1. Chapter 6 - Interacting Objects: Newton’s Lab
Bruce Chittenden

2. 6.1 The Starting Point: Newton’s Lab

3. Exercise 6.1
Right Click on space

4. Exercise 6.2
Sun and Planet

5. Exercise 6.2
Sun and Two Planets

6. Exercise 6.2
Sun, Planet, and Moon

7. Exercise 6.3

8. Exercise 6.4

9. Exercise 6.5 Space Class
import greenfoot.*; // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)
import java.awt.Color;
/**
* Space. The final frontier. *
Michael Kolling
1.0 */
public class Space extends World {

10. Exercise 6.5 Space Constructor
/**
* Create space. */
public Space() {
super(960, 620, 1);
// Uncomment one of the following method calls
// if you want the objects created automatically:
//sunAndPlanet();
//sunAndTwoPlanets();
//sunPlanetMoon(); }

11. Exercise 6.5 SunAndPlanet ()
/**
* Set up the universe with a sun and a planet. */
public void sunAndPlanet() {
removeAllObjects();
addObject (new Body (50, 240.0, new Vector(270, 0.03), new Color(255, 216, 0)), 460, 270);
addObject (new Body (20, 4.2, new Vector(90, 2.2), new Color(0, 124, 196)), 695, 260); }

12. Exercise 6.5 sunAndTwoPlanets ()
/**
* Set up the universe with a sun and two planets. */
public void sunAndTwoPlanets() {
removeAllObjects();
addObject (new Body (50, 240.0, new Vector(270, 0.0), new Color(255, 216, 0)), 460, 310);
addObject (new Body (20, 4.2, new Vector(90, 2.2), new Color(0, 124, 196)), 695, 300);
addObject (new Body (24, 4.6, new Vector(270, 1.8), new Color(248, 160, 86)), 180, 290); }

13. Exercise 6.5 sunPlanetMoon ()
/**
* Set up the universe with a sun, a planet, and a moon. */
public void sunPlanetMoon() {
removeAllObjects();
addObject (new Body (50, 240.0, new Vector(270, 0.0), new Color(255, 216, 0)), 460, 270);
addObject (new Body (20, 4.2, new Vector(90, 2.2), new Color(0, 124, 196)), 720, 260);
addObject (new Body (5, 0.8, new Vector(90, 3.25), new Color(240, 220, 96)), 748, 260); }

14. Exercise 6.5 removeAllObjects ()
/**
* Remove all objects currently in the world. */
private void removeAllObjects() {
removeObjects (getObjects(Actor.class)); }

15. 6.2 Helper Classes: SmoothMover and Vector
Some Helper Classes
Counter
Explosion
Mover
Plotter
Rotator
Slider
Vector
Wander

16. SmoothMover
SmoothMover can, for example, have the x-coordinate If we now move this actor along the x-coordinate in increments of 0.6, its successive locations will be
12.3, 12.9, 13.5, 14.1, 14.7, 15.3, 15.9, 16.5, 17.1, . . .
12, , 14, 14, , 15, 16, , 17,
and so on. We will see the actor on screen at rounded x-coordinates.

17. Cannot Create Objects for This Class, No Constructor
Abstract Classes
public abstract class SmoothMover extends Actor {
private Vector movement;
private double exactX;
private double exactY;
public SmoothMover()
this(new Vector()); }
/**
* Create new thing initialised with given speed. */
public SmoothMover(Vector movement)
this.movement = movement;
Cannot Create Objects for This Class, No Constructor

18. Exercise 6.6 accelerate addForce getExactX getExactY getMovement
getSpeed
move
setLocation

19. Exercise 6.7
/**
* Set the location using exact (double) co-ordinates. */
public void setLocation(double x, double y) {
exactX = x;
exactY = y;
super.setLocation((int) x, (int) y); }
* Set the location of this actor. Redefinition of the standard Greenfoot
* method to make sure the exact co-ordinates are updated in sync.
public void setLocation(int x, int y)
super.setLocation(x, y);
Can have the same name, as long as their parameters are different. This means that the methods (or constructors) have different signatures.

20. Overloading
It is perfectly legal to have two methods that have the same name, as long as their parameter lists are different. This is called Overloading (The name of the method is Overloaded - it refers to more than one method.)

21. Vectors dy dx Polar Representation = length and direction
Cartesian Representation = dx and dy

22. Exercise 6.8

23. Exercise 6.8

24. Exercise 6.9 Which methods can be called thru the object’s menu?
Which methods cannot?
Which

25. Exercise 6.9 public abstract class SmoothMover extends Actor {
private Vector movement;
private double exactX;
private double exactY;
public SmoothMover()
this(new Vector()); }
/**
* Create new thing initialised with given speed. */
public SmoothMover(Vector movement)
this.movement = movement;
smoothMover is not Callable since it is declared as an Abstract Class

26. Exercise 6.10
/**
* Construct a Body with default size, mass, movement and color. */
public Body() {
this (20, 300, new Vector(0, 1.0), defaultColor); }
* Construct a Body with a specified size, mass, movement and color.
public Body(int size, double mass, Vector movement, Color color)
this.mass = mass;
addForce(movement);
GreenfootImage image = new GreenfootImage (size, size);
image.setColor (color);
image.fillOval (0, 0, size-1, size-1);
setImage (image);
Body Class has Two Constructors This is Another Example of Overloading

27. 6.3 The Existing Body Class
A constructor without any parameters is also called a Default Constructor
The Body class has two constructors. One constructor has no parameters and the other constructor has four parameters. The default constructor makes it easy for us to create bodies interactively without having to specify all the details.

28. Code 6.1 public class Body extends SmoothMover { /**
* Construct a Body with default size, mass, movement and color. */
public Body()
this (20, 300, new Vector(0, 1.0), defaultColor); }
* Construct a Body with a specified size, mass, movement and color.
public Body(int size, double mass, Vector movement, Color color)
this.mass = mass;
addForce(movement);
GreenfootImage image = new GreenfootImage (size, size);
image.setColor (color);
image.fillOval (0, 0, size-1, size-1);
setImage (image);

29. this this (20, 300, new Vector (90, 1.0), defaultColor );
This line looks almost like a method call, except it uses the keyword this instead of a method name. Using this the constructor executes the other constructor, the one with parameters.
this.mass = mass;
When we write this.mass, we specify that we mean the mass field of the current object.

30. Exercise 6.11 Remove the “this” /**
* Construct a Body with default size, mass, movement and color. */
public Body() {
this (20, 300, new Vector(0, 1.0), defaultColor); }
* Construct a Body with a specified size, mass, movement and color.
public Body(int size, double mass, Vector movement, Color color)
this.mass = mass;
addForce(movement);
GreenfootImage image = new GreenfootImage (size, size);
image.setColor (color);
image.fillOval (0, 0, size-1, size-1);
setImage (image);
Remove the “this”

31. Exercise 6.11 Does is Compile? Does it Execute? What does the Code do?
What is its effect?

32. Code 6.2
private static final double GRAVITY = 5.8;
private static final Color defaultColor = new Color(255, 216, 0);
The term final defines this field to be a constant. A constant has similarities to a field, in that we can use the name in our code to refer to its value, but the value can never change (it is constant).
The effect of the static keyword is that this constant is shared between all actors of this class.

33. 6.4 First Extension: Creating Movement
The first obvious experiment is to make bodies move.
SmootherMover Class has a move () method and since Body is a SmoothMover, it too has access to this method.

34. Exercise 6.12 /** * Act. That is: apply the gravitation forces from
* all other bodies around, and then move. */
public void act() {
move(); }
Add the move () Method to the act () Method of the Body Class.”

35. Exercise 6.12 The Default Speed is 1.0
The Default Direction is 0 Degrees

36. Exercise 6.13
Multiple Objects Move from Left to Right at a Constant Speed

37. Exercies 6.14 The Sun Does Not Appear to Move
The Earth Moves Straight Down Rapidly

38. Exercise 6.14

39. Exercise 6.14

40. Exercise 6.15
/**
* Construct a Body with default size, mass, movement and color. */
public Body() {
this (20, 300, new Vector(-180, 1.0), defaultColor); }
Change the Direction in the Default Constructor from 0 to -180

41. Exercise 6.15
-90
270
-180
180
360
-270 90

42. 6.5 Using Java Library Classes
import java.awt.Color;
java.awt Package
Color Class
new Color (248, 160, 86) R G B

43. java.awt.Color

44. Exercise 6.16

45. Exercise 6.17

46. 6.6 Adding Gravitational Force
We can give an outline of the task in pseudo-code.
apply forces from other bodies:
get all other bodies in space;
for each of those bodies: {
apply gravity from that body to our own; }

47. Code 6.3 /** * Act. That is: apply the gravitation forces from
* all other bodies around, and then move. */
public void act() {
move (); }

48. Code 6.4 /*
* Act. For a body, that is: apply all the gravitation forces from
* all other bodies around, and then move. */
public void act() {
applyForces ();
move (); }
* Apply the forces of gravity from all other celestial bodies in this universe
private void applyForces()
// work to do here

49. Private Methods
Methods can be public or private. When methods are intended to be called from outside the class (either interactively by a user or from another class), the they must be public. When methods are intended to be called only from other methods within the same class, then they should be declared private.

50. Exercise 6.18

51. Exercise 6.18 getObjects ( java.lang.Class cls )
getObjectsAt ( int x, int y, java.lang.Class cls )
numberOfObjects ( )
removeObject ( Actor object)
removeObjects ( java.util.Collection objects )

52. java.util.List getObjects ( java.lang.Class cls )
Gives a list of all objects in the world of a particular class
getObjects ( Body.class )
Gets a list of all the objects in the world of class Body
getObjects ( null )
The keyword null is a special expression that means nothing, or no object

53. getWorld().getObjects
World getWorld()
There is a method in the Actor class that gives us access to the World class. It signature is
World getWorld ().
getWorld().getObjects ( Body.class )
getObjects is a method of the World class, so it must be called on a World object. We will write our code in the Body class, so we must first obtain the World object to call this method on.

54. 6.7 The List Type java.util.List
The List type is not a class, but an interface. Interfaces are a Java construct that provides an abstraction over different possible implementing classes.

55. Exercise 6.19

56. Exercise 6.19 add ( E o ) add ( int index, E element )
addAll ( Collection <? extends E> c )
addAll ( int index, Collection< ? extends E> c )
remove ( int index )
remove ( Object o )
size ()

57. Interface List<E>
Exercise 6.20
Interface List<E>

58. Interface List<E>
Interface is in the place of class and the notation <E> is after the type name. This is the Generic Type. This means that the type List needs an additional type specified as a parameter. This second type specifies the type of the elements held within the list.

59. Interface List<E>
List<String>
A list of Strings
List<Actor>
A list of Actors
List<Body> bodies
A list of bodies
List<Body> bodies = getWorld().getobjects (Body.class):
The variable bodies holds a list of all the bodies that exist in the World.

60. Code 6.5 import java.util.List; /*
* Apply the forces of gravity from all other celestial bodies in this universe */
private void applyForces() {
List<Body> bodies = getWorld().getObjects(Body.class); }

61. 6.8 The for-each Loop for ( Element Type variable : collection ) {
Java has a specialized loop for stepping through every element of a collection. It is called a for-each loop .
for ( Element Type variable : collection ) {
statements; }

62. The for-each Loop for ( Body body : bodies ) { body.move(); }
for each body in bodies do:
body = first element from ‘bodies’;
execute loop statements;
body = second element for ‘bodies’;
body = third element from ‘bodies’;

63. Code 6.6 /*
* Apply the forces of gravity from all other celestial bodies in this universe */
private void applyForces() {
List<Body> bodies = getWorld().getObjects(Body.class);
for (Body body : bodies)
if (body != this)
applyGravity (body); }
* Apply the gravity force of a given body to this one.
private void applyGravity (Body other)
// work to do here

64. 6.9 Applying Gravity Newton's Law of Universal Gravitation
states that every massive particle in the universe attracts every other massive particle with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

65. Code 6.7 /* * Apply the gravity force of a given body to this one. */
private void applyGravity(Body other) {
double dx = other.getExactX() - this.getExactX();
double dy = other.getExactY() - this.getExactY();
Vector force = new Vector (dx, dy);
double distance = Math.sqrt (dx*dx + dy*dy);
double strength = GRAVITY * this.mass * other.mass / (distance * distance);
double acceleration = strength / this.mass;
force.setLength (acceleration);
addForce (force); }

66. Pythagorean Theorem
a2 + b2 = c2

67. Exercise 6.21

68. Math Class sqrt

69. Exercise 6.22
max ( int a, int b )

70. Exercise 6.22

71. Acceleration force acceleration = mass
Once we have calculated the acceleration, we can set our force vector to the correct length and add this vector to the movement of our body.

72. Exercise 6.23 a2 + b2 = c2 dx2 + dy2 = distance2
(other.getExactX()-this.getExactX())2 + (other.getExactY()-this.getExactY())2 = distance2
distance = Math.sqrt ( (other.getExactX()-this.getExactX())2 + (other.getExactY()-this.getExactY())2 )

73. Exercise 6.23 G mass1 X mass2 force = distance
force = (mass1 * mass2 / distance2 ) * G
force = ( this.mass * other.mass / distance *distance ) * GRAVITY

74. Exercise 6.24

75. Exercise 6.24

76. Exercise 6.24

77. Exercise 6.25
If the Gravitational Constant is set Higher than 5.8, planets spiral into one another, and if it is set lower, planets fly off into deep space.

78. Exercise 6.26
size
mass
vector
movement x y r g b

79. Exercise 6.26
addObject (new Body (100, 500.0, new Vector(270, 0.0), new Color(255, 216, 0)), 460, 270);
addObject (new Body (20, 4.2, new Vector(90, 2.2), new Color(0, 124, 196)), 720, 260);
addObject (new Body (5, 0.8, new Vector(90, 3.25), new Color(240, 220, 96)), 748, 260);

80. Exercise 6.27
All Systems That I Tested Were Very Unstable

81. 6.11 Gravity and Music
The idea to add sound to a gravity project was inspired by Kepler’s Orrery (see )

82. Exercise 6.28

83. 6.12 Summary of Programming Techniques
One of the most important topics in the chapter was the use of additional classes from the Standard Java Class Library, Color, Math, and List.
Another new addition was the use of a new loop the for-each loop. This loop is used to do something to every element of a Java collection such as a List. If we need an index, or a loop independent of a collection, then we must use a for loop or a while loop instead.

84. Concept Summary