Commented the code.

double-dispatch/src/main/java/com/iluwatar/App.java

@@ -3,21 +3,43 @@ package com.iluwatar;
 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("");
     }

double-dispatch/src/main/java/com/iluwatar/FlamingAsteroid.java

@@ -1,5 +1,10 @@
 package com.iluwatar;
 
+/**
+ * 
+ * Flaming asteroid game object
+ *
+ */
 public class FlamingAsteroid extends Meteoroid {
 
 	public FlamingAsteroid(int left, int top, int right, int bottom) {

double-dispatch/src/main/java/com/iluwatar/GameObject.java

@@ -1,5 +1,11 @@
 package com.iluwatar;
 
+/**
+ * 
+ * Game objects have coordinates and some
+ * other status information.
+ *
+ */
 public abstract class GameObject extends Rectangle {
 	
 	private boolean damaged;

double-dispatch/src/main/java/com/iluwatar/Meteoroid.java

@@ -1,5 +1,10 @@
 package com.iluwatar;
 
+/**
+ * 
+ * Meteoroid game object
+ *
+ */
 public class Meteoroid extends GameObject {
 
 	public Meteoroid(int left, int top, int right, int bottom) {

double-dispatch/src/main/java/com/iluwatar/Rectangle.java

@@ -1,5 +1,11 @@
 package com.iluwatar;
 
+/**
+ * 
+ * Rectangle has coordinates and can be checked for overlap against
+ * other Rectangles.
+ *
+ */
 public class Rectangle {
 
 	private int left;

double-dispatch/src/main/java/com/iluwatar/SpaceStationIss.java

@@ -1,5 +1,10 @@
 package com.iluwatar;
 
+/**
+ * 
+ * Space station ISS game object
+ *
+ */
 public class SpaceStationIss extends SpaceStationMir {
 
 	public SpaceStationIss(int left, int top, int right, int bottom) {

double-dispatch/src/main/java/com/iluwatar/SpaceStationMir.java

@@ -1,5 +1,10 @@
 package com.iluwatar;
 
+/**
+ * 
+ * Space station Mir game object
+ *
+ */
 public class SpaceStationMir extends GameObject {
 
 	public SpaceStationMir(int left, int top, int right, int bottom) {

double-dispatch/src/test/java/com/iluwatar/RectangleTest.java

@@ -3,6 +3,11 @@ package com.iluwatar;
 import org.junit.Assert;
 import org.junit.Test;
 
+/**
+ * 
+ * Unit test for Rectangle
+ *
+ */
 public class RectangleTest {
 
 	@Test