Help me with My Game.

I am making a tower defense game. I followed a tutorial on how to make a game, but it was full screen. After making some of the game, I wanted to make it windowed. Now, I have a grey window that shows nothing. I think it is a problem with how I made the Graphics object:

Errors:

Exception in thread "main" java.lang.NullPointerException
at game.Main.movieLoop(Main.java:79)
at game.Main.run(Main.java:64)
at game.Main.<init>(Main.java:45)
at game.Main.main(Main.java:31)

Code :

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package game;
import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
import java.util.concurrent.CopyOnWriteArrayList;
import javax.swing.*;
 
public class Main extends JFrame {
 
	private static final long serialVersionUID = 1L;
 
	public Input listener;
	private boolean running;
	private Image green;
 
	private CopyOnWriteArrayList<Tower1> Tower1List1;
 
	private Image tower1Image;
	private Item Tower1Icon;
	private Image sidebar;
 
	private int icon1Clicked = 0;
	private int mousePressed = 0;
	private int mouseUsed = 0;
	private int drawTower1 = 0;
	private int money = 100;
 
	private Point mouse;
 
	public static void main(String[] args){
		new Main();
	}
 
	public Main(){
 
		JFrame f = new JFrame();
		f.setTitle("GAME!!!");
		f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		f.setSize(1024,768);
		f.setResizable(false);
		f.setVisible(true);
		f.createBufferStrategy(2);
		f.setBackground(Color.GREEN);
 
		run();
 
	}
 
	public void loadVariables(){
 
		mouse = new Point();
		Tower1List1 = new CopyOnWriteArrayList<Tower1>();
		tower1Image = new ImageIcon("C:\\test\\redGun.png").getImage();
		Tower1Icon = new Item(tower1Image, getWidth()-240,70);
		sidebar = new ImageIcon("C:\\test\\sidebar.png").getImage();
		green = new ImageIcon("C:\\test\\greenB.jpg").getImage();		
 
	}
 
	public void run(){
			running = true;
			loadVariables();
			listener = new Input();
			movieLoop();
	}
 
	public void movieLoop(){
		long startingTime = System.currentTimeMillis();
		long cumTime = startingTime;
 
		while(running){
 
			long timePassed = System.currentTimeMillis() - cumTime;
			cumTime += timePassed;
			update(timePassed);
			BufferStrategy s = getBufferStrategy();
LINE 79 -->>	        Graphics2D g =  (Graphics2D) s.getDrawGraphics();
			draw(g);
			s.show();
			g.dispose();
 
			try{
				Thread.sleep(20);
			}catch(Exception e){}
		}	
	}
 
	public void draw(Graphics g){
		setFont(new Font("Arial", Font.PLAIN, 20));		
 
		g.drawImage(green,0,0,null);		
		g.setColor(Color.WHITE);
		g.drawImage(sidebar, getWidth()-256,0, null);	
		g.drawString("$" + Integer.toString(money),  getWidth()-200, 45);
		g.drawString("$50", getWidth()-237,140);
 
		g.drawImage(Tower1Icon.getImage(), Tower1Icon.getX(), Tower1Icon.getY(),null);
 
		for (Tower1 tower1 : Tower1List1){
			g.drawImage(tower1.getI(),tower1.getX()-25,tower1.getY()-25,null);
		}
 
		if(icon1Clicked == 1){
			g.drawImage(Tower1Icon.getImage(),mouse.x-Tower1Icon.getWidth()/2,mouse.y-Tower1Icon.getHeight()/2,null);			
		}
	}		
 
	//update position
	public void update(long timePassed){
 
		if(drawTower1 == 1){
			Tower1 tower1 = new Tower1(mouse.x,mouse.y);
			Tower1List1.add(tower1);
			money -= 50;
			drawTower1 = 0;
		}
 
			Rectangle rect1 = new Rectangle(Tower1Icon.getX(), Tower1Icon.getY(),Tower1Icon.getHeight(),Tower1Icon.getWidth());
 
 
			if(mousePressed == 1 && mouseUsed == 0 ){
				if( rect1.contains(mouse.x,mouse.y)){
						if(!(money < 50)){
							icon1Clicked = 1;
							mouseUsed = 1;
						}
					}	
			}else{
				icon1Clicked = 0;			
		}
	}
 
	//user input
	public class Input implements KeyListener, MouseMotionListener, MouseListener, MouseWheelListener{
		Input(){	
			setFocusTraversalKeysEnabled(false);
			addKeyListener(this);
			addMouseMotionListener(this);
			addMouseListener(this);
			addMouseWheelListener(this);
		}
 
		//mouse click listener
		public void mousePressed(MouseEvent e){
			mousePressed = 1;
		}
 
		//unused mouse functions
		public void mouseReleased(MouseEvent e){
			mousePressed = 0;
 
			if(mouseUsed == 1){
				drawTower1 = 1;
				mouseUsed = 0;
			}
		}
 
		//record mouse position
		public void mouseDragged(MouseEvent e){
				mouseMoved(e);
		}
		public void mouseMoved(MouseEvent e){
			mouse.x = e.getX();
			mouse.y = e.getY();
		}
 
		//unused input functions
		public void keyPressed(KeyEvent e){e.consume();}
		public void keyReleased(KeyEvent e){e.consume();}		
		public void keyTyped(KeyEvent e){e.consume();}
		public void mouseClicked(MouseEvent e){e.consume();}
		public void mouseEntered(MouseEvent e){e.consume();}
		public void mouseExited(MouseEvent e){e.consume();}
		public void mouseWheelMoved(MouseWheelEvent e){
			money++;
			e.consume();}	
	} 
}

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A few tips:

1) If you want to do animation, I recommend reading a bit on threads and Swing (for example, sett Threads and Swing ). Swing is a single threaded model, and all drawing or updates to the GUI should be done on that single thread (the EDT - Event Dispatch Thread). Using a Swing timer if you want animation.

2) see Trail: 2D Graphics (The Java™ Tutorials) To paint to a component, do so by overriding the paintComponent method

3) Read the API's (for example, a quote from the getBufferedStrategy

Quote:

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Returns the BufferStrategy used by this component. This method will return null if a BufferStrategy has not yet been created or has been disposed.

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- quite probably the source of your exception - and another example: avoid using method names that may conflict with those defined in the API - eg update)

Quote:

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Originally Posted by copeg

A few tips:

1) If you want to do animation, I recommend reading a bit on threads and Swing (for example, sett Threads and Swing ). Swing is a single threaded model, and all drawing or updates to the GUI should be done on that single thread (the EDT - Event Dispatch Thread). Using a Swing timer if you want animation.

2) see Trail: 2D Graphics (The Java™ Tutorials) To paint to a component, do so by overriding the paintComponent method

3) Read the API's (for example, a quote from the getBufferedStrategy

- quite probably the source of your exception - and another example: avoid using method names that may conflict with those defined in the API - eg update)

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Thanks for the tips. The thing is, I made the buffer strategy in the Main constructor.