Decorator Design Pattern

package com.javaletters.sample.designpattern;

public interface Icecream {
  public String makeIcecream();
}

The above is an interface depicting an icecream. I have kept things as simple as possible so that the focus will be on understanding the design pattern. Following class is a concrete implementation of this interface. This is the base class on which the decorators will be added.

package com.javaletters.sample.designpattern;

public class SimpleIcecream implements Icecream {

  @Override
  public String makeIcecream() {
    return "Base Icecream";
  }

}

Following class is the decorator class. It is the core of the decorator design pattern. It contains an attribute for the type of interface. Instance is assigned dynamically at the creation of decorator using its constructor. Once assigned that instance method will be invoked.

package com.javaletters.sample.designpattern;

abstract class IcecreamDecorator implements Icecream {

  protected Icecream specialIcecream;

  public IcecreamDecorator(Icecream specialIcecream) {
    this.specialIcecream = specialIcecream;
  }

  public String makeIcecream() {
    return specialIcecream.makeIcecream();
  }
}

Following two classes are similar. These are two decorators, concrete class implementing the abstract decorator. When the decorator is created the base instance is passed using the constructor and is assigned to the super class. In the makeIcecream method we call the base method followed by its own method addNuts(). This addNuts() extends the behavior by adding its own steps.

package com.javaletters.sample.designpattern;

public class NuttyDecorator extends IcecreamDecorator {

  public NuttyDecorator(Icecream specialIcecream) {
    super(specialIcecream);
  }

  public String makeIcecream() {
    return specialIcecream.makeIcecream() + addNuts();
  }

  private String addNuts() {
    return " + cruncy nuts";
  }
}

package com.javaletters.sample.designpattern;

public class HoneyDecorator extends IcecreamDecorator {

  public HoneyDecorator(Icecream specialIcecream) {
    super(specialIcecream);
  }

  public String makeIcecream() {
    return specialIcecream.makeIcecream() + addHoney();
  }

  private String addHoney() {
    return " + sweet honey";
  }
}

Execution of the decorator pattern

I have created a simple icecream and decorated that with nuts and on top of it with honey. We can use as many decorators in any order we want. This excellent flexibility and changing the behaviour of an instance of our choice at runtime is the main advantage of the decorator design pattern.

package com.javaletters.sample.designpattern;

public class TestDecorator {

  public static void main(String args[]) {
    Icecream icecream = new HoneyDecorator(new NuttyDecorator(new SimpleIcecream()));
    System.out.println(icecream.makeIcecream());
  }

}

Output

Base Icecream + cruncy nuts + sweet honey