--- sort: 5 --- # Class diagrams > "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 translating 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. ``` classDiagram Animal <|-- Duck 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() } ``` ```mermaid classDiagram Animal <|-- Duck 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. ``` classDiagram class BankAccount BankAccount : +String owner BankAccount : +Bigdecimal balance BankAccount : +deposit(amount) BankAccount : +withdrawl(amount) ``` ```mermaid classDiagram class BankAccount BankAccount : +String owner BankAccount : +BigDecimal balance BankAccount : +deposit(amount) BankAccount : +withdrawl(amount) ``` ## Define a class There are two ways to define a class: - Explicitly defining a class using keyword **class** like `class Animal`. This defines the Animal class - Define two classes via a **relationship** between them `Vehicle <|-- Car`. This defines two classes Vehicle and Car along with their relationship. ``` classDiagram class Animal Vehicle <|-- Car ``` ```mermaid classDiagram class Animal Vehicle <|-- Car ``` Naming convention: a class name should be composed of alphanumeric (unicode allowed) and underscore characters. ## Defining Members of a class UML provides mechanisms to represent class members, such as attributes and methods, and 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 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: ``` class BankAccount BankAccount : +String owner BankAccount : +BigDecimal balance BankAccount : +deposit(amount) BankAccount : +withdrawal(amount) ``` ```mermaid classDiagram class BankAccount BankAccount : +String owner BankAccount : +BigDecimal balance BankAccount : +deposit(amount) BankAccount : +withdrawl(amount) ``` - Associate members of a class using **{}** brackets, where members are grouped within curly brackets. Suitable for defining multiple members at once. For example: ``` class BankAccount{ +String owner +BigDecimal balance +deposit(amount) bool +withdrawl(amount) } ``` ```mermaid classDiagram class BankAccount{ +String owner +BigDecimal balance +deposit(amount) bool +withdrawl(amount) int } ``` #### Return Type Optionally you can end the method/function definition with the data type that will be returned (note: there must be a space between the final `)` of the method definition and return type example: ``` class BankAccount{ +String owner +BigDecimal balance +deposit(amount) bool +withdrawl(amount) int } ``` ```mermaid classDiagram class BankAccount{ +String owner +BigDecimal balance +deposit(amount) bool +withdrawl(amount) int } ``` #### Generic Types Members can be defined using generic types, such as `List`, for fields, parameters and return types by enclosing the type within `~` (**tilde**). Note: **nested** type declarations (such as `List>`) are not currently supported This can be done as part of either class definition method: ``` 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~ ``` ```mermaid 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~ ``` #### Return Type Optionally you can end the method/function definition with the data type that will be returned #### Visibility To specify the visibility of a class member (i.e. any attribute or method), these notations may be placed before the member's name, but it is optional: - `+` Public - `-` Private - `#` Protected - `~` Package/Internal > _note_ you can also include additional _classifers_ to a method definition by adding the following notations to the end of the method, i.e.: after the `()`: > - `*` Abstract e.g.: `someAbstractMethod()*` > - `$` Static e.g.: `someStaticMethod()$` ## Defining Relationship A relationship is a general term covering the specific types of logical connections found on class and object diagrams. ``` [classA][Arrow][ClassB]:LabelText ``` There are different types of relations defined for classes under UML which are currently supported: | Type | Description | | ----- | ------------- | | <\|-- | Inheritance | | \*-- | Composition | | o-- | Aggregation | | --> | Association | | -- | Link (Solid) | | ..> | Dependency | | ..\|> | Realization | | .. | Link (Dashed) | ``` classDiagram classA <|-- classB classC *-- classD classE o-- classF classG <-- classH classI -- classJ classK <.. classL classM <|.. classN classO .. classP ``` ```mermaid 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 nature of relation between two classes. Also, arrowheads can be used in opposite directions 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) ``` ```mermaid 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 a label text to a relation: ``` [classA][Arrow][ClassB]:LabelText ``` ``` classDiagram classA <|-- classB : implements classC *-- classD : composition classE o-- classF : association ``` ```mermaid classDiagram classA <|-- classB : implements classE o-- classF : association ``` ## Cardinality / Multiplicity on relations Multiplicity or cardinality in class diagrams indicates the number of instances of one class linked to one instance of the other class. For example, one company will have one or more employees, but each employee works for just one company. Multiplicity notations are placed near the ends of an association. The different cardinality options are : - `0..1` Zero or one - `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 cardinality text within qoutes `"` before(optional) and after(optional) a given arrow. ``` [classA] "cardinality1" [Arrow] "cardinality2" [ClassB]:LabelText ``` ``` classDiagram Customer "1" --> "*" Ticket Student "1" --> "1..*" Course Galaxy --> "many" Star : Contains ``` ```mermaid classDiagram Customer "1" --> "*" Ticket Student "1" --> "1..*" Course Galaxy --> "many" Star : Contains ``` ## Annotations on classes It is possible to annotate classes with a specific marker text which is like meta-data for the class, giving a clear indication about its nature. Some common annotations examples could be: - `<>` To represent an Interface class - `<>` To represent an abstract class - `<>` To represent a service class - `<>` To represent an enum Annotations are defined within the opening `<<` and closing `>>`. There are two ways to add an annotation to a class and regardless of the syntax used output will be same. The two ways are : - In a **_separate line_** after a class is defined. For example: ``` classDiagram class Shape <> Shape ``` ```mermaid classDiagram class Shape <> Shape Shape : noOfVertices Shape : draw() ``` - In a **_nested structure_** along with class definition. For example: ``` classDiagram class Shape{ <> noOfVertices draw() } class Color{ <> RED BLUE GREEN WHITE BLACK } ``` ```mermaid classDiagram class Shape{ <> noOfVertices draw() } class Color{ <> 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 after the start of the comment to the next newline will be treated as a comment, including any class diagram syntax ``` classDiagram %% This whole line is a comment classDiagram class Shape <> class Shape{ <> noOfVertices draw() } ``` ## Interaction It is possible to bind a click event to a node, the click can lead to either a javascript callback or to a link which will be opened in a new browser tab. **Note**: This functionality is disabled when using `securityLevel='strict'` and enabled when using `securityLevel='loose'`. You would define these actions on a separate line after all classes have been declared. ``` action className "reference" "tooltip" ``` - _action_ is either `link` or `callback`, depending on which type of interaction you want to have called - _className_ is the id of the node that the action will be associated with - _reference_ is either the url link, or the function name for callback. (note: callback function will be called with the nodeId as parameter). - (_optional_) tooltip is a string to be displayed when hovering over element (note: The styles of the tooltip are set by the class .mermaidTooltip.) ### Examples _URL Link:_ ``` classDiagram class Shape link Shape "http://www.github.com" "This is a tooltip for a link" ``` _Callback:_ ``` classDiagram class Shape callback Shape "callbackFunction" "This is a tooltip for a callback" ``` ``` ``` ## Styling ### Styling a node It is possible to apply specific styles such as a thicker border or a different background color to individual nodes. This is done by predefining classes in css styles that can be applied from the graph definition as in the example below: ```html ``` Then attaching that class to a specific node as per below: ``` cssClass "nodeId1" cssClass; ``` It is also possible to attach a class to a list of nodes in one statement: ``` cssClass "nodeId1,nodeId2" cssClass; ``` A shorter form of adding a class is to attach the classname to the node using the `:::` operator as per below: ``` classDiagram class Animal:::cssClass ``` Or: ``` classDiagram class Animal:::cssClass { -int sizeInFeet -canEat() } ``` ?> cssClasses cannot be added using this shorthand method at the same time as a relation statement. ?> Due to limitations with existing markup for class diagrams, it is not currently possible to define css classes within the diagram itself. ***Coming soon!*** ### Default Styles The main styling of the class diagram is done with a preset number of css classes. During rendering these classes are extracted from the file located at src/themes/class.scss. The classes used here are described below: | Class | Description | | ------------------ | ----------------------------------------------------------------- | | g.classGroup text | Styles for general class text | | classGroup .title | Styles for general class title | | g.classGroup rect | Styles for class diagram rectangle | | g.classGroup line | Styles for class diagram line | | .classLabel .box | Styles for class label box | | .classLabel .label | Styles for class label text | | composition | Styles for componsition arrow head and arrow line | | aggregation | Styles for aggregation arrow head and arrow line(dashed or solid) | | dependency | Styles for dependency arrow head and arrow line | #### Sample stylesheet ```scss body { background: white; } g.classGroup text { fill: $nodeBorder; stroke: none; font-family: 'trebuchet ms', verdana, arial; font-family: var(--mermaid-font-family); font-size: 10px; .title { font-weight: bolder; } } g.classGroup rect { fill: $nodeBkg; stroke: $nodeBorder; } g.classGroup line { stroke: $nodeBorder; stroke-width: 1; } .classLabel .box { stroke: none; stroke-width: 0; fill: $nodeBkg; opacity: 0.5; } .classLabel .label { fill: $nodeBorder; font-size: 10px; } .relation { stroke: $nodeBorder; stroke-width: 1; fill: none; } @mixin composition { fill: $nodeBorder; stroke: $nodeBorder; stroke-width: 1; } #compositionStart { @include composition; } #compositionEnd { @include composition; } @mixin aggregation { fill: $nodeBkg; stroke: $nodeBorder; stroke-width: 1; } #aggregationStart { @include aggregation; } #aggregationEnd { @include aggregation; } #dependencyStart { @include composition; } #dependencyEnd { @include composition; } #extensionStart { @include composition; } #extensionEnd { @include composition; } ``` ## Configuration `Coming soon`