How to repaint a component when it moves?

Question

Hello, I have an application with many JPanels packed together, and one of them has a picture that I render with the paint() method, using the coordinates of the panel as reference. For example, suppose I were to draw the diagonal:
Dimension d = getSize();
int w = d.width;
int h = d.height;
g.clearRect(0, 0, w, h);
g.drawLine(0, 0, d.width - 1, d.height - 1);Now on events of user input, the size of other components may change and I revalidate the root pane. This may move or resize my JPanel where I drew.
So I thought paint() would automatically be called if the component is moved. But it's not! the JPanel moves around, with the diagonal staying in its old place on the screen (so it is no longer the diagonal of the moved JPanel).
Why and How to do this properly?      It would not be easy, if the user changes any of the many components, to have to manually keep track of everything else and repaint() everything else on the account that it might move, or have to compute whether it moved and repaint(). Wouldn't I be doing the job of the layout manager then? (I use many nested GridBag Layout managers).
Thank you for any insight. Mark

Answer 1

For example:
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JComponent;
* @version 1.0, 11/13/2008
* @author Pete
* Image from Funny Junk website: http://www.funnyjunk.com/funny_pictures/
public class MovePicPanel
  private static final Dimension PANEL_SIZE = new Dimension(800, 700);
  private JPanel mainPanel = new JPanel();
  private BufferedImage image;
  private JPanel picPanel = new JPanel()
    protected void paintComponent(Graphics g)
      super.paintComponent(g);
      if (image != null)
        //!! here's where I draw my image in the JPanel
        g.drawImage(image, 0, 0, this);
  public MovePicPanel()
    try
      image  = ImageIO.read(new URL("http://newmedia.funnyjunk.com/pictures/bills-in-context.jpg"));
    catch (MalformedURLException e)
      e.printStackTrace();
    catch (IOException e)
      e.printStackTrace();
    if (image != null)
      int width = image.getWidth();
      int height = image.getHeight();
      picPanel.setSize(new Dimension(width, height));
      picPanel.setLocation(0, 0);
    JLabel clickMeLabel = new JLabel("Click me and drag mouse");
    clickMeLabel.setFont(clickMeLabel.getFont().deriveFont(Font.BOLD, 32));
    clickMeLabel.setForeground(Color.red);
    picPanel.add(clickMeLabel);
    MouseAdapter mouseAdapter = new MyMouseAdapter();
    mainPanel.addMouseListener(mouseAdapter);
    mainPanel.addMouseMotionListener(mouseAdapter);
    mainPanel.setPreferredSize(PANEL_SIZE);
    mainPanel.setLayout(null);
    mainPanel.add(picPanel);
  public JComponent getComponent()
    return mainPanel;
  private class MyMouseAdapter extends MouseAdapter
    @Override
    public void mousePressed(MouseEvent e)
      picPanel.setLocation(e.getPoint());
    @Override
    public void mouseDragged(MouseEvent e)
      picPanel.setLocation(e.getPoint());
  private static void createAndShowUI()
    JFrame frame = new JFrame("MovePicPanel");
    frame.getContentPane().add(new MovePicPanel().getComponent());
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
  public static void main(String[] args)
    java.awt.EventQueue.invokeLater(new Runnable()
      public void run()
        createAndShowUI();
}