GridWorld User Guide

1. Your compiler or development environment, to compile your program
2. The GridWorld GUI, to run your program

import com.collegeboard.gridworld.actor.ActorWorld;

public
    class MyWorld extends ActorWorld
{
 public MyWorld()

    {
 // configure my world
 add(new MyCritter());
 }

    
 public static void main(String[] args)
 {
 // construct
    and show my world
 new MyWorld().show();
 }
}

Exploring Object State

Prerequisites:

• Instance fields
• The if statement
  

Activity 1: Tracing a Square

import com.collegeboard.gridworld.actor.Critter;

        
      public void act()
  
      {
      if (steps < SIDE_LENGTH)

      {
 move();
 steps++;
 }
 else
 {

      turn();
 turn();
 steps = 0;
 }

         }
      

    

Activity 2: Supplying a Construction Parameter

import com.collegeboard.gridworld.actor.Critter;

When you compile this class and select World -> Load Project, you don't get a SquareDancer object. The GridWorld doesn't know how to construct one since it doesn't know which value of n you might want. 

This is not a major obstacle. Simply click on an empty cell and select the SquareDancer(int) constructor. Then specify a value.

Activity 3: Making a World

When you design new actor classes, you may want to specify a world for them to live in. Extend the ActorWorld class. In the constructor of your class, add actors at the desired locations.

import com.collegeboard.gridworld.actor.ActorWorld;
import
    com.collegeboard.gridworld.grid.Location;
import
    java.awt.Color;

public class SquareDancerWorld extends
    ActorWorld
{
 public SquareDancerWorld()
 {

    SquareDancer alice = new SquareDancer(6);

    alice.setColor(Color.ORANGE);
 SquareDancer bob = new
    SquareDancer(3);
 add(new Location(7, 8), alice);
 add(new
    Location(5, 5), bob);
 }
}

import com.collegeboard.grid.BoundedGrid;
import
    com.collegeboard.gridworld.actor.ActorWorld;

public class
    SquareDancerWorld extends ActorWorld
{
 public
    SquareDancerWorld()
 {
 setGrid(new BoundedGrid(20,
    20));
 . . .
 }
}

import com.collegeboard.grid.UnboundedGrid;
import
    com.collegeboard.gridworld.actor.ActorWorld;

public class
    SquareDancerWorld extends ActorWorld
{
 public
    SquareDancerWorld()
 {
 setGrid(new
    UnboundedGrid());
 . . .
 }
}

Exploring Loops

Prerequisites:

• The for statement
  

Activity 1: Placing Rocks

You need two nested loops. The outer loop traverses the n rows:

for (int i = 1; i <= n; i++)

For each row, you place 2n + 1 rocks.

for (int i = 1; i <= n; i++)
 for (int j = 1; j <= 2 * n +
    1; j++)

public class TriangleWorld extends ActorWorld
{
 public
    TriangleWorld()
 { 
 for (int i = 1; i <= n; i++)
 for
    (int j = 1; j <= 2 * n + 1; j++) }
 {
 int row = i;
 int
    col = n - i + j;
 add(new Location(row, col), new Rock());

    }
 }
}

Activity 2: Stepping through a Loop

public class TriangleWorld extends StepWorld
{

    public void run()
 { 
 for (int i = 1; i <= n;
    i++)
 for (int j = 1; j <= 2 * n + 1; j++) }
 {
 int row
    = i;
 int col = n - i + j;
 add(new Location(row, col), new
    Rock());
 pause();
 }
 }
}

pause("i=" + i + ",j=" + j);

Exploring Inheritance

Prerequisites:

• The ArrayList class
  
• Java syntax for inheritance

Activity 1: Eaters

• You may override eat and move, but don't override act.
  
• In eat, don't be greedy. You may remove at most one edible object from the grid.
• In eat, don't move. Stay where you are, look around you, and eat if you find something edible.
  
• In move, don't eat. Just move to another location.
• In eat and move, look at your neighboring locations only. No peeking elsewhere . . .
  

Activity 2: Smart Eaters

public class SmartRockHound extends RockHound
{
 public
    SmartRockHound(Color color)
 {
 super(color);

    visitedLocations = new ArrayList<Location>(); 
 }
 

    public void move()
 {

    visitedLocations.add(getLocation());
 for (Location loc :
    getEmptyNeighborLocations())
 {
 if
    (!visitedLocations.contains(loc))
 {
 moveTo(loc);

    return;
 }
 }
 super.move();
 }
 
 private
    ArrayList<Location> visitedLocations;
}

Activity 3: Cheaters and Honest Eaters

• the number of objects in the grid before the call
• the position of this actor before the call

• If the number of objects in the grid has decreased by more than one, this actor was too greedy.
• If the number of objects in the grid has decreased by one, and the position has changed, this actor has both eaten and moved--a crime.
• If the actor has moved beyond the neighbors of the old location, it went too far.
  

Exploring Recursion

Activity 1: A triangle of rocks

TriangleWalker(int size)

public void act()
 {
 if (steps > 0) 
 {

    Grid<Actor> gr = getGrid();
 Location loc =
    getLocation();
 moveTo(gr.getNeighborLocation(loc, Grid.EAST));

    new Rock().putInGrid(gr, loc);
 steps--; 
 } 

    }

buddy = new TriangleWalker(size - 1);

This is a recursive solution. To do its job, the TriangleWalker asks another TriangleWalker to carry out a simpler instance of the job.

Here is the complete class: TriangleWalker.java

Exercise: Implement a triangle walker that makes a triangle like this:

*
 ***
*****

Activity 2: One-eyed pyramids

Here is the complete class: RockFinder.java

Other Worlds

Prerequisites:

• Inheritance
• Interfaces

It is easy to build other worlds, for example board games that live as a grid. Here is an example: a tile game to practice one's memory.

The game board contains covered tiles. The player's job is to uncover matching pairs. Click on a tile to flip it. Then click on another. If both tiles have the same color, they both stay exposed. If not, they are both covered again.



To implement this game, we don't need to subclass Actor . Simply provide a Tile class with a getColor and a flip method. The getColor method is called by the GridWorld framework to apply the correct color. Return black if the tile is covered and the actual color when it is exposed.

public class Tile
{
 private Color color;
 private
    boolean up;
 
 public Tile(Color color)
 {
 up =
    false;
 this.color = color;
 }

 public Color
    getColor()
 {
 if (up)
 return color;
 else 

    return Color.BLACK;
 }

 public void setColor(Color
    color)
 {
 this.color = color;
 }
 
 public void
    flip()
 {
 up = !up;
 }
}

The game class extends AbstractWorld<Tile> since the world contains a grid of tiles. The constructor populates the grid, and the locationClicked method is called whenever the user clicks on a location. 

public class TileGame extends AbstractWorld<Tile>
{

    public TileGame()
 {
 Color[] colors = 
 { 

    Color.RED, Color.BLUE, Color.GREEN, Color.CYAN,
 Color.PINK,
    Color.ORANGE, Color.GRAY, Color.MAGENTA,
 Color.WHITE,
    Color.YELLOW
 };
 for (Color color : colors)
 {

    add(new Tile(color));
 add(new Tile(color));
 }

    setMessage("Click on the first tile");
 }

 public boolean
    locationClicked(Location loc)
 {
 Grid<Tile> gr =
    getGrid();
 Tile t = gr.get(loc);
 if (t != null) 
 {
    
 t.flip(); 
 if (up == null)
 {
 setMessage("Click on
    the second tile");
 up = loc;
 }
 else
 {
 Tile
    first = gr.get(up);
 if
    (!first.getColor().equals(t.getColor()))
 {
 first.flip();

    t.flip();
 }
 setMessage("Click on the first tile");
 up =
    null;
 } 
 
 }
 return true; 
 }
 

    private Location up; 
}

The locationClicked method returns true to indicate to the framework that the game itself handles clicks. (Otherwise the framework would try to manipulate or add tiles.)

Note that the Step and Run buttons don't do anything in this game. 

Appendix: Installing the JAR file

Eclipse

You need to do the following for every project.

1. Start a project as usual
2. Select the project in the "Package explorer" window
3. Select Project -> Properties from the menu
4. Click on Java Build Path and select the Libraries tab
   
5. Click the Add External JARs... button
6. Navigate to the directory containing gridworld.jar. Select gridworld.jar.

BlueJ

You need to do the following once .

1. Select the menu option Tools -> Preferences
2. In the resulting dialog, click on the Libraries tab
   
3. Click the Add button
4. Navigate to the directory containing gridworld.jar. Select gridworld.jar.
5. Restart BlueJ

Command-line Compiler

When you compile or run programs, add both the current directory and gridworld.jar to the class path. On Windows, use a semicolon to separate . (i.e. the current directory) from the path to the JAR file:

javac -classpath .;\path\to\gridworld.jar
    *.java
 Replace \path\to\ with the path such as
    c:\GridWorld\

On Linux/Unix/Mac OS X, use a colon as a separator instead:

javac -classpath .: /path/to/ gridworld.jar *.java
Replace /path/to with the path such as  ~/GridWorld .