How to repaint a JPanel in bouncing balls game?

Question

I want to repaint the canvas panel in this bouncing balls game, but i do something wrong i don't know what, and the JPanel doesn't repaint?
The first class defines a BALL as a THREAD
If anyone knows how to correct the code please to write....
package fuck;
//THE FIRST CLASS
class CollideBall extends Thread{
    int width, height;
    public static final int diameter=15;
    //coordinates and value of increment
    double x, y, xinc, yinc, coll_x, coll_y;
    boolean collide;
    Color color;
    Rectangle r;
    bold BouncingBalls balls; //A REFERENCE TO SECOND CLASS
    //the constructor
    public CollideBall(int w, int h, int x, int y, double xinc, double yinc, Color c, BouncingBalls balls) {
        width=w;
        height=h;
        this.x=x;
        this.y=y;
        this.xinc=xinc;
        this.yinc=yinc;
        this.balls=balls;
        color=c;
        r=new Rectangle(150,80,130,90);
    public double getCenterX() {return x+diameter/2;}
    public double getCenterY() {return y+diameter/2;}
    public void move() {
        if (collide) {
        x+=xinc;
        y+=yinc;
        //when the ball bumps against a boundary, it bounces off
        //bounce off the obstacle
    public void hit(CollideBall b) {
        if(!collide) {
            coll_x=b.getCenterX();
            coll_y=b.getCenterY();
            collide=true;
    public void paint(Graphics gr) {
        Graphics g = gr;
        g.setColor(color);
        //the coordinates in fillOval have to be int, so we cast
        //explicitly from double to int
        g.fillOval((int)x,(int)y,diameter,diameter);
        g.setColor(Color.white);
        g.drawArc((int)x,(int)y,diameter,diameter,45,180);
        g.setColor(Color.darkGray);
        g.drawArc((int)x,(int)y,diameter,diameter,225,180);
        g.dispose(); ////////
    ///// Here is the buggy code/////
    public void run() {
        while(true) {
            try {Thread.sleep(15);} catch (Exception e) { }
            synchronized(balls)
                move();
                balls.repairCollisions(this);
            paint(balls.gBuffer);
            balls.canvas.repaint();
//THE SECOND CLASS
public class BouncingBalls extends JFrame{
    public Graphics gBuffer;
    public BufferedImage buffer;
    private Obstacle o;
    private List<CollideBall> balls=new ArrayList();
    private static final int SPEED_MIN = 0;
    private static final int SPEED_MAX = 15;
    private static final int SPEED_INIT = 3;
    private static final int INIT_X = 30;
    private static final int INIT_Y = 30;
    private JSlider slider;
    private ChangeListener listener;
    private MouseListener mlistener;
    private int speedToSet = SPEED_INIT;
    public JPanel canvas;
    private JPanel p;
    public BouncingBalls() {
        super("fuck");
        setSize(800, 600);
        p = new JPanel();
        Container contentPane = getContentPane();
        final BouncingBalls xxxx=this;
        o=new Obstacle(150,80,130,90);
        buffer=new BufferedImage(getSize().width, getSize().height, BufferedImage.TYPE_INT_RGB);
        gBuffer=buffer.getGraphics();
        //JPanel canvas start
        final JPanel canvas = new JPanel() {
            final int w=getSize().width-5;
            final int h=getSize().height-5;
            @Override
            public void update(Graphics g)
               paintComponent(g);
            @Override
            public void paintComponent(Graphics g) {
                super.paintComponent(g);
                gBuffer.setColor(Color.ORANGE);
                gBuffer.fillRect(0,0,getSize().width,getSize().height);
                gBuffer.draw3DRect(5,5,getSize().width-10,getSize().height-10,false);
                //paint the obstacle rectangle
                o.paint(gBuffer);
                g.drawImage(buffer,0,0, null);
                //gBuffer.dispose();
        };//JPanel canvas end
        addWindowListener(new WindowAdapter() {
            @Override
            public void windowClosing(WindowEvent e) {
                System.exit(0);
        addButton(p, "Start", new ActionListener() {
            public void actionPerformed(ActionEvent evt) {
                CollideBall b = new CollideBall(canvas.getSize().width,canvas.getSize().height
                        ,INIT_X,INIT_Y,speedToSet,speedToSet,Color.BLUE,xxxx);
                balls.add(b);
                b.start();
        contentPane.add(canvas, "Center");
        contentPane.add(p, "South");
    public void addButton(Container c, String title, ActionListener a) {
        JButton b = new JButton(title);
        c.add(b);
        b.addActionListener(a);
    public boolean collide(CollideBall b1, CollideBall b2) {
        double wx=b1.getCenterX()-b2.getCenterX();
        double wy=b1.getCenterY()-b2.getCenterY();
        //we calculate the distance between the centers two
        //colliding balls (theorem of Pythagoras)
        double distance=Math.sqrt(wx*wx+wy*wy);
        if(distance<b1.diameter)
            return true;
        return false;
    synchronized void repairCollisions(CollideBall a) {
        for (CollideBall x:balls) if (x!=a && collide(x,a)) {
            x.hit(a);
            a.hit(x);
    public static void main(String[] args) {
        JFrame frame = new BouncingBalls();
        frame.setVisible(true);
}  And when i press start button:
Exception in thread "Thread-2" java.lang.NullPointerException
at fuck.CollideBall.run(CollideBall.java:153)
Exception in thread "Thread-3" java.lang.NullPointerException
at fuck.CollideBall.run(CollideBall.java:153)
Exception in thread "Thread-4" java.lang.NullPointerException
at fuck.CollideBall.run(CollideBall.java:153)
and line 153 is: balls.canvas.repaint(); in Method run() in First class.
Please help.

Answer 1

public RepaintManager manager;
public BouncingBalls() {
        manager = new RepaintManager();
        manager.addDirtyRegion(canvas, 0, 0,canvas.getSize().width, canvas.getSize().height);
    public void run() {
        while(true) {
            try {Thread.sleep(15);} catch (Exception e) { }
            synchronized(balls)
                move();
                balls.repairCollisions(this);
            paint(balls.gBuffer);
            balls.manager.paintDirtyRegions(); //////// line 153
   but when push start:
Exception in thread "Thread-2" java.lang.IllegalMonitorStateException
at java.lang.Object.notifyAll(Native Method)
at fuck.CollideBall.run(CollideBall.java:153)
Exception in thread "Thread-3" java.lang.IllegalMonitorStateException
at java.lang.Object.notifyAll(Native Method)
at fuck.CollideBall.run(CollideBall.java:153)
i'm newbie with Concurrency and i cant handle this exceptons.
Is this the right way to do repaint?