18.9 Case Study: TicTacToe

18.9 Case Study: TicTacToe 629

The TicTacToe board consists of nine cells, created using new Cell[3][3]. To determine which player’s turn it is, you can introduce a variable named whoseTurn of char type. whoseTurn is initially X, then changes to O, and subsequently changes between X and O whenever a new cell is occupied. When the game is over, set whoseTurn to ' '.

How do you know whether the game is over, whether there is a winner, and who the winner, if any, is? You can create a method named isWon(char token) to check whether a specified token has won and a method named isFull() to check whether all the cells are occupied.

Clearly, two classes emerge from the foregoing analysis. One is the Cell class, which handles operations for a single cell; the other is the TicTacToe class, which plays the whole game and deals with all the cells. The relationship between these two classes is shown in Figure 18.13.

Cell javax.swing.JApplet

9

1

TicTacToe

-whoseTurn: char

-cell: Cell[][]

-jlblStatus: JLabel

+TicTacToe()

+isFull(): boolean

+isWon(token: char): boolean

FIGURE 18.13 The TicTacToe class contains nine cells.

Since the Cell class is only to support the TicTacToe class, it can be defined as an inner class in TicTacToe. The complete program is given in Listing 18.10:

LISTING 18.10 TicTacToe.java

10// Create and initialize cells

11private Cell[][] cells = new Cell[3][3];

13// Create and initialize a status label

14private JLabel jlblStatus = new JLabel("X's turn to play");

16/** Initialize UI */

17public TicTacToe() {

18// Panel p to hold cells

19JPanel p = new JPanel(new GridLayout(3, 3, 0, 0));

20for (int i = 0; i < 3; i++)

21for (int j = 0; j < 3; j++)

22p.add(cells[i][j] = new Cell());

18.9 Case Study: TicTacToe 631

83

84/** Return token */

85public char getToken() {

86return token;

87}

88

89/** Set a new token */

90public void setToken(char c) {

91token = c;

92repaint();

93}

99if (token == 'X') {

100g.drawLine(10, 10, getWidth() - 10, getHeight() - 10);

101g.drawLine(getWidth() - 10, 10, 10, getHeight() - 10);

102}

103else if (token == 'O') {

104g.drawOval(10, 10, getWidth() - 20, getHeight() - 20);

105}

106}

109/** Handle mouse click on a cell */

110public void mouseClicked(MouseEvent e) {

111// If cell is empty and game is not over

112if (token == ' ' && whoseTurn != ' ') {

113setToken(whoseTurn); // Set token in the cell

115// Check game status

116if (isWon(whoseTurn)) {

117jlblStatus.setText(whoseTurn + " won! The game is over");

118whoseTurn = ' '; // Game is over

119}

120else if (isFull()) {

121jlblStatus.setText("Draw! The game is over");

122whoseTurn = ' '; // Game is over

123}

124else {

125// Change the turn

126whoseTurn = (whoseTurn == 'X') ? 'O': 'X';

127// Display whose turn

128jlblStatus.setText(whoseTurn + "'s turn");

129}

130}

131}

132}

133}

The TicTacToe class initializes the user interface with nine cells placed in a panel of GridLayout (lines 19–22). A label named jlblStatus is used to show the status of the game (line 14). The variable whoseTurn (line 8) is used to track the next type of token to be placed in a cell. The methods isFull (lines 34-41) and isWon (lines 44–72) are for checking the status of the game.

Since Cell is an inner class in TicTacToe, the variable (whoseTurn) and methods (isFull and isWon) defined in TicTacToe can be referenced from the Cell class. The inner class makes