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Programming Exercises
For Chapter 8

THIS PAGE CONTAINS programming exercises based on material from Chapter 8 of this on-line Java textbook. Each exercise has a link to a discussion of one possible solution of that exercise.

Exercise 8.1: An example in Section 8.2 tried to answer the question, How many random people do you have to select before you find a duplicate birthday? The source code for that program can be found in the file Here are some related questions:

  • How many random people do you have to select before you find three people who share the same birthday? (That is, all three people were born on the same day in the same month, but not necessarily in the same year.)
  • Suppose you choose 365 people at random. How many different birthdays will they have? (The number could theoretically be anywhere from 1 to 365).
  • How many different people do you have to check before you've found at least one person with a birthday on each of the 365 days of the year?

Write three programs to answer these questions. Like the example program, BirthdayProblemDemo, each of your programs should simulate choosing people at random and checking their birthdays. (In each case, ignore the possibility of leap years.)

See the solution!

Exercise 8.2: Write a program that will read a sequence of positive real numbers entered by the user and will print the same numbers in sorted order from smallest to largest. The user will input a zero to mark the end of the input. Assume that at most 100 positive numbers will be entered.

See the solution!

Exercise 8.3: A polygon is a geometric figure made up of a sequence of connected line segments. The points where the line segments meet are called the vertices of the polygon. The Graphics class includes commands for drawing and filling polygons. For these commands, the coordinates of the vertices of the polygon are stored in arrays. If g is a variable of type Graphics then

  • g.drawPolygon(xCoords, yCoords, pointCt) will draw the outline of the polygon with vertices at (xCoords[0],yCoords[0]), (xCoords[1],yCoords[1]), ..., (xCoords[pointCt-1],yCoords[pointCt-1]). The third parameter, pointCt, is an int that specifies the number of vertices of the polygon. Its value should be 3 or greater. The first two parameters are arrays of type int[]. Note that the polygon automatically includes a line from the last point, (xCoords[pointCt-1],yCoords[pointCt-1]), back to the starting point (xCoords[0],yCoords[0]).
  • g.fillPolygon(xCoords, yCoords, pointCt) fills the interior of the polygon with the current drawing color. The parameters have the same meaning as in the drawPolygon() method. Note that it is OK for the sides of the polygon to cross each other, but the interior of a polygon with self-intersections might not be exactly what you expect.

Write a little applet that lets the user draw polygons. As the user clicks a sequence of points, count them and store their x- and y-coordinates in two arrays. These points will be the vertices of the polygon. Also, draw a line between each consecutive pair of points to give the user some visual feedback. When the user clicks near the starting point, draw the complete polygon. Draw it with a red interior and a black border. The user should then be able to start drawing a new polygon. When the user shift-clicks on the applet, clear it.

There is no need to store information about the contents of the applet. The paintComponent() method can just draw a border around the applet. The lines and polygons can be drawn using a graphics context, g, obtained with the command "g = getGraphics();".

You can try my solution. Note that as the user is drawing the polygon, lines are drawn between the points that the user clicks. Click within two pixels of the starting point to see a filled polygon.

See the solution!

Exercise 8.4: For this problem, you will need to use an array of objects. The objects belong to the class MovingBall, which I have already written. You can find the source code for this class in the file A MovingBall represents a circle that has an associated color, radius, direction, and speed. It is restricted to moving in a rectangle in the (x,y) plane. It will "bounce back" when it hits one of the sides of this rectangle. A MovingBall does not actually move by itself. It's just a collection of data. You have to call instance methods to tell it to update its position and to draw itself. The constructor for the MovingBall class takes the form

       new MovingBall(xmin, xmax, ymin, ymax)

where the parameters are integers that specify the limits on the x and y coordinates of the ball. In this exercise, you will want balls to bounce off the sides of the applet, so you will create them with the constructor call "new MovingBall(0, getWidth(), 0, getHeight())". The constructor creates a ball that initially is colored red, has a radius of 5 pixels, is located at the center of its range, has a random speed between 4 and 12, and is headed in a random direction. If ball is a variable of type MovingBall, then the following methods are available:

  • ball.draw(g) -- draw the ball in a graphics context. The parameter, g, must be of type Graphics. (The drawing color in g will be changed to the color of the ball.)
  • -- change the (x,y)-coordinates of the ball by an amount equal to its speed. The ball has a certain direction of motion, and the ball is moved in that direction. Ordinarily, you will call this once for each frame of an animation, so the speed is given in terms of "pixels per frame". Calling this routine does not move the ball on the screen. It just changes the values of some instance variables in the object. The next time the object's draw() method is called, the ball will be drawn in the new position.
  • ball.headTowards(x,y) -- change the direction of motion of the ball so that it is headed towards the point (x,y). This does not affect the speed.

These are the methods that you will need for this exercise. There are also methods for setting various properties of the ball, such as ball.setColor(color) for changing the color and ball.setRadius(radius) for changing its size. See the source code for more information.

For this exercise, you should create an applet that shows an animation of 25 balls bouncing around on a black background. Your applet can be defined as a subclass of SimpleAnimationApplet2, which was first introduced in Section 3.7. The drawFrame() method in your applet should move all the balls and draw them. (Alternatively, if you have read Chapter 7, you can program the animation yourself using a Timer.) Use an array of type MovingBall[] to hold the 25 balls.

In addition, your applet should implement the MouseListener and MouseMotionListener interfaces. When the user presses the mouse or drags the mouse, call each of the ball's headTowards() methods to make the balls head towards the mouse's location.

Here is my solution. Try clicking and dragging on the applet:

See the solution!

Exercise 8.5: The game of Go Moku (also known as Pente or Five Stones) is similar to Tic-Tac-Toe, except that it played on a much larger board and the object is to get five squares in a row rather than three. Players take turns placing pieces on a board. A piece can be placed in any empty square. The first player to get five pieces in a row -- horizontally, vertically, or diagonally -- wins. If all squares are filled before either player wins, then the game is a draw. Write an applet that lets two players play Go Moku against each other.

Your applet will be simpler than the Checkers applet from Section 8.5. Play alternates strictly between the two players, and there is no need to hilite the legal moves. You will only need two classes, a short applet class to set up the applet and a Board class to draw the board and do all the work of the game. Nevertheless, you will probably want to look at the source code for the checkers applet,, for ideas about the general outline of the program.

The hardest part of the program is checking whether the move that a player makes is a winning move. To do this, you have to look in each of the four possible directions from the square where the user has placed a piece. You have to count how many pieces that player has in a row in that direction. If the number is five or more in any direction, then that player wins. As a hint, here is part of the code from my applet. This code counts the number of pieces that the user has in a row in a specified direction. The direction is specified by two integers, dirX and dirY. The values of these variables are 0, 1, or -1, and at least one of them is non-zero. For example, to look in the horizontal direction, dirX is 1 and dirY is 0.

      int ct = 1;  // Number of pieces in a row belonging to the player.
      int r, c;    // A row and column to be examined.
      r = row + dirX;  // Look at square in specified direction.
      c = col + dirY;
      while ( r >= 0 && r < 13 && c >= 0 && c < 13
                                        && board[r][c] == player ) {
              // Square is on the board, and it
              // contains one of the players's pieces.
         r += dirX;  // Go on to next square in this direction.
         c += dirY;
      r = row - dirX;  // Now, look in the opposite direction.
      c = col - dirY;
      while ( r >= 0 && r < 13 && c >= 0 && c < 13
                                       && board[r][c] == player ) {
         r -= dirX;   // Go on to next square in this direction.
         c -= dirY;

Here is my applet. It uses a 13-by-13 board. You can do the same or use a normal 8-by-8 checkerboard.

See the solution!

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