A simple Snake clone implementation in Java

snake.java

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

class Game extends JPanel implements ActionListener {
    final int     TIMEOUT    = 125;
    final int     SIZE       = 25;
    final int     BOARD_COLS = 10;
    final int     BOARD_ROWS = 10;
    final int     BOARD_SIZE = (BOARD_COLS * BOARD_ROWS);
    int           head       = 0;
    int           apple      = 45;
    int           direction  = 1;
    int[]         tail       = new int[BOARD_SIZE];
    int           length     = 0;
    int           i          = 0;
    Timer         timer      = new Timer(TIMEOUT, this);
    Graphics      ctx        = null;

    public static void main(String[] args) {
        JFrame window = new JFrame();
        window.add(new Game());
        window.setResizable(false);
        window.pack();
        window.setTitle("Snake");
        window.setLocationRelativeTo(null);
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setVisible(true);
    }

    public Game() {
        addKeyListener(new KeyAdapter() {
            @Override public void keyPressed(KeyEvent e) {
                if      (e.getKeyCode() == KeyEvent.VK_UP)    setDirection(-BOARD_COLS);
                else if (e.getKeyCode() == KeyEvent.VK_DOWN)  setDirection( BOARD_COLS);
                else if (e.getKeyCode() == KeyEvent.VK_LEFT)  setDirection(-1);
                else if (e.getKeyCode() == KeyEvent.VK_RIGHT) setDirection( 1);
            }
        });
        setBackground(Color.black);
        setFocusable(true);
        setDoubleBuffered(true);
        setPreferredSize(new Dimension(BOARD_COLS * SIZE, BOARD_ROWS * SIZE));
        timer.start();
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        ctx = g;
        update();
        drawBoard();
    }

    @Override
    public void actionPerformed(ActionEvent e) { repaint(); }

    void draw(int position, Color color) {
        int x = position % BOARD_COLS;
        int y = position / BOARD_COLS;
        ctx.setColor(color);
        ctx.fillRect( x * SIZE, y * SIZE, SIZE, SIZE );
    }

    int random() {
        return (int) (Math.random() * BOARD_SIZE);
    }

    boolean isOnLimits() {
        return (direction ==  1 && head % BOARD_COLS == 0) ||
                (direction == -1 && (head + 1) % BOARD_COLS == 0);
    }

    boolean hasCollisionWithTail(int position) {
        for (int i = 0; i < length; i++)
            if (tail[i] == position) return true;
        return false;
    }

    void placeApple() {
        do apple = random();
        while (head == apple || hasCollisionWithTail(apple));
    }

    void setDirection(int dir) {
        if (dir != -direction || length == 0)
            direction = dir;
    }

    void drawBoard() {
        for (int i = 0; i < length; ++i)
            draw(tail[i], Color.GREEN);

        draw(head, Color.BLUE);
        draw(apple, Color.RED);
    }

    void move() {
        tail[i] = head;
        head += direction;

        if (++i >= length) i = 0;

        if (isOnLimits())
            head -= BOARD_COLS * direction;
        else if (head < 0)
            head += BOARD_SIZE;
        else if (head >= BOARD_SIZE)
            head -= BOARD_SIZE;
    }

    void update() {
        move();

        if (head == apple) {
            placeApple();
            tail[length++] = head;
        } else if (hasCollisionWithTail(head)) timer.stop();
    }
}