Object diagrams: instance snapshots

简介 / Introduction

An Object Diagram records concrete instances and their attribute values at a specific point in time, serving as a structural snapshot of a class diagram. While class diagrams model abstract types and relationships, object diagrams answer the question: what real instances exist right now and what are their states? PlantUML has a native objectdiagram keyword, while Mermaid approximates it with class diagrams plus notes.

PlantUML 版

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@startuml
object "Alice (ID:001)" as A {
  name = Alice
  role = Admin
  active = true
}
object "Bob (ID:002)" as B {
  name = Bob
  role = Editor
  active = false
}
object "Order #20261020" as O {
  id = 20261020
  status = shipped
}

A --> O : places
B --> O : reviews
@enduml

PlantUML’s object keyword declares an instance directly, as assigns a shorthand alias, and curly braces hold key = value pairs. Arrows (-->) connect instances; instance labels can be quoted with a display name after the colon.

Mermaid 版

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classDiagram
    class User {
        +String name
        +String role
        +Boolean active
    }
    class Order {
        +Integer id
        +String status
    }
    User "2" o-- "1" Order : places
    User "1" ..> "1" Order : reviews
    note for User "Alice(role=Admin)\nBob(role=Editor)"
    note for Order "id=20261020\nstatus=shipped"

Mermaid has no dedicated object diagram type; it uses classDiagram with note blocks to attach instance values. Multiplicity labels ("2" / "1") indicate multiple instances of the same class, and property lists show field names.

对比 / Side-by-side

小结 / Wrap-up

PlantUML’s objectdiagram is the most direct way to express runtime snapshots, ideal for debugging and traceability. Mermaid approximates object diagrams with class diagrams and note annotations, leveraging its unified renderer at the cost of less precise instance representation. Using both together — class diagrams for design, object diagrams for runtime inspection — covers the full modeling lifecycle.