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Solution for Programmming Exercise 6.5

This page contains a sample solution to one of the exercises from Introduction to Programming Using Java.

Exercise 6.5:

In Exercise 3.6, you drew a checkerboard. For this exercise, write a checkerboard applet where the user can select a square by clicking on it. Hilite the selected square by drawing a colored border around it. When the applet is first created, no square is selected. When the user clicks on a square that is not currently selected, it becomes selected. If the user clicks the square that is selected, it becomes unselected. Assume that the size of the applet is exactly 160 by 160 pixels, so that each square on the checkerboard is 20 by 20 pixels. Here is a working version of the applet:


See the solution to Exercise 3.6 for a discussion of how to draw the checkerboard. In that exercise, the code for drawing the board was in the paint() method of a plain Applet. Since then, we have switched to using JApplet exclusively. However, the same code works in the paintComponent() method that I use in the solution to this exercise.

As always, there are many ways to organize the program. In this case, I decided to put all the programming into a drawing surface class named Board. Besides this nested class, there is nothing in the main applet class but an init() method. The init() method simply creates a Board and uses if for the content pane of the JApplet This can be done in a single line:

setContentPane( new Board() );

The rest of this discussion concerns the Board class. This drawing surface class is a subclass of JPanel and will also be used to respond to mouse clicks, so its definition begins

class Board extends JPanel implements MouseListener {

To keep track of which square is selected, if any, the Board class contains instance variables, selectedRow and selectedCol. When no square is selected, selectedRow is -1 (and I don't care what selectedCol is). When a square is selected, selectedRow is the number of the row that contains that square and selectedCol is the number of the column that contains the selected square. Remember that rows and columns are numbered from 0 to 7. This makes some of the calculations easier than numbering them from 1 to 8.

After drawing the checkerboard, the paintComponent() method has to hilite the selected square, if there is one. I do this by drawing a cyan border around the inside of the selected square. This is the new code that is added to the checkerboard-drawing code:

if (selectedRow >= 0) {
      // Since there is a selected square, draw a cyan
      // border around it.
   y = selectedRow * 20;
   x = selectedCol * 20;
   g.drawRect(x, y, 19, 19);
   g.drawRect(x+1, y+1, 17, 17);

Since the squares are 20 pixels on each side, you might wonder why the first drawRect() command specifies a width and height of 19 instead of 20. In the fillRect() method that is used earlier in the paintComponent() method to fill in the square, a width and height of 20 is used. Remember that the drawRect() method actually draws a rectangle whose width and height are one more than the values specified in the parameters. (Remember the bit about the pen that hangs one pixel outside the rectangle?)

To respond to user mouse clicks, the board must implement the MouseListener interface. The constructor in the Board class calls addMouseListener(this) to register the board to listen for mouse events on itself. (Remember that calling addMouseListener(this) is the same as calling this.addMouseListener(this).) Of the five methods specified in the MouseListener interface, only mousePressed has a non-empty definition. This method must figure out which square the user clicked and adjust the values of the instance variables selectedRow and selectedCol accordingly.

Let's say that the user clicked at the point (x,y). The problem is to determine which square on the checkerboard contains that point. The column number of the square is obtained by dividing the x coordinate by the width of the squares. Since the squares are 20 pixels wide, the row number of the clicked square is x/20. For values of x between 0 and 19, this gives a column number of 0, which is correct. For the next 20 pixels, from 20 to 39, x/20 is 1, which is the correct column number. For the next strip of pixels, from 40 to 59, the answer is 2. And so on. Similarly, y/20 gives the row number of the square where the user clicked.

Once we know the row and column where the user clicked, we can compare them to selectedRow and selectedCol. If the values are the same, then the user clicked in a square that was already selected. We want to remove the hiliting. That can be done by setting selectedRow = -1, the value that indicates that no square is selected. Otherwise, the values of selectedRow and selectedCol are set to the row and column that the user clicked.

All this explains the reasoning behind the mousePressed() routine, which you can see below.

The Solution

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

 *  This applet draws a red-and-black checkerboard.
 *  It is assumed that the size of the applet is 160
 *  by 160 pixels.  When the user clicks a square, that
 *  square is selected, unless it is already selected.
 *  When the user clicks the selected square, it is
 *  unselected.  If there is a selected square, it is
 *  hilited with a cyan border.
public class ClickableCheckerboard extends JApplet{
    * Note:  I have added a main() routine to the program, although it is
    * not required by the exercise.  This has nothing at all to do with
    * the function of the class as an applet, but it does use the nested
    * Board class.  When this class is run as a stand-alone application,
    * this main() routine opens a window that shows a Board as its content
    * pane.  The window is sized by calling its pack() routine.  For this
    * to work, the preferred size of the Board must be set correctly;
    * this is done in the constructor of the Board class, which sets 
    * the preferred size to 160-by-160.
   public static void main(String[] args) {
      JFrame window = new JFrame("Clickable Checkerboard");
      Board content = new Board();
      window.pack(); // Size the window to the preferred size of its content.
      window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
      window.setResizable(false);  // User can't change the window's size.

    * This does nothing but make an object belonging
    * to the nested class, Board, and add set it
    * to be the content pane of the applet.
   public void init() {
       setContentPane( new Board() );

    *  This nested class represents the checkerboard.  Besides drawing
    *  the board, it listens for mouse events on itself so that it
    *  can select or unselect the square that the user clicks.
   private static class Board extends JPanel implements MouseListener {

      int selectedRow; // Row and column of selected square.  If no
      int selectedCol; //      square is selected, selectedRow is -1.
       * Constructor.  Set selectedRow to -1 to indicate that
       * no square is selected.  And set the board object
       * to listen for mouse events on itself.
      Board() {
         selectedRow = -1;     
         setPreferredSize( new Dimension(160,160) );

      * Draw the checkerboard and hilite selected square, if any.
      * (Note: paintComponent() is not necessary, since this
      * method already paints the entire surface of the object.
      * This assumes that the object is exactly 160-by-160 pixels.
      public void paintComponent(Graphics g) {

         int row;   // Row number, from 0 to 7
         int col;   // Column number, from 0 to 7
         int x,y;   // Top-left corner of square

         for ( row = 0;  row < 8;  row++ ) {

            for ( col = 0;  col < 8;  col++) {
                x = col * 20;
                y = row * 20;
                if ( (row % 2) == (col % 2) )
                g.fillRect(x, y, 20, 20);

         } // end for row
         if (selectedRow >= 0) {
               // Since there is a selected square, draw a cyan
               // border around it.  (If selectedRow < 0, then
               // no square is selected and no border is drawn.)
            y = selectedRow * 20;
            x = selectedCol * 20;
            g.drawRect(x, y, 19, 19);
            g.drawRect(x+1, y+1, 17, 17);

      }  // end paint()

       * When the user clicks on the applet, figure out which
       * row and column the click was in and change the
       * selected square accordingly.
      public void mousePressed(MouseEvent evt) {

         int col = evt.getX() / 20;   // Column where user clicked.
         int row = evt.getY() / 20;   // Row where user clicked.

         if (selectedRow == row && selectedCol == col) {
              // User clicked on the currently selected square.
              // Turn off the selection by setting selectedRow to -1.
            selectedRow = -1;
         else {
               // Change the selection to the square the user clicked on.
            selectedRow = row;
            selectedCol = col;

      }  // end mouseDown()

      public void mouseReleased(MouseEvent evt) { }
      public void mouseClicked(MouseEvent evt) { }
      public void mouseEntered(MouseEvent evt) { }
      public void mouseExited(MouseEvent evt) { }

   }   // end nested class Board

} // end class ClickableCheckerboard

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