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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."


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.

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.

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.

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:

You can also use backticks to escape special characters in the label:

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

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:

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.

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.


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

--Link (Solid)
..Link (Dashed)

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:

Labels on Relations ​

It is possible to add label text to a relation:


Two-way relations ​

Relations can logically represent an N:M association:

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.

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

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:
  • In a nested structure along with the class definition:

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.

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:

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"
click className call callback() "tooltip"
click className href "url" "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.
  • (optional) tooltip is a string to be displayed when hovering over element (note: The styles of the tooltip are set by the class .mermaidTooltip.)
  • note: callback function will be called with the nodeId as parameter.

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".

Examples ​

URL Link:


  const callbackFunction = function () {
    alert('A callback was triggered');

Success The tooltip functionality and the ability to link to urls are available from version 0.5.2.

Beginner's tipβ€”a full example using interactive links in an HTML page:

  <pre class="mermaid">
    Animal <|-- Duck
    Animal <|-- Fish
    Animal <|-- Zebra
    Animal : +int age
    Animal : +String gender
    Animal: +isMammal()
    Animal: +mate()
    class Duck{
      +String beakColor
    class Fish{
      -int sizeInFeet
    class Zebra{
      +bool is_wild

      callback Duck callback "Tooltip"
      link Zebra "" "This is a link"

    const callback = function () {
      alert('A callback was triggered');
    const config = {
      startOnLoad: true,
      securityLevel: 'loose',

Styling ​

Styling a node (v10.7.0+) ​

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.

Classes ​

More convenient than defining the style every time is to define a class of styles and attach this class to the nodes that should have a different look. This is done by predefining classes in css styles that can be applied from the graph definition using the cssClass statement or the ::: short hand.

  .styleClass > rect {
    fill: #ff0000;
    stroke: #ffff00;
    stroke-width: 4px;

Then attaching that class to a specific node:

    cssClass "nodeId1" styleClass;

It is also possible to attach a class to a list of nodes in one statement:

    cssClass "nodeId1,nodeId2" styleClass;

A shorter form of adding a class is to attach the classname to the node using the ::: operator:


?> 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:

g.classGroup textStyles for general class text
classGroup .titleStyles for general class title
g.classGroup rectStyles for class diagram rectangle
g.classGroup lineStyles for class diagram line
.classLabel .boxStyles for class label box
.classLabel .labelStyles for class label text
compositionStyles for composition arrow head and arrow line
aggregationStyles for aggregation arrow head and arrow line(dashed or solid)
dependencyStyles for dependency arrow head and arrow line

Sample stylesheet ​

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!