# Object diagram

Where a class diagram shows types, an object diagram shows actual instances with actual values. It is a snapshot, and it is unusually good at explaining a confusing structure by example.

Use it when a class diagram has left people unsure what the data actually looks like.

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## When to draw one

* Explaining a model by example rather than in the abstract.
* Documenting a specific test scenario or fixture.
* Teaching, where a concrete instance beats a general rule.

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## What is on the diagram

| **Element**      | **Meaning**                                           |
| ---------------- | ----------------------------------------------------- |
| Object           | name : Type, underlined                               |
| Attribute values | Actual values, not types                              |
| Link             | A line between objects, an instance of an association |

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## A worked example

One order, with its real values:

![Object diagram](https://help.gocapable.com/images/att1418199060.svg)

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

```
@startuml
object "order42 : Order" as O {
  id = 4c1e-88fa
  status = AWAITING_PAYMENT
  total = 148.50
  placedAt = 2026-09-01T14:22Z
}
object "item1 : OrderItem" as I1 {
  sku = KB-104
  quantity = 1
  unitPrice = 129.00
}
object "item2 : OrderItem" as I2 {
  sku = CB-USB-2M
  quantity = 2
  unitPrice = 9.75
}
object "alice : Customer" as C {
  email = alice@example.com
  tier = STANDARD
}

C -- O : places
O *-- I1
O *-- I2
@enduml
```

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## A few things that catch people out

* Use real, plausible values. Foo and bar defeat the purpose of an object diagram.
* It is a snapshot at one instant. Say which instant if it matters.
* Pair it with the class diagram rather than replacing it.

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## Related

[Package diagramModules, and which depends on which.](https://help.gocapable.com/diagrams/package-diagram.html)

[UML timing diagramState against a time axis.](https://help.gocapable.com/diagrams/uml-timing-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._