"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:

Code :

---

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

---

2. Game:

Code :

---

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:

Code :

---

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?

Quote:

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"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

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

Quote:

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Originally Posted by poshcandlesticks

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

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You must know what should be in there to execute your code properly. It totally depends upon your logic and rules of program.

Code :

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rowofalien.drawAlien(g);

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In your code, rowofalien is actually an object of RowOfAlien but it's never initialized.