SwingTicTacToe.java

package tictactoegame.ui;

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

public class SwingTicTacToe extends JFrame {

    /**
     * The font used for the buttons.
     */
    private static final Font FONT = new Font("Arial", Font.PLAIN, 50);

    /**
     * The board map (2D array, each char is one of '-', 'x', 'o').
     */
    private final char[][] board = {{'-', '-', '-'}, {'-', '-', '-'}, {'-', '-', '-'}};

    /**
     * The buttons corresponding to the board.
     */
    private final JButton[][] buttons = new JButton[3][3];

    /**
     * Whether it is X's turn currently.
     */
    private boolean turnX = true;

    private SwingTicTacToe(String title, int width, int height) {
        super(title);
        // Sets the size of the window
        setSize(width, height);
        // Puts the window in the middle of the screen, instead of the top left
        setLocationRelativeTo(null);
        // Makes the application exit when the window is closed
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        // Makes the window not resizable
        setResizable(false);

        Container pane = getContentPane();
        // Makes the grid 3x3
        pane.setLayout(new GridLayout(3, 3));

        // Initializes the buttons
        for (int y = 0; y < 3; y++) {
            for (int x = 0; x < 3; x++) {
                JButton button = new JButton();
                button.setFont(FONT);

                // Loop cursors are copied to final values, so they can be used inside anonymous classes
                int finalX = x;
                int finalY = y;
                // Adds action listeners for each button
                button.addActionListener(e -> {
                    // Sets the board value
                    board[finalY][finalX] = turnX ? 'x' : 'o';
                    updateButtons();
                    // Switches whose turn it is
                    turnX = !turnX;
                    // Checks if the game has ended
                    checkBoard();
                });
                // Removes the outline on buttons
                button.setFocusable(false);

                // Adds the button to the grid
                pane.add(button);
                buttons[y][x] = button;
            }
        }
        updateButtons();
    }

    private void checkBoard() {
        if (!someoneHasWon() && isBoardFull()) {
            // If nobody won and the board is full, the game is a tie
            JOptionPane.showMessageDialog(this, "It's a tie!");
            // Start over
            resetBoard();
        }
        if (someoneHasWon()) {
            // Someone won, announce the winner
            if (!turnX) {
                JOptionPane.showMessageDialog(this, "X won!");
            } else {
                JOptionPane.showMessageDialog(this, "O won!");
            }
            // Start over
            resetBoard();
        }
    }

    /**
     * Checks if whether someone has completed a winning move.
     */
    private boolean someoneHasWon() {
        // Diagonals
        if (board[1][1] != '-') {
            if (board[0][0] == board[1][1] && board[1][1] == board[2][2])
                return true;

            if (board[0][2] == board[1][1] && board[1][1] == board[2][0])
                return true;
        }

        // Test rows
        for (int i = 0; i < 3; ++i) {
            if (board[i][0] != '-' && board[i][0] == board[i][1] && board[i][1] == board[i][2])
                return true;
        }

        // Test columns
        for (int j = 0; j < 3; ++j) {
            if (board[0][j] != '-' && board[0][j] == board[1][j] && board[1][j] == board[2][j])
                return true;
        }
        return false;
    }

    /**
     * Checks whether no tiles are left empty.
     */
    private boolean isBoardFull() {
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 3; j++) {
                if (board[i][j] == '-') {
                    return false;
                }
            }
        }
        return true;
    }

    /**
     * Resets the board to its original (empty) state.
     */
    private void resetBoard() {
        for (int y = 0; y < 3; y++) {
            for (int x = 0; x < 3; x++) {
                board[y][x] = '-';
            }
        }
        turnX = true;
        updateButtons();
    }

    /**
     * Updates the buttons to reflect the board.
     */
    private void updateButtons() {
        for (int y = 0; y < 3; y++) {
            for (int x = 0; x < 3; x++) {
                char tile = board[y][x];
                if (tile == '-') {
                    buttons[y][x].setEnabled(true);
                    buttons[y][x].setText("");
                } else {
                    buttons[y][x].setText(String.valueOf(board[y][x]));
                    buttons[y][x].setEnabled(false);
                }
            }
        }
    }

    public static void main(String[] args) throws Exception {
        // Overrides the Swing look to the OS look.
        UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());

        // Start
        SwingTicTacToe window = new SwingTicTacToe("TicTacToe", 500, 500);
        window.setVisible(true);
    }
}