classDiagramv0.9.4

What is a Class diagram?

Object-oriented relationships and inheritance.

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classDiagram

When to use it

Use a Class when…

Object-oriented domain modeling

API client/server contracts

Inheritance and composition diagrams

Syntax basics

The minimal syntax

Copy this and you have a working class. Build from here.

class.mmd

classDiagram
    class User {
      +UUID id
      +string email
      +login()
      +logout()
    }
    class Org {
      +UUID id
      +string name
    }
    class Session {
      +string token
      +revoke()
    }
    User class="code-str">"1" -- class="code-str">"*" Session : owns
    Org class="code-str">"1" -- class="code-str">"*" User : employs

Common patterns

Patterns worth memorizing

Visibility prefixes

`+` public, `-` private, `#` protected, `~` package — exactly like UML.

Cardinality

`User "1" -- "*" Post : writes` — quoted multiplicities flank the relation.

Generics

`class List~T~` renders the angle brackets; useful for typed collections.

Common gotchas

The things that will trip you up

Mermaid silently ignores misspellings — these are the failures we see most.

01
Method bodies aren't supported — only signatures.
02
The relation arrow direction matters: `<|--` is inheritance pointing to the parent.
03
Notes attach to a class with `note for ClassName "text"`.

Related diagram types