Decorator Pattern



The Decorator Pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality. 
  • Decorators have the same supertype as the objects they decorate.
  • You can use one or more decorators to wrap an object.
  • Given that the decorator has the same supertype as the object it decorates, we can pass around a decorated object in place of the original (wrapped) object.
  • The decorator adds its own behavior either before and/or after delegating to the object it decorates to do the rest of the job.
  • Objects can be decorated at any time, so we can decorate objects dynamically at runtime with as many decorators as we like.
Example

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public abstract class Item {
 public String name;
 public Item(){
  name = "No Name";
 }
 public String getName(){
  return name;
 }
 public abstract int getCost();
}
=========
public abstract class ItemDecorator extends Item {
 public abstract String getName();
}
=========
public class Dinner extends Item{
 Dinner(){
  name = "Dinner";
 }
 public String getName(){
  return name;
 }
 @Override
 public int getCost() {  
  return 110;
 }
}
==========
public class IceCream extends ItemDecorator{
 Item item;
 IceCream(Item item){
  this.item = item;
 }
 @Override
 public String getName() {
  return item.getName()+ " + Ice-Cream";
 }
 @Override
 public int getCost() {
  return item.getCost() + 46;
 }
}
==========
public class Juice extends ItemDecorator{
 Item item;
 Juice(Item item){
  this.item = item;
 }
 @Override
 public String getName() {
  return item.getName()+" + Juice";
 }
 @Override
 public int getCost() {
  return item.getCost() + 40;
 }
}
==========
public class DecoratorFlight {
 public static void main(String[] bag){
  Item dinner = new Dinner();
  dinner = new Juice(dinner);
  dinner = new IceCream(dinner);
  
  String names = dinner.getName();
  int totalCost = dinner.getCost();
  
  System.out.println(names+" = Rs: "+totalCost);
 }
}