Question

In this part of the Jython Swing programming tutorial, we will create a Nibbles game clone.

Nibbles is an older classic video game. It was first created in late 70s. Later it was brought to PCs. In this game the player controls a snake. The objective is to eat as many apples as possible. Each time the snake eats an apple, its body grows. The snake must avoid the walls and its own body.

Development

The size of each of the joints of a snake is 10px. The snake is controlled with the cursor keys. Initially, the snake has three joints. The game starts immediately. When the game is finished, we display "Game Over" message in the center of the window.

import random

from java.awt import Color
from java.awt import Font
from java.awt import Toolkit
from java.awt.event import ActionListener
from java.awt.event import KeyEvent
from java.awt.event import KeyListener
from javax.swing import ImageIcon
from javax.swing import JPanel
from javax.swing import Timer

WIDTH = 300
HEIGHT = 300
DOT_SIZE = 10
ALL_DOTS = WIDTH * HEIGHT / (DOT_SIZE * DOT_SIZE)
RAND_POS = 29
DELAY = 140

x = [0] * ALL_DOTS
y = [0] * ALL_DOTS


class Board(JPanel, KeyListener, ActionListener):
  def __init__(self):
    super(Board, self).__init__()

    self.initUI()


  def initUI(self):
    
    self.setBackground(Color.black)

    iid = ImageIcon("dot.png")
    self.ball = iid.getImage()

    iia = ImageIcon("apple.png")
    self.apple = iia.getImage()

    iih = ImageIcon("head.png")
    self.head = iih.getImage()

    self.setFocusable(True)
    self.addKeyListener(self)
    self.initGame()


  def initGame(self):

    self.left = False
    self.right = True
    self.up = False
    self.down = False
    self.inGame = True
    self.dots = 3

    for i in range(self.dots):
      x[i] = 50 - i * 10
      y[i] = 50


    self.locateApple()

    self.timer = Timer(DELAY, self)
    self.timer.start()
  


  def paint(self, g):

    # due to bug, cannot call super()
    JPanel.paint(self, g)

    if self.inGame:
      self.drawObjects(g)

    else:
      self.gameOver(g)

  def drawObjects(self, g):
    
    g.drawImage(self.apple, self.apple_x, self.apple_y, self)

    for z in range(self.dots):
      if (z == 0):
        g.drawImage(self.head, x[z], y[z], self)
      else:
        g.drawImage(self.ball, x[z], y[z], self)

    Toolkit.getDefaultToolkit().sync()
    g.dispose()

  def gameOver(self, g):

    msg = "Game Over"
    small = Font("Helvetica", Font.BOLD, 14)
    metr = self.getFontMetrics(small)

    g.setColor(Color.white)
    g.setFont(small)
    g.drawString(msg, (WIDTH - metr.stringWidth(msg)) / 2,
           HEIGHT / 2)


  def checkApple(self):

    if x[0] == self.apple_x and y[0] == self.apple_y:
      self.dots = self.dots + 1
      self.locateApple()


  def move(self):

    z = self.dots

    while z > 0:
      x[z] = x[(z - 1)]
      y[z] = y[(z - 1)]
      z = z - 1

    if self.left:
      x[0] -= DOT_SIZE

    if self.right:
      x[0] += DOT_SIZE

    if self.up:
      y[0] -= DOT_SIZE

    if self.down:
      y[0] += DOT_SIZE


  def checkCollision(self):

    z = self.dots

    while z > 0:
      if z > 4 and x[0] == x[z] and y[0] == y[z]:
        self.inGame = False
      z = z - 1

    if y[0] > HEIGHT - DOT_SIZE:
      self.inGame = False

    if y[0] < 0:
      self.inGame = False

    if x[0] > WIDTH - DOT_SIZE:
      self.inGame = False

    if x[0] < 0:
      self.inGame = False

  def locateApple(self):

    r = random.randint(0, RAND_POS)
    self.apple_x = r * DOT_SIZE
    r = random.randint(0, RAND_POS)
    self.apple_y = r * DOT_SIZE

#  public void actionPerformed(ActionEvent e) {

  def actionPerformed(self, e):

    if self.inGame:
      self.checkApple()
      self.checkCollision()
      self.move()
    else:
      self.timer.stop()
    
    self.repaint()
  

  def keyPressed(self, e):

    key = e.getKeyCode()

    if key == KeyEvent.VK_LEFT and not self.right:
      self.left = True
      self.up = False
      self.down = False


    if key == KeyEvent.VK_RIGHT and not self.left:
      self.right = True
      self.up = False
      self.down = False

    if key == KeyEvent.VK_UP and not self.down:
      self.up = True
      self.right = False
      self.left = False

    if key == KeyEvent.VK_DOWN and not self.up:
      self.down = True
      self.right = False
      self.left = False

First we will define some constants used in our game.

The WIDTH and HEIGHT constants determine the size of the Board. The DOT_SIZE is the size of the apple and the dot of the snake. The ALL_DOTS constant defines the maximum number of possible dots on the Board. The RAND_POS constant is used to calculate a random position of an apple. The DELAY constant determines the speed of the game.

x = [0] * ALL_DOTS
y = [0] * ALL_DOTS

These two arrays store x, y coordinates of all possible joints of a snake.

The initGame() method initialises variables, loads images and starts a timeout function.

def paint(self, g):

  JPanel.paint(self, g)

  if self.inGame:
    self.drawObjects(g)

  else:
    self.gameOver(g)

Inside the paint() method, we check the inGame variable. If it is true, we draw our objects. The apple and the snake joints. Otherwise we display "Game over" text.

def drawObjects(self, g):
  
  g.drawImage(self.apple, self.apple_x, self.apple_y, self)

  for z in range(self.dots):
    if (z == 0):
      g.drawImage(self.head, x[z], y[z], self)
    else:
      g.drawImage(self.ball, x[z], y[z], self)

  Toolkit.getDefaultToolkit().sync()
  g.dispose()

The drawObjects() method draws the apple and the joints of the snake. The first joint of a snake is its head, which is represented by a red circle. The Toolkit.getDefaultToolkit().sync() method ensures that the display is up-to-date. It is useful for animation.

def checkApple(self):

  if x[0] == self.apple_x and y[0] == self.apple_y:
    self.dots = self.dots + 1
    self.locateApple()

The checkApple() method checks if the snake has hit the apple object. If so, we add another snake joint and call the locateApple() method, which randomly places a new apple object.

In the move() method we have the key algorithm of the game. To understand it, look at how the snake is moving. You control the head of the snake. You can change its direction with the cursor keys. The rest of the joints move one position up the chain. The second joint moves where the first was, the third joint where the second was etc.

while z > 0:
  x[z] = x[(z - 1)]
  y[z] = y[(z - 1)]
  z = z - 1

This code moves the joints up the chain.

if self.left:
  x[0] -= DOT_SIZE

Move the head to the left.

In the checkCollision() method, we determine if the snake has hit itself or one of the walls.

while z > 0:
  if z > 4 and x[0] == x[z] and y[0] == y[z]:
    self.inGame = False
  z = z - 1

We finish the game if the snake hits one of its joints with the head.

if y[0] > HEIGHT - DOT_SIZE:
  self.inGame = False

We finish the game if the snake hits the bottom of the Board.

The locateApple() method locates an apple randomly on the board.

r = random.randint(0, RAND_POS)

We get a random number from 0 to RAND_POS - 1.

self.apple_x = r * DOT_SIZE
...
self.apple_y = r * DOT_SIZE

These lines set the x, y coordinates of the apple object.

def actionPerformed(self, e):

  if self.inGame:
    self.checkApple()
    self.checkCollision()
    self.move()
  else:
    self.timer.stop()
  
  self.repaint()

Every DELAY ms, the actionPerformed() method is called. If we are in the game, we call three methods that build the logic of the game. Otherwise we stop the timer.

In the keyPressed() method of the Board class, we determine the keys that were pressed.

if key == KeyEvent.VK_LEFT and not self.right:
  self.left = True
  self.up = False
  self.down = False

If we hit the left cursor key, we set left variable to true. This variable is used in the move() method to change coordinates of the snake object. Notice also that when the snake is heading to the right, we cannot turn immediately to the left.

#!/usr/local/bin/jython
# -*- coding: utf-8 -*-

"""
ZetCode Jython Swing tutorial

This is a simple Nibbles game
clone.

author: Jan Bodnar
website: zetcode.com
last edited: December 2010
"""

from java.awt import Dimension
from javax.swing import JFrame
from Board import Board


class Nibbles(JFrame):
  def __init__(self):
    super(Nibbles, self).__init__()

    self.initUI()

  def initUI(self):

    self.board = Board()
    self.board.setPreferredSize(Dimension(300, 300))
    self.add(self.board)

    self.setTitle("Nibbles")

    self.pack()
    self.setResizable(False)
    self.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)
    self.setLocationRelativeTo(None)
    self.setVisible(True)


if __name__ == '__main__':
  Nibbles()

In this class, we set up the Nibbles game.

Figure: Nibbles

This was the Nibbles computer game programmed with the Swing library and the Jython programming language.