Solution for
Programming Exercise 8.6
THIS PAGE DISCUSSES ONE POSSIBLE SOLUTION to the following exercise from this on-line Java textbook.
Exercise 8.6: 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 GoMokuCanvas 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, Checkers.java, 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. ct++; 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 ) { ct++; 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.
Discussion
This is a fairly complicated program, but its possible to design and build it in stages, testing each stage separately. The first stage, the general layout of the applet, is already done in the Checkers applet. With just a few changes, the main applet class, the layout of the applet, and the button and message handling come directly from that applet. Let's take the rest of the Go Moku game one stage at a time.
A two-dimensional array is used to store the contents of the board. This array is of type int[][] and is named board. It is defined as an instance variable in the GoMokuCanvas class, and it is initialized in the constructor of that class to be a 13-by-13 array. The value in each position of the array is one of three constants: EMPTY, WHITE, or BLACK. When a game begins, each of the entries in the array is set to empty. When a player clicks on an empty square, the corresponding entry in the array is changed from EMPTY to the player's color, BLACK or WHITE. In the paint() method, the contents of the board array are used to decide what pieces to draw on the board.
Drawing the Board
We need a paint() method for the GoMokuCanvas class that can draw the board. The board has 13 rows and 13 columns of spaces. How wide should the board be? If each square in the board is x pixels wide, we need a total of 13*x pixels just for the spaces. But there are also lines between the spaces. These require another 12 pixels. And there is a 2-pixel border on each side, for another 4 pixels added to the width. So, with squares of side x, we need a board that is 13*x+16 pixels wide. Since I wanted something about the same size as the original checkerboard, I choose x to be 12, giving a board width of 172. The height is the same. The dimension of the board is set in the main applet class.
The left edge of the col-th column of squares in the board is 2+13*col. This allows for the two-pixel border on the left and for 13 pixels for each of the preceding columns of squares. (That's 12 pixels for the square plus one pixel for the line between that column and the next.) The lines between the columns are drawn one pixel to the left of each column, at x values 1 + 13*i, for i from 1 to 12. Rows work the same way. To draw a piece in row number row and column number col, the command
g.fillOval(3 + 13*col, 3 + 13*row, 10, 10);can be used. This allows a one-pixel border between the oval the represents that piece and the side of the square. In my applet, I defined a method to draw a piece:
private void drawPiece(Graphics g, int piece, int row, int col) { // Draw a piece in the square at (row,col). The color // is specified by the "piece" parameter, which should be // either BLACK or WHITE. if (piece == WHITE) g.setColor(Color.white); else g.setColor(Color.black); g.fillOval(3 + 13*col, 3 + 13*row, 10, 10); }The background of the canvas is gray, so the paint() method only has to draw the black border around the board, the lines between the squares, and all the pieces on the board. This can be done with the following paint() method:
public void paint(Graphics g) { /* Draw grid lines in darkGray. */ g.setColor(Color.darkGray); for (int i = 1; i < 13; i++) { g.drawLine(1 + 13*i, 0, 1 + 13*i, getSize().height); g.drawLine(0, 1 + 13*i, getSize().width, 1 + 13*i); } /* Draw a two-pixel black border around the edges of the board. */ g.setColor(Color.black); g.drawRect(0,0,getSize().width-1,getSize().height-1); g.drawRect(1,1,getSize().width-3,getSize().height-3); /* Draw the pieces that are on the board. */ for (int row = 0; row < 13; row++) for (int col = 0; col < 13; col++) if (board[row][col] != EMPTY) drawPiece(g, board[row][col], row, col); } // end paint()
Playing the Game
The logic of the GoMoku game itself is mostly in the method "void doClickSquare(int row, int col)", which is called by the mousePressed() method when the user clicks on the square in row number row and column number col. This method must check whether the move is legal. If so, the move is made. The method then checks whether the move wins the game. If so, the game ends. The game will also end if the board has become completely full. Otherwise, play passes to the other player.
The current player is recorded in an instance variable named currentPlayer. The value of this variable is one of the two constants WHITE or BLACK. The game can be ended by calling a method named gameOver(). I wrote a boolean-valued method called winner() to check whether a move wins the game. (When I first wrote this method, it did nothing but "return false". This let me try out the applet at this stage of development, before I started working on the difficult problem of testing for a winner.) The doClickSquare() method can be written:
void doClickSquare(int row, int col) { // This is called by mousePressed() when a player clicks // on the square in the specified row and col. It has already // been checked that a game is, in fact, in progress. /* Check that the user clicked an empty square. If not, show an error message and exit. */ if ( board[row][col] != EMPTY ) { if (currentPlayer == BLACK) message.setText("BLACK: Please click an empty square."); else message.setText("WHITE: Please click an empty square."); return; } /* Make the move. Check if the board is full or if the move is a winning move. If so, the game ends. If not, then it's the other user's turn. */ board[row][col] = currentPlayer; // Make the move. Graphics g = getGraphics(); drawPiece(g, currentPlayer, row, col); // Draw the new piece. g.dispose(); if (winner(row,col)) { // First, check for a winner. if (currentPlayer == WHITE) gameOver("WHITE wins the game!"); else gameOver("BLACK wins the game!"); return; } boolean emptySpace = false; // Check if the board is full. for (int i = 0; i < 13; i++) for (int j = 0; j < 13; j++) if (board[i][j] == EMPTY) emptySpace = true; // The board contains an empty space. if (emptySpace == false) { gameOver("The game ends in a draw."); return; } /* Continue the game. It's the other player's turn. */ if (currentPlayer == BLACK) { currentPlayer = WHITE; message.setText("WHITE: Make your move."); } else { currentPlayer = BLACK; message.setText("BLACK: Make your move."); } } // end doClickSquare()
Determining the Winner
The winner() method is certainly the hardest part of the applet. The method must look in each of the four possible directions from the square where the user has placed a piece. If the player has five or more pieces in a row in that direction, then the player has won. As indicated in the exercise, a direction can be indicated by two variables, dirX and dirY. The values of these variables for each of the four directions are:
dirX dirY Why? ---- ---- -------------------------------- horizontal direction 1 0 Only x changes. vertical direction 0 1 Only y changes. first diagonal 1 1 Both x and y change. second diagonal 1 -1 Change in opposing directions.I wrote a method "int count(int player, int row, int col, int dirX, int dirY) that counts the number of pieces the specified player has in a row, starting from the square in row number row and column number col and looking in the direction indicated by dirX and dirY. This method contains the code given in the exercise. It returns the number of pieces found. My winner method just calls this method for each of the four directions:
private boolean winner(int row, int col) { // This is called just after a piece has been played on the // square in the specified row and column. It determines // whether that was a winning move by counting the number // of squares in a line in each of the four possible // directions from (row,col). If there are 5 squares (or more) // in a row in any direction, then the game is won. if (count( board[row][col], row, col, 1, 0 ) >= 5) return true; if (count( board[row][col], row, col, 0, 1 ) >= 5) return true; if (count( board[row][col], row, col, 1, -1 ) >= 5) return true; if (count( board[row][col], row, col, 1, 1 ) >= 5) return true; /* When we get to this point, we know that the game is not won. */ return false; } // end winner()When I first wrote this method, I checked whether the number of pieces was "== 5" instead of ">= 5". This was a bug. It's possible for a player to get more than 5 pieces in a row, if the player plays a piece in an empty square that joins two shorter rows of pieces together.
Marking the Winning Pieces
In my applet, when a player wins, the row of pieces that wins the game is marked with a red line. To do this, I added four instance variables of type int to the GoMokuCanvas class. The instance variables are named win_r1, win_c1, win_r2, and win_c2. If the game has not yet been won, then the value of win_r1 is -1. the paint() method uses this value as a signal that it should not draw any red line. After a player has won the game, the values of these variables are set to mark the squares at the two ends of the winning row of pieces. The positions of these squares are given by (win_r1,win_c1) and (win_r2,win_c2). If win_r1 is greater than -1, then the paint() method draws a red line between these two squares. (The doClickSquare() method also draws a red line when the game is won. It draws this line directly on the board just as it draws the piece that was played directly on the board. I do this, rather than call repaint(), to avoid the flicker that would be visible if the entire board were repainted.)
I added some code to the winner() and count() methods to set the values of these variables properly. As the count() method counts pieces in a row, it sets win_r1, win_c1, win_r2, and win_c2 to mark the location of the last piece it finds in the two directions it checks. If the game is won, this will set the values correctly. To handle the case where the game is not won, I added the line "win_r1 = -1;" to the winner() method, just before the "return false;" statement. This ensures that the value of this variable will be -1 whenever the game is not yet won. (Perhaps this is all too tricky, but I really wanted to mark the winning pieces...)
The Solution
/* This applet lets two uses play GoMoku (a.k.a Pente) against each other. Black always starts the game. When a player gets five-in-a-row, that player wins. The game ends in a draw if the board is filled before either player wins. This file defines two classes: the main applet class, GoMuku, and a canvas class, GoMokuCanvas. It is assumed that this applet is 330 pixels wide and 240 pixels high! */ import java.awt.*; import java.awt.event.*; import java.applet.*; public class GoMoku extends Applet { /* The main applet class only lays out the applet. The work of the game is all done in the GoMokuCanvas object. Note that the Buttons and Label used in the applet are defined as instance variables in the GoMokuCanvas class. The applet class gives them their visual appearance and sets their size and positions.*/ public void init() { setLayout(null); // I will do the layout myself. setBackground(new Color(0,150,0)); // Dark green background. /* Create the components and add them to the applet. */ GoMokuCanvas board = new GoMokuCanvas(); // Note: The constructor creates the buttons board.resignButton // and board.newGameButton and the Label board.message. add(board); board.newGameButton.setBackground(Color.lightGray); add(board.newGameButton); board.resignButton.setBackground(Color.lightGray); add(board.resignButton); board.message.setForeground(Color.green); board.message.setFont(new Font("Serif", Font.BOLD, 14)); add(board.message); /* Set the position and size of each component by calling its setBounds() method. */ board.setBounds(16,16,172,172); // Note: size MUST be 172-by-172 ! board.newGameButton.setBounds(210, 60, 100, 30); board.resignButton.setBounds(210, 120, 100, 30); board.message.setBounds(0, 200, 330, 30); } } // end class GoMoku class GoMokuCanvas extends Canvas implements ActionListener, MouseListener { Button resignButton; // Current player can resign by clicking this button. Button newGameButton; // This button starts a new game. It is enabled only // when the current game has ended. Label message; // A label for displaying messages to the user. int[][] board; // The data for the board is kept here. The values // in this array are chosen from the following constants. static final int EMPTY = 0, // Represents an empty square. WHITE = 1, // A white piece. BLACK = 2; // A black piece. boolean gameInProgress; // Is a game currently in progress? int currentPlayer; // Whose turn is it now? The possible values // are WHITE and BLACK. (This is valid only while // a game is in progress.) int win_r1, win_c1, win_r2, win_c2; // When a player wins by getting five or more // pieces in a row, the squares at the // ends of the row are (win_r1,win_c1) // and (win_r2,win_c2). A red line is // drawn between these squares. When there // are no five pieces in a row, the value of // win_r1 is -1. The values are set in the // count() method. The value of win_r1 is // tested in the paint() method. public GoMokuCanvas() { // Constructor. Create the buttons and label. Listen for mouse // clicks and for clicks on the buttons. Create the board and // start the first game. setBackground(Color.lightGray); addMouseListener(this); setFont(new Font("Serif", Font.BOLD, 14)); resignButton = new Button("Resign"); resignButton.addActionListener(this); newGameButton = new Button("New Game"); newGameButton.addActionListener(this); message = new Label("",Label.CENTER); board = new int[13][13]; doNewGame(); } public void actionPerformed(ActionEvent evt) { // Respond to user's click on one of the two buttons. Object src = evt.getSource(); if (src == newGameButton) doNewGame(); else if (src == resignButton) doResign(); } void doNewGame() { // Begin a new game. if (gameInProgress == true) { // This should not be possible, but it doesn't // hurt to check. message.setText("Finish the current game first!"); return; } for (int row = 0; row < 13; row++) // Fill the board with EMPTYs for (int col = 0; col < 13; col++) board[row][col] = EMPTY; currentPlayer = BLACK; // BLACK moves first. message.setText("BLACK: Make your move."); gameInProgress = true; newGameButton.setEnabled(false); resignButton.setEnabled(true); win_r1 = -1; // This value indicates that no red line is to be drawn. repaint(); } void doResign() { // Current player resigns. Game ends. Opponent wins. if (gameInProgress == false) { // This should not be possible. message.setText("There is no game in progress!"); return; } if (currentPlayer == WHITE) message.setText("WHITE resigns. BLACK wins."); else message.setText("BLACK resigns. WHITE wins."); newGameButton.setEnabled(true); resignButton.setEnabled(false); gameInProgress = false; } void gameOver(String str) { // The game ends. The parameter, str, is displayed as a message. message.setText(str); newGameButton.setEnabled(true); resignButton.setEnabled(false); gameInProgress = false; } void doClickSquare(int row, int col) { // This is called by mousePressed() when a player clicks on the // square in the specified row and col. It has already been checked // that a game is, in fact, in progress. /* Check that the user clicked an empty square. If not, show an error message and exit. */ if ( board[row][col] != EMPTY ) { if (currentPlayer == BLACK) message.setText("BLACK: Please click an empty square."); else message.setText("WHITE: Please click an empty square."); return; } /* Make the move. Check if the board is full or if the move is a winning move. If so, the game ends. If not, then it's the other user's turn. */ board[row][col] = currentPlayer; // Make the move. Graphics g = getGraphics(); drawPiece(g, currentPlayer, row, col); g.dispose(); if (winner(row,col)) { // First, check for a winner. if (currentPlayer == WHITE) gameOver("WHITE wins the game!"); else gameOver("BLACK wins the game!"); Graphics w = getGraphics(); drawWinLine(w); w.dispose(); return; } boolean emptySpace = false; // Check if the board is full. for (int i = 0; i < 13; i++) for (int j = 0; j < 13; j++) if (board[i][j] == EMPTY) emptySpace = true; if (emptySpace == false) { gameOver("The game ends in a draw."); return; } /* Continue the game. It's the other player's turn. */ if (currentPlayer == BLACK) { currentPlayer = WHITE; message.setText("WHITE: Make your move."); } else { currentPlayer = BLACK; message.setText("BLACK: Make your move."); } } // end doClickSquare() private boolean winner(int row, int col) { // This is called just after a piece has been played on the // square in the specified row and column. It determines // whether that was a winning move by counting the number // of squares in a line in each of the four possible // directions from (row,col). If there are 5 squares (or more) // in a row in any direction, then the game is won. if (count( board[row][col], row, col, 1, 0 ) >= 5) return true; if (count( board[row][col], row, col, 0, 1 ) >= 5) return true; if (count( board[row][col], row, col, 1, -1 ) >= 5) return true; if (count( board[row][col], row, col, 1, 1 ) >= 5) return true; /* When we get to this point, we know that the game is not won. The value of win_r1, which was changed in the count() method, has to be reset to -1, to avoid drawing a red line on the board. */ win_r1 = -1; return false; } // end winner() private int count(int player, int row, int col, int dirX, int dirY) { // Counts the number of the specified player's pieces starting at // square (row,col) and extending along the direction specified by // (dirX,dirY). It is assumed that the player has a piece at // (row,col). This method looks at the squares (row + dirX, col+dirY), // (row + 2*dirX, col + 2*dirY), ... until it hits a square that is // off the board or is not occupied by one of the players pieces. // It counts the squares that are occupied by the player's pieces. // Furthermore, it sets (win_r1,win_c1) to mark last position where // it saw one of the player's pieces. Then, it looks in the // opposite direction, at squares (row - dirX, col-dirY), // (row - 2*dirX, col - 2*dirY), ... and does the same thing. // Except, this time it sets (win_r2,win_c2) to mark the last piece. // Note: The values of dirX and dirY must be 0, 1, or -1. At least // one of them must be non-zero. 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 contains one of the players's pieces. ct++; r += dirX; // Go on to next square in this direction. c += dirY; } win_r1 = r - dirX; // The next-to-last square looked at. win_c1 = c - dirY; // (The LAST one looked at was off the board or // did not contain one of the player's pieces. r = row - dirX; // Look in the opposite direction. c = col - dirY; while ( r >= 0 && r < 13 && c >= 0 && c < 13 && board[r][c] == player ) { // Square is on the board and contains one of the players's pieces. ct++; r -= dirX; // Go on to next square in this direction. c -= dirY; } win_r2 = r + dirX; win_c2 = c + dirY; // At this point, (win_r1,win_c1) and (win_r2,win_c2) mark the endpoints // of the line of pieces belonging to the player. return ct; } // end count() public void paint(Graphics g) { /* Draw a two-pixel black border around the edges of the canvas, and draw grid lines in darkGray. */ g.setColor(Color.darkGray); for (int i = 1; i < 13; i++) { g.drawLine(1 + 13*i, 0, 1 + 13*i, getSize().height); g.drawLine(0, 1 + 13*i, getSize().width, 1 + 13*i); } g.setColor(Color.black); g.drawRect(0,0,getSize().width-1,getSize().height-1); g.drawRect(1,1,getSize().width-3,getSize().height-3); /* Draw the pieces that are on the board. */ for (int row = 0; row < 13; row++) for (int col = 0; col < 13; col++) if (board[row][col] != EMPTY) drawPiece(g, board[row][col], row, col); /* If the game has been won, then win_r1 >= 0. Draw a line to mark the five winning pieces. */ if (win_r1 >= 0) drawWinLine(g); } // end paint() private void drawPiece(Graphics g, int piece, int row, int col) { // Draw a piece in the square at (row,col). The color is specified // by the piece parameter, which should be either BLACK or WHITE. if (piece == WHITE) g.setColor(Color.white); else g.setColor(Color.black); g.fillOval(3 + 13*col, 3 + 13*row, 10, 10); } private void drawWinLine(Graphics g) { // Draw a 2-pixel wide red line from the middle of the square at // (win_r1,win_c1) to the middle of the square at (win_r2,win_c2). // This routine is called to mark the 5 pieces that won the game. // The values of the variables are set in the count() method. g.setColor(Color.red); g.drawLine( 8 + 13*win_c1, 8 + 13*win_r1, 8 + 13*win_c2, 8 + 13*win_r2 ); if (win_r1 == win_r2) g.drawLine( 8 + 13*win_c1, 7 + 13*win_r1, 8 + 13*win_c2, 7 + 13*win_r2 ); else g.drawLine( 7 + 13*win_c1, 8 + 13*win_r1, 7 + 13*win_c2, 8 + 13*win_r2 ); } public Dimension getPreferredSize() { // Specify desired size for this component. Note: // the size MUST be 172 by 172. return new Dimension(172, 172); } public Dimension getMinimumSize() { return new Dimension(172, 172); } public void mousePressed(MouseEvent evt) { // Respond to a user click on the board. If no game is // in progress, show an error message. Otherwise, find // the row and column that the user clicked and call // doClickSquare() to handle it. if (gameInProgress == false) message.setText("Click \"New Game\" to start a new game."); else { int col = (evt.getX() - 2) / 13; int row = (evt.getY() - 2) / 13; if (col >= 0 && col < 13 && row >= 0 && row < 13) doClickSquare(row,col); } } public void mouseReleased(MouseEvent evt) { } public void mouseClicked(MouseEvent evt) { } public void mouseEntered(MouseEvent evt) { } public void mouseExited(MouseEvent evt) { } } // end class GoMokuCanvas
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