PlantUML Class Diagram: Design Patterns

Design Patterns in PlantUML Class Diagrams

PlantUML class diagrams are the standard way to document design patterns. Each pattern has a characteristic structure that becomes immediately recognizable in diagram form.

Observer Pattern

@startuml
interface Observer {
  +update(event: Event): void
}

abstract class Subject {
  -observers: List<Observer>
  +subscribe(o: Observer): void
  +unsubscribe(o: Observer): void
  +notify(event: Event): void
}

class EventBus extends Subject {
  +publish(topic: String, data: any): void
}

class Logger implements Observer {
  +update(event: Event): void
}

class MetricCollector implements Observer {
  +update(event: Event): void
}

Subject o-- Observer
@enduml

Strategy Pattern

interface SortStrategy {
  +sort(data: List): List
}

class QuickSort implements SortStrategy
class MergeSort implements SortStrategy
class BubbleSort implements SortStrategy

class Sorter {
  -strategy: SortStrategy
  +setStrategy(s: SortStrategy): void
  +doSort(data: List): List
}

Sorter --> SortStrategy

The Strategy pattern uses a --> association from the context to the strategy interface, with concrete strategies implementing the interface.

Factory Method Pattern

abstract class Creator {
  +{abstract} createProduct(): Product
  +doSomething(): void
}

interface Product {
  +use(): void
}

class ConcreteCreatorA extends Creator {
  +createProduct(): Product
}

class ConcreteProductA implements Product

ConcreteCreatorA ..> ConcreteProductA : creates

The dashed dependency arrow (..>) with "creates" label shows the factory relationship clearly.

When to Use Each Arrow

The key to readable pattern diagrams is using the correct arrow type. Inheritance (<|--) for "is-a", composition (*--) for "has-a and owns", association (-->) for "uses", and dependency (..>) for "creates or depends on temporarily".