# Class diagram

A class diagram shows the structure of a model: what types exist, what they hold, and how they relate. It is most useful close to the code, and least useful as a whole-system drawing.

Draw the part of the model under discussion, not all of it.

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## When to draw one

* Explaining a domain model during design.
* Documenting a library's public types.
* Teaching, where the notation itself is the subject.

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## What is on the diagram

| **Element**  | **Meaning**                                   |
| ------------ | --------------------------------------------- |
| Class        | A type, with attributes and methods           |
| Inheritance  | A hollow triangle pointing at the parent      |
| Composition  | A filled diamond: the part cannot exist alone |
| Aggregation  | A hollow diamond: the part can exist alone    |
| Multiplicity | 1, 0..1, 1..\* on each end of an association  |

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## A worked example

A small domain model with inheritance and composition:

![Class diagram](https://help.gocapable.com/images/att1418035207.svg)

The source, which you can paste into a new diagram and edit:

```
classDiagram
  class Order {
    +UUID id
    +OrderStatus status
    +Money total
    +addItem(Product, int)
    +submit()
  }
  class OrderItem {
    +int quantity
    +Money unitPrice
  }
  class Payment {
    <<abstract>>
    +Money amount
    +authorise()
  }
  class CardPayment
  class BankTransfer

  Order "1" *-- "1..*" OrderItem : contains
  Order "1" o-- "0..*" Payment : settled by
  Payment <|-- CardPayment
  Payment <|-- BankTransfer
```

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## A few things that catch people out

* Multiplicity is the part that carries information. Without it, an association says almost nothing.
* Do not list every field. List the ones the reader needs to understand the relationships.
* Class diagrams age badly. Keep them close to the code or regenerate them.

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## Related

[Sequence diagramWho calls whom, in what order.](https://help.gocapable.com/diagrams/sequence-diagram.html)

[Use case diagramActors, a boundary, and what the system does for them.](https://help.gocapable.com/diagrams/use-case-diagram.html)

[UML diagramsBack to the UML overview.](https://help.gocapable.com/diagrams/uml-diagrams.html)

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_Four types cover most of it._