Class diagrams: abstract vs concrete classes

简介 / Introduction

An abstract class cannot be instantiated directly — it defines a skeleton of attributes and methods that its subclasses must implement. A concrete class inherits from the abstract class and provides full implementations. This article shows how to model this pattern in both PlantUML and Mermaid.

PlantUML 版

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abstract class Animal {
  {abstract} +makeSound(): void
  +eat(): void
}

class Dog extends Animal {
  +makeSound(): void
}

class Cat extends Animal {
  +makeSound(): void
}

Animal <|-- Dog
Animal <|-- Cat

The abstract keyword declares the abstract class, and {abstract} marks abstract methods. extends denotes inheritance.

Mermaid 版

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classDiagram
  class Animal {
    <<abstract>>
    +makeSound() void
    +eat() void
  }
  class Dog {
    +makeSound() void
  }
  class Cat {
    +makeSound() void
  }
  Animal <|-- Dog : extends
  Animal <|-- Cat : extends

Mermaid uses the <<abstract>> stereotype label to mark abstract classes, and <|-- for generalization (inheritance).

对比 / Side-by-side

小结 / Wrap-up

PlantUML’s abstract keyword makes abstract class syntax more compact, while Mermaid requires an explicit <<abstract>> label. Both clearly express the core OOP concept: an abstract class defines a contract, and concrete classes provide the implementation.