Can't do paint computation only outside EDT

Question

Hello, I am really sorry, this must be obvious, but I just can't do myself, or find on the web, an example of doing paintComponent() "the proper way". That is, if I have a lot of computation of the image, before the actual painting occurs, I should first get out of EDT, do this outside, and then come back with invokeLater(). That is what I read in the docs and on the web - please correct me if I am misunderstanding something. Under java 1.4.2 (most browsers still run that, so I have to use it for applet development), there is no SwingWorker, so I thought I would just spawn a thread. But the following code fails to paint anything - if I resize the window, I expect the diagonal to appear. This is what happens if I just put g.drawLine(x0, y0, x1, y1); directly inside paintComponent(). But to do it the right way, as if in a computational intensive situation, does not work. What mistake am I making? Here is the isolated faulty code:
import javax.swing.*;
import java.awt.*;
public class Foobar
     public static void main(String[] args)
          JFrame frame = new JFrame();
          FoobarPanel panel = new FoobarPanel();
          frame.setContentPane(panel);
          frame.pack();
          frame.setVisible(true);
class FoobarPanel extends JPanel
     public void paintComponent(final Graphics g)
          new Thread(new Runnable()
               public void run()
                    paintOutsideEDT(g);
          }).run();
     void paintOutsideEDT(final Graphics g)
          final int x0 = 0;
          final int y0 = 0;
          Dimension d = getSize();
          final int x1 = d.width - 1;
          final int y1 = d.height - 1;
          SwingUtilities.invokeLater(new Runnable()
               public void run()
                    paintBackInEDT(g, x0, y0, x1, y1);
     void paintBackInEDT(Graphics g, int x0, int y0, int x1, int y1)
          g.drawLine(x0, y0, x1, y1);
}

Answer 1

Here's an example that draws random ellipses on a JPanel using a background thread. The time-intensive processing is mimicked by a call to Thread.sleep(...):
import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.geom.Ellipse2D;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JComponent;
import javax.swing.SwingWorker;
public class BackgroundPaint
  private static final int SIDE = 400;
  private static final Dimension APP_SIZE = new Dimension(SIDE, SIDE);
  protected static final long SLEEP_DELAY = 500;
  private Random random = new Random();
  private List<EllipseColor> ellipseColorList = new ArrayList<EllipseColor>();
  //private SwingWorker<Void, Void> swingworker;
  private JPanel mainPanel = new JPanel()
    @Override
    protected void paintComponent(Graphics g)
      super.paintComponent(g);
      myPaint(g);
  public BackgroundPaint()
    mainPanel.setPreferredSize(APP_SIZE);
    new Thread(new Runnable()
      public void run()
        while (true) // lord I hate this
          try
            Thread.sleep(SLEEP_DELAY);
          catch (InterruptedException e){}
          Color c = new Color(
              random.nextInt(256),
              random.nextInt(256),
              random.nextInt(256));
          int centerX = random.nextInt(SIDE);
          int centerY = random.nextInt(SIDE);
          int radiusX = random.nextInt(Math.min(Math.abs(SIDE - centerX), centerX));
          int radiusY = random.nextInt(Math.min(Math.abs(SIDE - centerY), centerY));
          Ellipse2D ellipse = new Ellipse2D.Double(
              centerX - radiusX, centerY - radiusY,
              2*radiusX, 2*radiusY);
          ellipseColorList.add(new EllipseColor(ellipse, c));
          mainPanel.repaint();
    }).start();
  private void myPaint(Graphics g)
    Graphics2D g2 = (Graphics2D) g;
    g2.setStroke(new BasicStroke(4));
    g2.setRenderingHint(
        RenderingHints.KEY_ANTIALIASING,
        RenderingHints.VALUE_ANTIALIAS_ON);
    for (EllipseColor ec : ellipseColorList)
      g2.setColor(ec.getColor());
      g2.draw(ec.getEllipse());
  public JComponent getComponent()
    return mainPanel;
  private class EllipseColor
    private Ellipse2D ellipse;
    private Color color;
    public EllipseColor(Ellipse2D ellipse, Color color)
      this.ellipse = ellipse;
      this.color = color;
    public Ellipse2D getEllipse()
      return ellipse;
    public Color getColor()
      return color;
  private static void createAndShowUI()
    JFrame frame = new JFrame("BackgroundPaint");
    frame.getContentPane().add(new BackgroundPaint().getComponent());
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setResizable(false);
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
  public static void main(String[] args)
    java.awt.EventQueue.invokeLater(new Runnable()
      public void run()
        createAndShowUI();
}Edited by: Encephalopathic on Nov 23, 2008 8:30 AM