Class diagrams: stereotypes and interfaces

简介 / Introduction

Class diagrams are the core of object-oriented modeling. Beyond ordinary classes, stereotypes (such as <<interface>>, <<abstract>>, <<service>>) mark special semantic roles. The corresponding UML concepts are Interface and Abstract Class. This post shows how to write these in both PlantUML and Mermaid.

PlantUML 版

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@startuml
skinparam classAttributeIconSize 0

interface "<<interface>>\nIProduct" as IProduct {
  + getPrice(): double
  + getName(): string
}

abstract class "<<abstract>>\nItem" as Item {
  + id: int
  + calculateDiscount(): double
}

class "Product" as Product {
  + price: double
  + name: string
  + getPrice(): double
  + getName(): string
}

class "Service" as Service {
  + fee: double
  + getPrice(): double
}

IProduct <|.. Product : implements
IProduct <|.. Service : implements
Item <|-- Product : extends
@enduml

Use the interface keyword, or annotate a class name with a stereotype. <<abstract>> is declared with abstract class. Implementation uses the dashed triangle <|..; inheritance uses the solid triangle <|--.

Mermaid 版

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classDiagram
    class IProduct {
        <<interface>>
        + getPrice(): double
        + getName(): string
    }

    class Item {
        <<abstract>>
        - id: int
        + calculateDiscount(): double
    }

    class Product {
        + price: double
        + name: string
        + getPrice(): double
        + getName(): string
    }

    class Service {
        + fee: double
        + getPrice(): double
    }

    IProduct <|.. Product : implements
    IProduct <|.. Service : implements
    Item <|-- Product : extends

In Mermaid’s classDiagram, annotate a class with <<interface>> or <<abstract>> inside the class body. Implementation uses <|.. and inheritance uses <|--. Note Mermaid only supports + (public) and - (private) visibility modifiers.

对比 / Side-by-side

小结 / Wrap-up

Stereotypes distinguish roles like interface, abstract class, and service in UML class diagrams. PlantUML renders stereotypes more richly (directly on the class name), while Mermaid uses inline {<<interface>>} annotations. Both engines share the same arrow syntax for implementation and inheritance. Choose based on your team’s tooling and the level of visual customization needed.