Accessing graphics object?

Question

Hi,
I was wondering, is the only place the graphics object relevent inside the paintComponent method?
I'm asking because I have a method that draws a custom progress bar. I have another method that draws that progress bar filling up over a arbitrary period of time. I want to use Thread.sleep() to provide a delay between redraws.
Now I have the first method inside the paintComponent() method of a JPanel. Thats fine, works great! Now I didn't want to put the redraw method inside the paintComponent because the Thread delay will manifest itself in the painting of the component. So I thought I could solve this by making the second method a public one and using the getGraphics() method of the JPanel call the method once the object is instantiated. However this gives me a NullPointerException as soon as the method calls on the graphics object. Any ideas where I'm going wrong? Thanks!
Method inside extended JPanel object
public void testRun(Graphics g)
        // Cast to Graphics2D
        Graphics2D g2 = (Graphics2D)g;
        while(percentComplete < 100)
            try {
                Thread.sleep(30);
            } catch (java.lang.InterruptedException ie) {
                System.err.println("We got a problem");
            drawProgressBar(g2,percentComplete);
            percentComplete += 1;+
+        }+
+        percentComplete+ = 1;
        drawProgressBar(g2,percentComplete); // contains a repaint() method call to object
    }This inside the method of another object
private void setupContentPanel()
        cpb = new CustomProgressBar();
        mainWindow.add(pb, BorderLayout.CENTER);
        cpb.testRun( pb.getGraphics() );
    }

Answer 1

No, don't use update(...), that's not at all what the method is for. Also, avoid getGraphics() like the plague.
What you need to do is increment an instance field using a Swing Timer, and invoke repaint(). The paintComponent override should use the current value of the instance field for appropriate painting. Example:import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
public class PseudoProgressBar {
   int progress = 0;
   JPanel panel= new JPanel() {
         @Override
         protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.setColor(Color.BLUE);
            int w = getWidth();
            int h = getHeight();
            g.fillRect(0, h/2-10, (w * progress)/100, 20);
   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         @Override
         public void run() {
            new PseudoProgressBar().makeUI();
   public void makeUI() {
      JFrame frame = new JFrame();
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setSize(400, 100);
      frame.setContentPane(panel);
      frame.setLocationRelativeTo(null);
      frame.setVisible(true);
      new Timer(100, new ActionListener() {
         public void actionPerformed(ActionEvent e) {
            progress %= 100;
            progress++;
            panel.repaint();
      }).start();
}db