content: package com.iluwatar.doubledispatch;
  
  import java.util.ArrayList;
  import java.util.List;
  
  /**
   * 
   * When a message with a parameter is sent to an object, the resultant behaviour is defined by the 
   * implementation of that method in the receiver. Sometimes the behaviour must also be determined 
   * by the type of the parameter.
   * 
   * One way to implement this would be to create multiple instanceof-checks for the methods parameter.
   * However, this creates a maintenance issue. When new types are added we would also need to change
   * the method's implementation and add a new instanceof-check. This violates the single responsibility
   * principle - a class should have only one reason to change.
   * 
   * Instead of the instanceof-checks a better way is to make another virtual call on the parameter
   * object. This way new functionality can be easily added without the need to modify existing
   * implementation (open-closed principle).
   * 
   * In this example we have hierarchy of objects (GameObject) that can collide to each other. Each
   * object has its own coordinates which are checked against the other objects' coordinates. If
   * there is an overlap, then the objects collide utilizing the Double Dispatch pattern.
   *
   */
  public class App {
  	
      public static void main( String[] args ) {
      	// initialize game objects and print their status
      	List<GameObject> objects = new ArrayList<>();
      	objects.add(new FlamingAsteroid(0, 0, 5, 5));
      	objects.add(new SpaceStationMir(1, 1, 2, 2));
      	objects.add(new Meteoroid(10, 10, 15, 15));
      	objects.add(new SpaceStationIss(12, 12, 14, 14));
      	objects.stream().forEach(o -> System.out.println(o));
      	System.out.println("");
      	
      	// collision check
      	objects.stream().forEach(o1 -> objects.stream().forEach(o2 -> { if (o1 != o2 && o1.intersectsWith(o2)) o1.collision(o2); } ));
      	System.out.println("");
      	
      	// output eventual object statuses
      	objects.stream().forEach(o -> System.out.println(o));
      	System.out.println("");
      }
  }
  