Decorator Pattern

Tags
Design Pattern, Java

Introduction

• Purpose
  • Allows for the dynamic wrapping of objects in order to modify their existing responsibilities and behaviors.
• Use When
  • Object responsibilities and behaviors should be dynamically modifiable.
  • Concrete implementions should be decoupled from responsibilities and behaviors.
  • Subclassing to achieve modification is impratical or impossible.
  • Specific functionality should not reside high in the object hierarchy.
  • A lot of little objects surrounding a concrete implementation is acceptable.
• Characteristics
  • We can use one or more decorators to wrap an object.
  • Decorators have the same super type as the objects they decorate.
  • We can decorate objects dynamically at runtime with as many decorators as we want.
• Design Principle
  • Open-Closed Principle (OCP)
• Mechanisms
  • Uses object composition and delegation
  • Decorator class mirrors the type of components they are decorating.
    • We can wrap a component with any number of decorators
• Advantages
  • attaches additional responsibilities to an object dynamically
  • flexible alternative to subclassing for extending functionality
• Disadvantages
  • can generate a lot of small classes (ex. Java I/O)
  • hard to understand if not familiar with the pattern

How to Use (Example)

• Decorated Class

  public abstract class Beverage {
      protected String description = "Unknown Beverage";
  
      public String getDescription() {
          return description;
      }
  
      public abstract double cost();
  }
  
  public class Espresso extends Beverage {
      public Espresso() {
          description = "Espresso";
      }
  
      public double cost() {
          return 1.99;
      }
  }
  

• Decorator Class

  public abstract class CondimentDecorator extends Beverage {
      protected Beverage beverage;
  
      public abstract String getDescription();
  }
  
  public class Mocha extends CondimentDecorator {
      public Mocha(Beverage beverage) {
          this.beverage = beverage;
      }
  
      public String getDescription() {
          return beverage.getDescription() + ", Mocha";
      }
  
      public double cost() {
          return 0.2 + beverage.cost();
      }
  }
  
  public class Whip extends CondimentDecorator {
      public Mocha(Beverage beverage) {
          this.beverage = beverage;
      }
  
      public String getDescription() {
          return beverage.getDescription() + ", Whip";
      }
  
      public double cost() {
          return 0.3 + beverage.cost();
      }
  }
  

• Main

  public class Main {
      public static void main(String args[]) {
          Beverage beverage = new Espresso();
          beverage = new Mocha(beverage);
          beverage = new Mocha(beverage);
          beverage = new Whip(beverage);
          System.out.println(beverage.getDescription() + " $" + beverage.cost());
      }
  }