1. Problem Solving with Data Structures using Java: A Multimedia Approach
Chapter 14: Using an Existing Simulation Package

2. Chapter Objectives

3. Simulations
“A simulation is a representation of a system of objects in a real or fantasy world. The purpose of creating a computer simulation is to provide a framework in which to understand the simulated situation, for example, to understand the behavior of a waiting line, the workload of clerks, or the timeliness of service to customers. A computer simulation makes it possible to collect statistics about these situations, and to test out new ideas about their organization.”
Adele Goldberg & David Robson, Smalltalk-80: The Language and Its Implementation (Addison-Wesley, 1989)

4. Simulations and Objects
Object-oriented programming was invented, in part, to make simulations easier to build!
The characteristics of objects make them more like real world objects, e.g.,
Each thing knows some stuff and knows how to do some stuff.
Objects get things done by asking each other to do things.
Your internals are private, unless you want to make them otherwise.

5. Continuous vs. Discrete Simulations
Two main kinds of simulations in the world.
Continuous: Each moment of time is simulated.
When every moment counts.
Discrete: Skip to the important moments.
Want to simulate 100 years?

6. Actors: Those that act in the simulations
Actors do things, take time, and request and use resources.
In continuous simulations, actors are told to act().
In discrete event simulations, actors do something, then reschedule themselves in the simulation.l

7. Resources Resources are points of coordination in a simulation.
Examples: A cashier, a library book, a parking space on a ferry, a jelly bean.
Some resources are fixed and others are produced and consumed.
Some resources are renewable and shared.
Others are coordinated.
Example: For a surgeon to do a surgery, the patient must meet the surgeon at the operating table (the resource)

8. When an object has to wait…
What happens if you (or your proxy object) need a resource and it’s not available?
You wait in a queue
A list that is first-in-first-out (FIFO)

9. A simulation is an executed model
Setting up a simulation is a process of modeling the world (real or fantasy) to be simulated.
That model is realized in terms of objects.
We want our model to:
Reflect the world.
Be easy to extend and change.
Some of our modeling techniques:
Aggregation
Generalization and specialization

10. Aggregation Some objects are made up of other objects.
Cars have engines
People have livers and lungs
These internal things are objects, too!
Livers don’t directly mess with the innards of lungs!
We call this aggregation
Putting references to some objects inside of other objects.

11. Generalization and Specialization
There are general and specialized forms of real world objects.
Cells are biological objects that have membranes and a nucleus and mitochondria and…
Blood, lung, and liver cells are all cells but have specialized functions.
The superclass-subclass relationship is a way of modeling general forms of objects and specialized forms of objects

12. Start Greenfoot Click on the Greenfoot icon to start it
The first time it will ask if you want to do the tutorial
After that it will open the last scenario
Click on Scenario and open to open another one
Like balloons
Like wombat
The Greenfoot Tutorial is always available from the Help menu.

13. Objects and Classes Classes define what objects know and can do
There is one Balloon class
Objects do the action
There can be many objects of the same class
There are many balloon objects
Objects
Classes

14. Worlds and Actors Greenfoot has two main classes World Actor
Place to hold and display actors
Has a width and height and cell size
Of type integer (int)
Has a background image
Actor
Actors know how to act
Actors have a x and y location in the world

15. Balloon Code and Doc Double-click on the Balloon class
Select Documentation on the right
You can switch between the source code and documentation

16. Documentation Starts with the class name Then the superclasses
Should be an uppercase letter
Then the superclasses
Then the class definition
And a short description
Author
Constructors
Methods

17. Methods Provide behavior for objects Balloons can act and pop
public is the visibility
Who can see and execute this method
void is the return type
Nothing is returned from the method
The () means that the methods take no values
Select “Source Code” for the Balloon to see the source code.

18. Text Questions:
Which method in what class adds all bombs and dart to the world?
What method in what class increases the score and by how much?
What does an explosion object do when it acts?
What does a balloon object do when it acts?

19. Open the Wombat Scenario
You may need to click the Compile button to update everything if classes looked “hashed”

20. Make a Wombat, and some Leaves
Right click on Wombat class,
Choose New Wombat()
Now, have the wombat act()

21. Current Wombat act() method
Can you see why the Wombat won’t necessarily find all leaves?

22. Create a new method for random walk

23. New Wombat act() method

24. Greenfoot worlds World class Actor class Abstract
Subclass it to create a new class
Actor class
Subclass it to create Wombats and Balloons

25. WombatWorld Class

28. Making a Wall class

29. Use the randomLeaves method as starting place

30. New randomWombats

31. New randomWalls

32. New WombatWorld Constructor

33. New Wombat act() so random turns

34. Teach wombats to avoid walls

35. Questions Does at least one wombat find the leaf?
How long does it take a wombat to find a leaf?
Can you make a smarter wombat?
Can you modify the scenario so that wombats can smell leaves that are within a certain radius of their current position?

36. Making Breakout

37. Making the BreakoutWorld
import greenfoot.*; // (World, Actor, GreenfootImage, and Greenfoot) import java.awt.Color; import java.util.List; public class BreakoutWorld extends World { /////////////////// constants ////////////////////////// /** the width of the bricks in pixels (cells) */ public static final int BRICK_WIDTH = 36; /** the height of the bricks in pixels (cells) */ public static final int BRICK_HEIGHT = 8; /** the distance between bricks in pixels (cells) */ public static final int BRICK_SEP = 4; /** the number of bricks per row */ public static final int NUM_BRICKS_PER_ROW = 10; /** distance from the top edge in pixels (cells) */ public static final int BRICK_Y_OFFSET = 70; /** the number of pixels per cell */
Making the BreakoutWorld

38. public static final int RESOLUTION = 1; /
public static final int RESOLUTION = 1; /** world width in pixels (cells) */ public static final int WIDTH = (BRICK_WIDTH + BRICK_SEP) * NUM_BRICKS_PER_ROW + BRICK_SEP; /** world height in pixels (cells) */ public static final int HEIGHT = 600; /** number of rows of bricks */ public static final int NUM_ROWS = 10; /** the colors to use for each row of bricks */ public static final Color[] colorArray = {Color.RED, Color.RED, Color.ORANGE, Color.ORANGE, Color.GREEN,Color.GREEN, Color.YELLOW,Color.YELLOW, Color.CYAN, Color.CYAN}; ////////////////// instance fields /////////////////////////////// /** the number of balls created in the game so far */ private int numBalls = 0; /** a message displayed for the user */ private Message message = null;

39. Constructor

40. setUpBreakout()

41. Create and set background image

42. setUpBricks()

43. Creating the brick
public class Brick extends Actor { /** the width of the brick */ private int width = 36; /** the height of the brick */ private int height = 8; /** the color of the brick */ private Color color;
/////////// constructor //////////////// /** * Constructor that takes the color for the brick theColor the color to use for this brick */ public Brick(Color theColor) { color = theColor; updateImage(); }

44. Constructor for Brick
/** * Constructor that takes the width, height, and color for this * brick. */ public Brick(int theWidth, int theHeight, Color theColor) { width = theWidth; height = theHeight; color = theColor; updateImage(); }

45. What Bricks do /** * Method to act during a time step */
public void act() {}
* Method to create the image and set it for this brick.
* If you change the width, height, or color invoke this
* method.
public void updateImage() {
GreenfootImage image = new GreenfootImage(width,height);
image.setColor(this.color);
image.fillRect(0,0,width,height);
setImage(image); }
What Bricks do

46. public class Ball extends Actor { //////////// fields ///////////////////////////// /** the radius of this ball */ private int radius = 10; /** the width of this ball (diameter) */ private int width = radius * 2; /** the color of the ball */ private Color color = Color.BLACK; /** the amount of change in x during each act */ private int velX; /** the amount of change in y during each act */ private int velY = 3;
Ball class

49. End of Ball class

50. Making newBall in BreakoutWorld

51. checkIfWon()
/** * Method to check if the game is over and if so tell the user * and stop the simulation */ public void checkIfWon() { List brickList = this.getObjects(Brick.class); if (brickList.size() == 0) message.setText("You Won!!!!!!"); Greenfoot.stop(); }