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Class diagram

Mermaid keyword: classDiagram

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Sample

classDiagram
  class Customer {
    +int id
    +string email
    +list~Order~ orders
    +placeOrder(items) Order
  }
  class Order {
    +int id
    +Date placedAt
    +string status
    +addItem(item)
  }
  class LineItem {
    +int qty
    +Money unitPrice
  }
  Customer "1" --> "*" Order : has
  Order "1" --> "*" LineItem : contains

When to reach for a class diagram

Class diagrams are ideal for domain models: showing entities, their fields, and how they relate. They double as a fast onboarding tool (“here are the five core types you’ll encounter in this codebase”) and as a precise way to specify a library’s public surface.

For a single function’s call graph, a flowchart is usually clearer. For database schemas with foreign keys and cardinality, prefer an ER diagram.

Cheat sheet

classDiagram
  class Animal {
    +string name
    +int age
    +eat() void
  }
  class Dog
  Animal <|-- Dog                  # inheritance
  Owner "1" --> "*" Dog : owns     # association with cardinality
  Engine *-- Car                    # composition (filled diamond)
  Wheel o-- Car                     # aggregation (open diamond)

Common mistakes

1. Listing every field. Class diagrams are not exhaustive specs. Show the 3–5 fields that matter to the audience.
2. Mixing visibility prefixes. +, -, #, ~ mean public/private/protected/package. Pick the ones your team uses.
3. Drawing classes that are really states. If “Pending”, “Shipped”, “Delivered” are showing up as classes, you want a state diagram.

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