Design Patterns

• Design Principles: General Guidelines
• Design Patterns: Specific Solutions
• Design Patterns are expressed by a definition, a class diagram and collected into a catalog of patterns
• Some references:
  • https://java-design-patterns.com/patterns/
  • https://github.com/topics/low-level-design

Popular Design Patterns

Important

• Singleton (multi-threading is important)
• Factory
• Abstract Factory
• Observer
• Builder

Other

• Decorator
• Adapter
• Strategy
• Facade

Practical Design Patterns

• Singleton
  • property: global access + must have only one instance (hurts flexibility)
  • No true example able to find!! possibly anti-pattern.
  • pseudo examples:
    • logger object (they are flexible hence not singleton)
    • Java Beans (they are restricted to each application context hence not really single instance)
• Builder
  • Lombok’s @Builder annotation
• Decorator (aka Wrapper, Adapter Pattern is also known as Wrapper)
  • java.util.Collections.unmodifiableList(List)
    • convert list to unmodifiable list
• Facade
  • Importing libraries
  • SLF4J, JPA??
• Iterator
  • Interface: java.util.Iterator
    • hasNext(): boolean
    • next(): E
  • For-Each loops use Iterator under the hood for Collections
• Observer/Reactor for Project Reactor