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Thread: "Exception in thread "main" java.lang.NullPointerException" Error in code. Help?

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    Default "Exception in thread "main" java.lang.NullPointerException" Error in code. Help?

    I had my code working (at least somewhat) and I must have changed something, because now it won't even launch. There's no shown errors within the code, but when I try to run it this is what appears:
    "Exception in thread "main" java.lang.NullPointerException
    at Version2.Game.moveAllObjects(Game.java:117)
    at Version2.Game.playGame(Game.java:63)
    at Version2.GameDemo.main(GameDemo.java:8)"

    How do I fix it?

    Here's my relevant code:

    1.GameDemo:
     
    package Version2;
     
     
    public class GameDemo {
     
    	public static void main(String argv[]) {
    		Game g = new Game();
    		g.playGame();
     
    	}
     
    }



    2. Game:
    package Version2;
     
    import java.awt.Canvas;
    import java.awt.Color;
    import java.awt.Dimension;
    import java.awt.Graphics2D;
    import java.awt.event.KeyAdapter;
    import java.awt.event.KeyEvent;
    import java.awt.event.WindowAdapter;
    import java.awt.event.WindowEvent;
    import java.awt.image.BufferStrategy;
     
    import javax.swing.JFrame;
    import javax.swing.JPanel;
     
    import Version2.RowOfAlien;
     
    /**
     * @original author Kevin Glass
     * @adapted by: Nikunj Raghuvanshi
     * @adapted by: Kye Hedlund 3/2010
     */
    public class Game extends Canvas {
     
    	// CONSTANTS
    	public final int ALIEN_WIDTH = 40;
    	public final int SCREEN_WIDTH = 800;
    	public final int SCREEN_HEIGHT = 600;
     
    	// Time (in ms) to pause after each step of the game loop.
    	// This, in part, determines the speed at which the game is
    	// updated on the screen.
    	public final int PAUSE_INTERVAL = 10;
     
    	// VARIABLES
    	private RowOfAlien rowofalien;
    	private Base base;
    	private Missile missile;
    	private Alien alien;
     
    	// Keyboard stuff - will be used in a future assignment
    	// True if we're holding up game play until a key has been pressed
    	private boolean waitingForKeyPress = true;
    	private boolean leftCursorCurrentlyPressed = false;
    	private boolean rightCursorCurrentlyPressed = false;
    	private boolean fireCurrrentlyPressed = false;
    	private boolean gameRunning = true;
    	// Graphics stuff
    	private Graphics2D g;
    	private BufferStrategy strategy; // Use accelerated page flipping
     
    	/*
    	 * Create a new game that is ready to play
    	 */
    	public Game() {
    		graphicsInitialization();
    		initAllObjects();
    	}
     
    	public void playGame() {
     
    		while (gameRunning) {
    			moveAllObjects();
    			graphicsUpdateStartOfLoop();
    			drawAllObjects();
    			graphicsUpdateEndOfLoop();
     
     
    		}
    	}
     
    	private void drawAllObjects() {
    		rowofalien.drawAlien(g);
    		base.draw(g);
    		missile.draw(g);
    		alien.draw(g);
    	}
     
    	private void initAllObjects() {
    		final int ALIEN_X_POSITION = 20;
    		final int ALIEN_Y_POSITION = -40; // ?????????????? negative
    		alien = new Alien(ALIEN_X_POSITION, ALIEN_Y_POSITION, SCREEN_WIDTH, SCREEN_HEIGHT);
     
    		final int BASE_X_POSITION = 400;
    		final int BASE_Y_POSITION = 560;
    		base = new Base(BASE_X_POSITION, BASE_Y_POSITION);
     
    		final int MISSILE_X_POSITION = 400;
    		final int MISSILE_Y_POSITION = 560;
    		missile = new Missile(MISSILE_X_POSITION, MISSILE_Y_POSITION);
     
     
     
    	}
     
    	private void moveAllObjects() {
     
     
    		if (leftCursorCurrentlyPressed == true) {
    			base.moveLeft();
     
    		}
    		if (rightCursorCurrentlyPressed == true) {
    			base.moveRight();
     
    		}
     
    		if ((fireCurrrentlyPressed == true) && (missile.getUpperLeftY() >= 600)) {
    			missile.setUpperLeftX(base.getUpperLeftX());
    			missile.move();
    		}
    		if (missile.getUpperLeftY() < 600)
    			missile.move();
    		gameRunning = false;
     
    		for (int index = 0; index < 6; index++) {
    			if (rowofalien.getAlien(index).getUpperLeftX() <= missile.getUpperLeftX()
    					&& missile.getUpperLeftX() <= rowofalien.getAlien(index).getUpperLeftX() + 40
    					&& rowofalien.getAlien(index).getUpperLeftY() <= missile.getUpperLeftY()
    					&& missile.getUpperLeftY() <= rowofalien.getAlien(index).getUpperLeftY() + 40
    					&& missile.getUpperLeftY() > 0) {
     
    				System.out.println("Hit!");
    				rowofalien.setAlive(index, false);
     
    			}
    			rowofalien.move();
     
    			gameRunning = gameRunning || rowofalien.getAlive(index);
    		}
     
     
    		}
     
     
     
     
    	// **************************************************************
    	// Do not change any of the methods whose names begin with "graphics" or
    	// "key".
    	// From here to the end of the class - don't change anything.
    	public void graphicsUpdateEndOfLoop() {
     
    		// Finally, we've completed drawing so clear up the graphics
    		// and flip the buffer over
    		g.dispose();
    		strategy.show();
     
    		// Finally pause for a bit. Note: this should run us at about
    		// 100 fps but on windows this might vary each loop due to
    		// a bad implementation of timer
    		try {
    			Thread.sleep(PAUSE_INTERVAL);
    		} catch (Exception e) {
    		}
     
    	}
     
    	// Do no change any of the methods whose names begin with "graphics"
    	public void graphicsUpdateStartOfLoop() {
     
    		// Get hold of a graphics context for the accelerated
    		// surface and blank it out
    		g = (Graphics2D) strategy.getDrawGraphics();
    		g.setColor(Color.black);
    		g.fillRect(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT);
    	}
     
    	public void graphicsInitialization() {
     
    		// Create a frame to contain our game
    		JFrame container = new JFrame("Space Invaders 110");
     
    		// Get the content of the frame and set up the resolution of the
    		// game
    		JPanel panel = (JPanel) container.getContentPane();
    		panel.setPreferredSize(new Dimension(SCREEN_WIDTH, SCREEN_HEIGHT));
    		panel.setLayout(null);
     
    		// Setup our canvas size and put it into the content of the frame
    		setBounds(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT);
    		panel.add(this);
     
    		// Tell AWT not to bother repainting our canvas since we're
    		// going to do that ourself in accelerated mode
    		setIgnoreRepaint(true);
     
    		// Make the window visible
    		container.pack();
    		container.setResizable(false);
    		container.setVisible(true);
     
    		// Add a listener to respond to the user closing the window. If they
    		// do we'd like to exit the game
    		container.addWindowListener(new WindowAdapter() {
    			public void windowClosing(WindowEvent e) {
    				System.exit(0);
    			}
    		});
     
    		// Request the focus so key events come to us
    		requestFocus();
     
    		// Create the buffering strategy which will allow AWT
    		// to manage our accelerated graphics
    		createBufferStrategy(2);
    		strategy = getBufferStrategy();
     
    		// Add a key input system (defined below) to our canvas
    		// so we can respond to key pressed
    		addKeyListener(new KeyInputHandler());
    	}
     
    	/**
    	 * A class to handle keyboard input from the user. The class handles both
    	 * dynamic input during game play, i.e. left/right and shoot, and more
    	 * static type input (i.e. press any key to continue)
    	 * 
    	 * This has been implemented as an inner class more through habbit then
    	 * anything else. Its perfectly normal to implement this as seperate class
    	 * if slight less convienient.
    	 * 
    	 * @author Kevin Glass
    	 */
    	private class KeyInputHandler extends KeyAdapter {
    		/**
    		 * The number of key presses we've had while waiting for an "any key"
    		 * press
    		 */
    		private int pressCount = 1;
     
    		/**
    		 * Notification from AWT that a key has been pressed. Note that a key
    		 * being pressed is equal to being pushed down but *NOT* released. Thats
    		 * where keyTyped() comes in.
    		 * 
    		 * @param e
    		 *            The details of the key that was pressed
    		 */
    		public void keyPressed(KeyEvent e) {
    			// if we're waiting for an "any key" typed then we don't
    			// want to do anything with just a "press"
    			if (waitingForKeyPress) {
    				return;
    			}
     
    			if (e.getKeyCode() == KeyEvent.VK_LEFT) {
    				leftCursorCurrentlyPressed = true;
    			}
    			if (e.getKeyCode() == KeyEvent.VK_RIGHT) {
    				rightCursorCurrentlyPressed = true;
    			}
    			if (e.getKeyCode() == KeyEvent.VK_SPACE) {
    				fireCurrrentlyPressed = true;
    			}
    		}
     
    		/**
    		 * Notification from AWT that a key has been released.
    		 * 
    		 * @param e
    		 *            The details of the key that was released
    		 */
    		public void keyReleased(KeyEvent e) {
    			// if we're waiting for an "any key" typed then we don't
    			// want to do anything with just a "released"
    			if (waitingForKeyPress) {
    				return;
    			}
    			if (e.getKeyCode() == KeyEvent.VK_LEFT) {
    				leftCursorCurrentlyPressed = false;
    			}
    			if (e.getKeyCode() == KeyEvent.VK_RIGHT) {
    				rightCursorCurrentlyPressed = false;
    			}
    			if (e.getKeyCode() == KeyEvent.VK_SPACE) {
    				fireCurrrentlyPressed = false;
    			}
    		}
     
    		/**
    		 * Notification from AWT that a key has been typed. Note that typing a
    		 * key means to both press and then release it.
    		 * 
    		 * @param e
    		 *            The details of the key that was typed.
    		 */
    		public void keyTyped(KeyEvent e) {
    			// if we're waiting for a "any key" type then
    			// check if we've recieved any recently. We may
    			// have had a keyType() event from the user releasing
    			// the shoot or move keys, hence the use of the "pressCount"
    			// counter.
    			if (waitingForKeyPress) {
    				if (pressCount == 1) {
    					// since we've now recieved our key typed
    					// event we can mark it as such and start
    					// our new game
    					waitingForKeyPress = false;
    					// startGame();
    					pressCount = 0;
    				} else {
    					pressCount++;
    				}
    			}
     
    		}
    	}
     
    }

    3. RowOfAlien:
    package Version2;
     
    import java.awt.Graphics;
     
    public class RowOfAlien {
     
    	// CONSTANTS
    	public final int NUMBER_OF_ALIENS = 5;
     
    	// Size of the canvas in which alien moves
    	private final int MAX_WIDTH;
    	private final int MAX_HEIGHT;
     
    	// VARIABLES
    	private Alien[] row;
    	private boolean[] alive;
    	private int upperLeftX, upperLeftY;
    	//private boolean moveRight = true;
    	private final int INITIAL_X_SPEED = 5;
    	private final int DROP_DOWN_AMOUNT = 50;
     
     
    	public RowOfAlien(int numberOfAliens, int theMaxWidth, int theMaxHeight) {
    		MAX_WIDTH = theMaxWidth;
    		MAX_HEIGHT = theMaxHeight;
    		row = new Alien[6];
     
     
    		for (int index = 0; index < 6; index++) {
    			row[index] = new Alien(index * 50, 50, theMaxWidth, theMaxHeight);
    			alive[index] = true;
    		}
    		alive = new boolean[6];
    	}
     
    	public void move() {
    		for (int index = 0; index < row.length; index++) {
    			if (alive[index]) {
    				row[index].move();
    			}
    		}
    		//for (int index = 0; index < 6; index++) {
    			//if (alive[index])
    				//if(row[index].getUpperLeftY() < 580){
    					//if (moveRight)
    						//row[index].setUpperLeftX(row[index].getUpperLeftX() + INITIAL_X_SPEED);
    					//else
    						//row[index].setUpperLeftX(row[index].getUpperLeftX() - INITIAL_X_SPEED);
    				//}
     
     
    				}
    		//for (int index = 0; index < 6; index++) {
    			//if (alive[index])
    				//if (row[index].getUpperLeftX() >= 780){
    					//for (int j = 0; j < 6; j++)
    						//row[j].setUpperLeftY(row[j].getUpperLeftY() + DROP_DOWN_AMOUNT);
    					//moveRight = false;
    					//if (row[index].getUpperLeftY() >= 580)
    						//System.out.println("DONE");
    				//} else if (row[index].getUpperLeftX() <= 0) {
    					//for (int j = 0; j< 6; j++)
    						//row[j].setUpperLeftY(row[j].getUpperLeftY() + DROP_DOWN_AMOUNT);
    					//moveRight = true;
     
     
    				//}
     
     
     
     
     
    	public void drawAlien(Graphics g) {
    		for (int index = 0; index < row.length; index++) {
    			if (alive[index]) {
    				row[index].draw(g);
     
    			}
    		}
    	}
     
     
     
     
    	public void setAlive(int i, boolean val)
    	{ 
    		alive[i] = val;
     
    	}
    	public boolean getAlive(int i)
    	{
    		return alive[i];
    	}
    	public Alien getAlien(int index) {
    		return row[index];
    	}
     
    }







    Any suggestions?
    Last edited by poshcandlesticks; December 6th, 2011 at 09:26 PM.


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: "Exception in thread "main" java.lang.NullPointerException" Error in code. Help?

    "Exception in thread "main" java.lang.NullPointerException
    at Version2.Game.moveAllObjects(Game.java:117)
    In the code at line 117 a variable has a null value. Look at line 117 and find the variable with the null value. Then backtrack in the code to see why the variable does not have a valid value and change the code so it does.
    If you can not see what variable is null, add a println() just before line 117 and print out the values of all the variables.

    Please edit your post and wrap your code in code tags: [code]
    See:BB Code List - Java Programming Forums - The Java Community

  3. The Following User Says Thank You to Norm For This Useful Post:

    poshcandlesticks (December 6th, 2011)

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    Default Re: "Exception in thread "main" java.lang.NullPointerException" Error in code. Help?

    I took your advice and found that the rowofalien variable is null. How do I go about fixing that?

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    Default Re: "Exception in thread "main" java.lang.NullPointerException" Error in code. Help?

    Quote Originally Posted by poshcandlesticks View Post
    I took your advice and found that the rowofalien variable is null. How do I go about fixing that?
    You must know what should be in there to execute your code properly. It totally depends upon your logic and rules of program.
    rowofalien.drawAlien(g);
    In your code, rowofalien is actually an object of RowOfAlien but it's never initialized.

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