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Solution for
Programming Exercise 6.6


THIS PAGE DISCUSSES ONE POSSIBLE SOLUTION to the following exercise from this on-line Java textbook.

Exercise 6.6: For this exercise, you should modify the SubKiller game from Section 6.5. You can start with the existing source code, from the file SubKillerGame.java. Modify the game so it keeps track of the number of hits and misses and displays these quantities. That is, every time the depth charge blows up the sub, the number of hits goes up by one. Every time the depth charge falls off the bottom of the screen without hitting the sub, the number of misses goes up by one. There is room at the top of the applet to display these numbers. To do this exercise, you only have to add a half-dozen lines to the source code. But you have to figure out what they are and where to add them. To do this, you'll have to read the source code closely enough to understand how it works.


Discussion

Here is the applet:

You can do this exercise by adding exactly seven lines to the original version, SubKillerGame.java. I used two lines to declare instance variables named hits and misses. These variables have to be updated whenever the depth charge hits the sub or falls off the bottom of the applet. These events are already detected by the applet, in the doBombFrame() method. At the point where this method detects that the depth charge has hit the sub, I add the command "hits++;" to chalk up another hit for the user. At the point in doBombFrame() where it is determined that the y-coordinate of the depth charge has exceeded the height of the applet, I add the command "misses++;" to record the fact that the sub has escaped destruction this time.

The only other thing to do is to display the number of hits and misses at the top of the applet. This is part of drawing the frame, so it is done in the drawFrame() method. The information is output with two drawString commands. We need one more command to make sure that the strings are displayed in a color that can be seen:

           g.setColor(Color.black);
           g.drawString("Number of hits:   " + hits, 15, 22);
           g.drawString("Number of misses: " + misses, 15, 40);

I made one further change when I decided that I wanted the output to be displayed in a larger font. I create a new font, store it in an instance variable named infoFont, and use the command "g.setFont(infoFont);" before drawing the strings.

The source code is shown below. Changes from the original version are shown in red.


The Solution


    /*
       This is a simple arcade game in which the user tries to blow up
       a submarine (a black oval) by dropping depth charges from a
       ship (a blue roundrect).  The user moves the boat with the
       left- and right-arrow keys and drops the depth charge with the
       down-arrow key.
       
       The game displays the number of times that the user has hit the
       sub and the number of times that the user has missed it.
     
       The sub killer applet is based on KeyboardAnimationApplet2, which is
       a framework for applets that display animated action while allowing
       keyboard input from the user to affect the action.  See the file
       KeyboardAnimationApplet2.java for more information.
       
       Note that the KeyboardAnimationApplet2 class already implements
       KeyListener, FocusListener, and MouseListener.
    */
    
    
    import java.awt.*;        // for access to GUI classes
    import java.awt.event.*;  // for access to the KeyEvent class
    
    
    public class SubKillerGame2 extends KeyboardAnimationApplet2 {
    
       
       int hits = 0;    // Number of times the user has hit the sub.
       int misses = 0;  // Number of times the user has missed the sub.
       
       Font infoFont = new Font("Monospaced", Font.PLAIN, 14);
              // A font for displaying the numbers of hits and misses.
    
    
       protected void doInitialization(int width, int height) {
             // This routine will be called once, when the animation applet
             // is first created.  The three objects that appear in the
             // applet are initialized, by calling methods that are defined
             // later in this class.  The parameters width and height
             // give the width and height of the drawing area.
             
          initSubmarine(width, height);  
          initBoat(width, height); 
          initBomb(width,height);
          
       } // end doInitialization()
    
    
       synchronized public void drawFrame(Graphics g, int width, int height) {
              // This routine is called to draw the next frame of the animation.
              // The parameter, g, is a graphics context for drawing the frame.
              // The parameters width and height give the size of the frame.
              // Here, methods are called to process and draw each of the
              // three objects in the scene.
       
           // First, fill the whole drawing area with green
           
           g.setColor(Color.green);
           g.fillRect(0,0,width,height);
           
           // Display the number of hits and the number of misses.
           
           g.setColor(Color.black);
           g.setFont(infoFont);
           g.drawString("Number of hits:   " + hits, 15, 22);
           g.drawString("Number of misses: " + misses, 15, 40);
           
           // Now, draw the objects in the scene. These subroutines
           // are defined below.  They also change values of instance
           // variables so that the scene will change from one frame to
           // the next.
           
           doBoatFrame(g, width, height);
           doSubmarineFrame(g, width, height);
           doBombFrame(g, width, height);
           
       } // end drawFrame()
       
       
       synchronized public void keyPressed(KeyEvent evt) {
              // This routine is called each time the user presses
              // a key on the keyboard (while the applet has the keyboard
              // focus).  The value of evt.getKeyCode() tells which
              // key was pressed.  For this applet, only the three of the
              // arrow keys will have any effect.  The codes for the
              // four arrow keys are KeyEvent.VK_UP, KeyEvent.VK_DOWN,
              // KeyEvent.VK_LEFT, and KeyEvent.VK_RIGHT.
    
           int code = evt.getKeyCode();  // which key was pressed
    
           if (code == KeyEvent.VK_LEFT) {
                   // Move the boat left.  (If this moves the boat out of the 
                   // frame, its position will be adjusted in the doBoatFrame() method.)
              boatCenterX -= 15;
           }
           else if (code == KeyEvent.VK_RIGHT) {  
                   // Move the boat right.  (If this moves the boat out of the 
                   // frame, its position will be adjusted in the doBoatFrame() method.)
              boatCenterX += 15;
           }
           else if (code == KeyEvent.VK_DOWN) {
                   // Start the bomb falling, it is not already falling.
                   // (The test is not really necessary, since if bombIsFalling
                   // is already true, the assignment bombIsFalling = true has
                   // no effect anyway.)
              if ( bombIsFalling == false )
                 bombIsFalling = true;
           }
       
       } // end keyPressed()
       
    
       // -------------- Instance variables and method for the bomb -------------
       
       int bombCenterX;  // Horizontal coordinate of the center of the bomb.
       int bombCenterY;  // Vertical coordinate of the center of the bomb.
    
       boolean bombIsFalling;  // This is set to true when the boat releases the
                               //    bomb.  While it is false, the bomb is 
                               //    attached to the boat.  While it is true,
                               //    the vertical coordinate of the bomb increases
                               //    in each frame, so the bomb falls.
    
       void initBomb(int width, int height) {
              // Initialize instance variables that pertain to the bomb.
           bombIsFalling = false;
           bombCenterY = 100;
       }
       
       void doBombFrame(Graphics g, int width, int height) {
              // If the bomb is NOT falling, just draw it under the center of the boat.
              // If it is falling, move it downwards and then check whether
              // it has either hit the sub or fallen off the applet.  In either
              // case, its state changes to not falling (and so it reappears
              // back under the boat in the next frame).
           if (bombIsFalling) {
              if (bombCenterY > height) {
                    // Bomb has missed the submarine.  It is returned to its initial state.
                    // It's not drawn in this frame, but it will appear in the next.
                 initBomb(width, height);
                 bombCenterX = boatCenterX;
                 misses++;  // Add 1 to the number of misses, since user has missed the sub.
              }
              else if (Math.abs(bombCenterX - subCenterX) <= 36 &&
                          Math.abs(bombCenterY - subCenterY) <= 21) {
                    // Bomb has hit the submarine.  It is not
                    // drawn in this frame, but an explosion starts,
                    // and the bomb is returned to its initial state.
                 explosionFrameNumber = 1;
                 initBomb(width, height);
                 bombCenterX = boatCenterX;
                 hits++;  // Add 1 to the number of hits, since user has hit the sub.
              }
              else {
                 bombCenterY += 10;
                 g.setColor(Color.red);
                 g.fillOval(bombCenterX - 8, bombCenterY - 8, 16, 16);
              }
           }
           else {
                 // Bomb is not falling.  It is attached to the boat.
                 // Use the boat's horizontal position for the bomb.
              bombCenterX = boatCenterX;
              g.setColor(Color.red);
              g.fillOval(bombCenterX - 8, bombCenterY - 8, 16, 16);
           }
       }
    
    
    
       // -------------- Instance variables and method for the boat -------------
       
       int boatCenterX;  // Horizontal coordinate of the center of the boat.
       int boatCenterY;  // Vertical coordinate of the center of the boat.
    
       void initBoat(int width, int height) {
              // Initialize instance variables that pertain to the boat.
          boatCenterX = width/2;
          boatCenterY = 80;
       }
    
       void doBoatFrame(Graphics g, int width, int height) {
              // Draw the boat.  Make sure that the boat's horizontal
              // position is within the range 0 to width.  This prevents
              // the user from moving the boat off the screen.
          if (boatCenterX < 0)
             boatCenterX = 0;
          else if (boatCenterX > width)
             boatCenterX = width;
          g.setColor(Color.blue);
          g.fillRoundRect(boatCenterX - 40, boatCenterY - 20, 80, 40, 20, 20);
       }
    
    
       // -------------- Instance variables and method for the submarine -------------
       
       int subCenterX;  // Horizontal coordinate of the center of the submarine.
       int subCenterY;  // Vertical coordinate of the center of the submarine.
       
       boolean subIsMovingLeft;  // Sub can be moving left or right; this variable tells which.
       
       int explosionFrameNumber; // While the sub is exploding, this tells for how many
                                 //    frames the explosion has been in progress.  When the
                                 //    sub is not exploding, the value of this variable is 0.
                                 //    when the bomb hits the sub, the value is set to 1 in
                                 //    the doBombFrame method.  When explosiongFrameNumber
                                 //    reaches 14, the explosion ends and a new sub is
                                 //    created.
      
       void initSubmarine(int width, int height) {
              // Initialize the instance variables that pertain to the submarine.
              // The sub is placed in a random horizontal position, 40 pixels from
              // the bottom of the drawing area.  Its direction of motion is
              // set randomly to either left or right.  At the beginning, the
              // sub is not exploding.
          subCenterX = (int)(width * Math.random());
          subCenterY = height - 40;
          explosionFrameNumber = 0;
          if (Math.random() < 0.5)
             subIsMovingLeft = true;
          else
             subIsMovingLeft = false;
       }
       
       void doSubmarineFrame(Graphics g, int width, int height) {
             // Draw the submarine and update the instance variables that
             // describe it.  The sub has two very different states,
             // depending on whether or not it is exploding.  Note that
             // at the end of an explosion, the bomb is reinitialized so
             // that it appears again under the boat.
    
          if (explosionFrameNumber > 0) {
    
             if (explosionFrameNumber == 14) {  // In the 14-th frame...
                initSubmarine(width,height);    //    ... create a new sub (also sets explosionFrameNumber to 0)
             }
                
             else if (explosionFrameNumber > 10) { // For frames 11, 12, and 13,
                                                   //   there is no sub
                 explosionFrameNumber++;  // Just go to next frame of explosion sequence
             }
             
             else {  // For frames 1 through 10, draw the sub under an expanding explosion
                g.setColor(Color.black);
                g.fillOval(subCenterX - 30, subCenterY - 15, 60, 30);
                g.setColor(Color.yellow);
                g.fillOval(subCenterX - 4*explosionFrameNumber,
                           subCenterY - 2*explosionFrameNumber,
                           8*explosionFrameNumber,
                           4*explosionFrameNumber);
                g.setColor(Color.red);
                g.fillOval(subCenterX - 2*explosionFrameNumber,
                           subCenterY - explosionFrameNumber/2,
                           4*explosionFrameNumber,
                           explosionFrameNumber);
                explosionFrameNumber++;  // Go on to next frame of explosion.
             }
                
          }
                
          else { // Sub is not exploding.  Move the sub and draw it in its new position
          
             if (Math.random() < 0.04)               // One time out of 25, on average...
                subIsMovingLeft = !subIsMovingLeft;  //   ...randomly reverse direction
                
             if (subIsMovingLeft) { 
                subCenterX -= 5;        // Move the sub left 5 pixels.
                if (subCenterX <= 0) {  // If sub has moved off left hand edge of applet...
                   subCenterX = 0;      //       ...move it back to the left edge...
                   subIsMovingLeft = false; //  ...and start it moving right.
                }
             }
             else {
                subCenterX += 5;           // Move the sub right 5 pixels.
                if (subCenterX > width) {  // If the sub has moved off the right edge...
                   subCenterX = width;     //    ...move it back to the right edge...
                   subIsMovingLeft = true; //    ...and start it moving left.
                }
             }
             g.setColor(Color.black);  // Draw the sub
             g.fillOval(subCenterX - 30, subCenterY - 15, 60, 30);
          }
    
       }  // end doSubmarineFrame 
       
       
    } // end class SubKiller


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