Module 6: Loops

import java.awt.*;
import java.awt.event.*;
import java.applet.Applet;

public class BouncingBallApplet2 extends Applet implements Runnable, MouseListener {

  int delay;
  int frameNumber;
  Thread animatorThread;


  public void init ()
  {
    delay = 500;
    animatorThread = null;

    // Need to tell Java to call "us" when a mouseclick occurs.
    addMouseListener (this);
  }


  public void start ()
  {
    frameNumber = 0;

    if (animatorThread == null) {
      animatorThread = new Thread (this);
    }

    animatorThread.start ();
  }


  public void stop () 
  {
    animatorThread = null;
  }


  public void run ()
  {
    while (animatorThread != null) {

      frameNumber ++;

      // Draw next frame.
      repaint ();

      // Delay
      try {
        Thread.sleep (delay);
      } 
      catch (InterruptedException e) {
        break;
      }
    }

  }


  public void paint (Graphics g) 
  {
    // Set foreground and background colors.
    setBackground (Color.white);
    g.setColor (Color.blue);

    // Here's how to get the size of the applet bounds within the program:
    // d.width is the width, d.height is the height.
    Dimension d = getSize();

    if (frameNumber % 2 == 1) {
      // On odd frames, draw the ball at the top.
      g.fillOval (0, 0, d.width/2, d.height/2);
    }
    else {
      // On even frames, draw the ball at the bottom.
      g.fillOval (0, d.height/2, d.width/2, d.height/2);
    }
  }


  // This method gets called when mouse click occurs.

  public void mouseClicked (MouseEvent e) 
  {
    // Draw a small red dot where the click occurred.
    int x = e.getX ();
    int y = e.getY ();
    Graphics g = getGraphics ();
    g.setColor (Color.red);
    g.fillOval (x-5, y-5, 10, 10);

    // If no thread is running, we'll create one.
    if (animatorThread == null) {
      start ();
    }
    else {
      stop ();
    }
  }
  
  // We need to have these methods, but
  // don't actually have to do anything inside.
  public void mouseEntered (MouseEvent m) {}
  public void mouseExited (MouseEvent m) {}
  public void mousePressed (MouseEvent m) {}
  public void mouseReleased (MouseEvent m) {}

}

import java.awt.*;
import java.awt.event.*;
import java.applet.Applet;

public class GridBallApplet extends Applet implements Runnable {

  // How many milliseconds between frames:
  int delay;

  // The current frame:
  int frameNumber;

  // A thread object.
  Thread animatorThread;

  // Track the current cell containing the ball.
  int currentRow = 0;
  int currentColumn = 0;

  public void init ()
  {
    // We'll pause 500 ms between frames.
    delay = 500;

    // We are going to use "null" as a true/false indicator.
    // Need to initialize to "false" (null).
    animatorThread = null;
  }


  // Start is called by the browser each time the applet
  // comes into view.

  public void start ()
  {
    // Initially, we're at frame 0.
    frameNumber = 0;
    
    // If there isnt' an animator thread, create one.
    if (animatorThread == null) {
      animatorThread = new Thread (this);
    }

    // Start the independent thread.
    animatorThread.start ();
  }


  public void stop () 
  {
    // Stop the animation by indicating "false".
    animatorThread = null;
  }


  public void run ()
  {
    while (animatorThread != null) {

      frameNumber ++;

      // Draw next frame.
      repaint ();

      // Delay
      try {
        Thread.sleep (delay);
      } 
      catch (InterruptedException e) {
        break;
      }
    }

  }


  public void paint (Graphics g) 
  {
    // Set foreground and background colors.
    setBackground (Color.white);

    // Draw a grid:
    g.setColor (Color.black);
    
    // Draw the vertical lines of the grid:
    for (int x=0; x<=240; x+=30) {
      g.drawLine (x,0, x,240);
    }

    // Draw the horizontal lines of the grid:
    for (int y=0; y<=240; y+=30) {
      g.drawLine (0,y, 240,y);
    }
    
    // Choose a random new row, wrapping around 
    // if necessary.
    int newRow = currentRow + 1;
    if (Math.random () < 0.5)
      newRow = currentRow -1;
    if (newRow > 7)
      newRow = 0;
    else if (newRow < 0)
      newRow = 7;
    
    // Choose a random new column, wrapping around 
    // if necessary.
    int newColumn = currentColumn + 1;
    if (Math.random () < 0.5)
      newColumn = currentColumn -1;
    if (newColumn > 7)
      newColumn = 0;
    else if (newColumn < 0)
      newColumn = 7;
    
    // Compute where the top left corner of the ball's
    // bounding square should be.
    int x = newRow * 30;
    int y = newColumn * 30;

    // Draw it.
    g.setColor (Color.blue);
    g.fillOval (x, y, 30, 30);

    // Update current cell coordinates.
    currentRow = newRow;
    currentColumn = newColumn;
  }

}

• Our grid is the same 8 x 8 grid from before, with 64 cells.

• The rows are numbered 0,...,7 and the columns are numbered 0,...,7. Thus, the bottom-right cell has coordinates (7,7).

• We now have two variables currentRow and currentColumn to keep the current position of the ball.

• Aside from declaring the two variables at the top, all the changes are in paint.

• A call to Math.random() returns a random number between 0 and 1.0.

• Notice that we pick a new neighboring cell at random by generating a random row (top or below) and a random column (left or right).

• If the new cell threatens to go off-grid, we wrap around.