>>
    noOfVertices
    draw()
}

Setting the direction of the diagram

With class diagrams you can use the direction statement to set the direction in which the diagram will render:

classDiagram
  direction RL
  class Student {
    -idCard : IdCard
  }
  class IdCard{
    -id : int
    -name : string
  }
  class Bike{
    -id : int
    -name : string
  }
  Student "1" --o "1" IdCard : carries
  Student "1" --o "1" Bike : rides

classDiagram
  direction RL
  class Student {
    -idCard : IdCard
  }
  class IdCard{
    -id : int
    -name : string
  }
  class Bike{
    -id : int
    -name : string
  }
  Student "1" --o "1" IdCard : carries
  Student "1" --o "1" Bike : rides

Notes

It is possible to add notes on the diagram using note "line1\nline2". A note can be added for a specific class using note for <CLASS NAME> "line1\nline2".

Styling

Styling a node

It is possible to apply specific styles such as a thicker border or a different background color to an individual node using the style keyword.

classDiagram
  class Animal
  class Mineral
  style Animal fill:#f9f,stroke:#333,stroke-width:4px
  style Mineral fill:#bbf,stroke:#f66,stroke-width:2px,color:#fff,stroke-dasharray: 5 5

classDiagram
  class Animal
  class Mineral
  style Animal fill:#f9f,stroke:#333,stroke-width:4px
  style Mineral fill:#bbf,stroke:#f66,stroke-width:2px,color:#fff,stroke-dasharray: 5 5