Thread: In need of assistance!

I'm trying to make the classic game of Mini Tennis, but can't seem to overcome this problem...
This is the code:

package java_1;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import javax.swing.JFrame;
import javax.swing.JPanel;
 
@SuppressWarnings("serial")
public class MiniTennis_5 extends JPanel {
 
	Ball ball = new Ball(this);
	Racquet racquet = new Racquet(this);
 
	public MiniTennis_5() {
		addKeyListener(new KeyListener() {
			@Override
			public void keyTyped(KeyEvent e) {
			}
 
			@Override
			public void keyReleased(KeyEvent e) {
				racquet.keyReleased(e);
			}
 
			@Override
			public void keyPressed(KeyEvent e) {
				racquet.keyPressed(e);
			}
		});
		setFocusable(true);
	}
 
	private void move() {
		ball.move();
		racquet.move();
	}
 
	@Override
	public void paint(Graphics g) {
		super.paint(g);
		Graphics2D g2d = (Graphics2D) g;
		g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
				RenderingHints.VALUE_ANTIALIAS_ON);
		ball.paint(g2d);
		racquet.paint(g2d);
	}
 
	public static void main(String[] args) throws InterruptedException {
		JFrame frame = new JFrame("Mini Tennis");
		MiniTennis_5 game = new MiniTennis_5();
		frame.add(game);
		frame.setSize(300, 400);
		frame.setVisible(true);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
		while (true) {
			game.move();
			game.repaint();
			Thread.sleep(10);
		}
	}
}

I compiled, but there was some kind of error while running it. I've tried debugging it, but I can't find the problem...

please paste the error message

Yes, we need to see the error, but there are other problems that should be corrected:

1 - A while(true) loop on the main thread (or any, for that matter) is not the way to stimulate a Swing app. However, it is very effective at causing the Swing app to become locked up and unresponsive.

2 - Swing apps should be run on the EDT.

3 - Do not override the paint() method in Swing apps. Instead, override the paintComponent() method. You can learn more about the proper way to paint in Swing apps by reviewing the "Swing custom painting" tutorial.

4 - Use a container like a JFrame as the main app frame/window, and to that add a JPanel or other JComponent on which to do the painting and animation.

There are other minor issues to resolve - like program structure - but this is a good start.

In my opinion you might be better off by first going through a simple Java game tutorial. E.g., you can go through Space Invaders 101 | Coke And Code. You can also look through the follow-up tutorial at Space Invaders 102 | Coke And Code for a better way to handle the game loop.

On EDT (Event Dispatching Thread), see Event dispatching thread - Wikipedia, the free encyclopedia and Lesson: Concurrency in Swing (The Java™ Tutorials > Creating a GUI With JFC/Swing).

On custom painting, see Lesson: Performing Custom Painting (The Java™ Tutorials > Creating a GUI With JFC/Swing). While you're at it, notice the way example programs in this tutorial are started:

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }

You'd understand the significance of this if you have gone through the EDT links above.

I'll try to fix the problems in the code and will try out the simpler Java Tutorials as jashburn has suggested and will post the errors right here:
Exception in thread "main" java.lang.NullPointerException
at java_1.Ball.move(Ball.java:22)
at java_1.MiniTennis_5.move(MiniTennis_5.java:36)
at java_1.MiniTennis_5.main(MiniTennis_5.java:59)


Also, as you can see there is "at java_1.Ball.move(Ball.java:22). And that is referring to another code which is this:

package java_1;
import java.awt.Graphics2D;
 
public class Ball {
	int x = 0;
	int y = 0;
	int xa = 1;
	int ya = 1;
	private MiniTennis_4 game;
 
	public Ball(MiniTennis_4 game) {
		this.game= game;
	}
 
	public Ball(MiniTennis_5 miniTennis_5) {
		// TODO Auto-generated constructor stub
	}
 
	void move() {
		if (x + xa < 0)
			xa = 1;
		if (x + xa > game.getWidth() - 30)
			xa = -1;
		if (y + ya < 0)
			ya = 1;
		if (y + ya > game.getHeight() - 30)
			ya = -1;
 
		x = x + xa;
		y = y + ya;
	}
 
	public void paint(Graphics2D g) {
		g.fillOval(x, y, 30, 30);
	}
}

I'll try out the other Tutorials and fix the code...

You'll have to tell us which line is line 22, but look at that line and determine which variables have not been initialized and fix that.