Nibbles in JRuby Swing

In this part of the JRuby 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.

#!/usr/local/bin/jruby

# ZetCode JRuby Swing tutorial
# 
# In this program, we create
# a Nibbles game clone.
# 
# author: Jan Bodnar
# website: www.zetcode.com
# last modified: December 2010


include Java

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


NWIDTH = 300
NHEIGHT = 300
DOT_SIZE = 10
ALL_DOTS = NWIDTH * NHEIGHT / (DOT_SIZE * DOT_SIZE)
RAND_POS = 25
DELAY = 140

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


class Board < JPanel
    include KeyListener, ActionListener
    
    def initialize 
        super
        
        self.setFocusable true
       
        self.initGame
    end


    def initGame
    
        @left = false
        @right = true
        @up = false
        @down = false
        @inGame = true
        @dots = 3
        
        begin
            iid = ImageIcon.new "dot.png"
            @ball = iid.getImage

            iia = ImageIcon.new "apple.png"
            @apple = iia.getImage

            iih = ImageIcon.new "head.png"
            @head = iih.getImage
        rescue
            puts "cannot load images"
        end

        for i in 0..@dots
            $x[i] = 50 - i * 10
            $y[i] = 50
        end

        self.locateApple
        self.setBackground Color.black
        self.addKeyListener self
        
        @timer = Timer.new DELAY, self
        @timer.start
        
    end

    def paint g

        super g      

        if @inGame

            self.drawObjects g
            
            Toolkit.getDefaultToolkit.sync
            g.dispose

        else
            self.gameOver g
        end
    end


    def drawObjects g

        g.drawImage @apple, @apple_x, @apple_y, self

        for z in 0..@dots
            if z == 0
                g.drawImage @head, $x[z], $y[z], self
            else
                g.drawImage @ball, $x[z], $y[z], self
            end
        end
    end
    

    def gameOver g

        msg = "Game Over"
        small = Font.new "Helvetica", Font::BOLD, 14
        metr = self.getFontMetrics small

        g.setColor Color.white
        g.setFont small
        g.drawString msg,  (NWIDTH - metr.stringWidth(msg)) / 2,
                     NHEIGHT / 2
        @timer.stop
    end
    

    def checkApple

        if $x[0] == @apple_x and $y[0] == @apple_y 
            @dots = @dots + 1
            self.locateApple
        end
    end
    
    
    def move

        z = @dots

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

        if @left
            $x[0] -= DOT_SIZE
        end

        if @right 
            $x[0] += DOT_SIZE
        end

        if @up
            $y[0] -= DOT_SIZE
        end

        if @down
            $y[0] += DOT_SIZE
        end
        
     end


    def checkCollision

        z = @dots
       
        while z > 0
            if z > 4 and $x[0] == $x[z] and $y[0] == $y[z]
                @inGame = false
            end
            z = z - 1
        end

        if $y[0] > NHEIGHT - DOT_SIZE
            @inGame = false
        end
        
        if $y[0] < 0
            @inGame = false
        end
        
        if $x[0] > NWIDTH - DOT_SIZE
            @inGame = false
        end
        
        if $x[0] < 0
            @inGame = false
        end    
        
    end


    def locateApple
    
        r = rand RAND_POS
        @apple_x = r * DOT_SIZE
        r = rand RAND_POS
        @apple_y = r * DOT_SIZE
    end

    
    def actionPerformed e

        if @inGame
            self.checkApple
            self.checkCollision
            self.move
        end
        
        self.repaint
        
    end
    
    def keyReleased e
    end

    def keyPressed e
        
        key = e.getKeyCode

        if key == KeyEvent::VK_LEFT and not @right
            @left = true
            @up = false
            @down = false
        end
        
        if key == KeyEvent::VK_RIGHT and not @left
            @right = true
            @up = false
            @down = false
        end

        if key == KeyEvent::VK_UP and not @down
            @up = true
            @right = false
            @left = false
        end

        if key == KeyEvent::VK_DOWN and not @up
            @down = true
            @right = false
            @left = false
        end
    end    
end


class Example < JFrame
  
    def initialize
        super "Nibbles"
        
        self.initUI
    end
      
    def initUI
      
        board = Board.new
        board.setPreferredSize Dimension.new NWIDTH, NHEIGHT
        self.add board     

        self.pack
        
        self.setResizable false
        self.setDefaultCloseOperation JFrame::EXIT_ON_CLOSE
        self.setLocationRelativeTo nil
        self.setVisible true
    end
end

Example.new

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 g

    super g      

    if @inGame

        self.drawObjects g
        
        Toolkit.getDefaultToolkit.sync
        g.dispose

    else
        self.gameOver g
    end
end

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. The Toolkit.getDefaultToolkit.sync method ensures that the display is up-to-date. It is useful for animation.

def drawObjects g

    g.drawImage @apple, @apple_x, @apple_y, self

    for z in 0..@dots
        if z == 0
            g.drawImage @head, $x[z], $y[z], self
        else
            g.drawImage @ball, $x[z], $y[z], self
        end
    end
end

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.

def gameOver g

    msg = "Game Over"
    small = Font.new "Helvetica", Font::BOLD, 14
    metr = self.getFontMetrics small

    g.setColor Color.white
    g.setFont small
    g.drawString msg,  (NWIDTH - metr.stringWidth(msg)) / 2,
                  NHEIGHT / 2
    @timer.stop
end

In the gameOver method, we display "Game Over" message in the center of the window. We also stop the timer.

def checkApple

    if $x[0] == @apple_x and $y[0] == @apple_y 
        @dots = @dots + 1
        self.locateApple
    end
end

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
end

This code moves the joints up the chain.

if @left
    $x[0] -= DOT_SIZE
end

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]
        @inGame = false
    end
    z = z - 1
end

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

if $y[0] > NHEIGHT - DOT_SIZE
    @inGame = false
end

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

The locateApple method locates an apple randomly on the board.

r = rand RAND_POS

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

@apple_x = r * DOT_SIZE
...
@apple_y = r * DOT_SIZE

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

def actionPerformed e

    if @inGame
        self.checkApple
        self.checkCollision
        self.move
    end
    
    self.repaint
    
end

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.

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

if key == KeyEvent::VK_LEFT and not @right
    @left = true
    @up = false
    @down = false
end

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.

class Example < JFrame
  
    def initialize
        super "Nibbles"
        
        self.initUI
    end
      
    def initUI
      
        board = Board.new
        board.setPreferredSize Dimension.new NWIDTH, NHEIGHT
        self.add board     

        self.pack
        
        self.setResizable false
        self.setDefaultCloseOperation JFrame::EXIT_ON_CLOSE
        self.setLocationRelativeTo nil
        self.setVisible true
    end
end

In this class, we set up the Nibbles game.

Nibbles
Figure: Nibbles

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