Semi-opaque panel gets more and more opaque

Question

Hello forum members.
I'm writing a semi-opaque panel with an undesired behaviour and is hoping for clues to what I should do about it.
Desired functionality: A semiopaque panel that can change colour and opacity.
Problem: when I press the 'change colour' button in the code below, the colour changes but the panel gets more opaque for every press on the button. When I resize the panel manually, opacity and colours are drawn as they are supposed to be.
Is there som event to be fired that redraws the panel from the beginning?
import java.awt.*;
import java.awt.event.*;
import java.awt.color.*;
import javax.swing.*;
public class TransparentPanelTest implements ActionListener {
  private JDialog dialog = null;
  private Color bgColour = Color.YELLOW;
  private TransparentPanel panel = null;
  public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
      public void run() {
        new TransparentPanelTest().show();
  private void show() {
    dialog = new JDialog();
    dialog.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE);
    panel = new TransparentPanel();
    panel.add(new JLabel("Hello world"));
    dialog.add(panel, BorderLayout.CENTER);
    JButton changeButton = new JButton("change colour");
    changeButton.addActionListener(this);
    dialog.add(changeButton, BorderLayout.PAGE_END);
    dialog.pack();
    dialog.setVisible(true);
  public void actionPerformed(ActionEvent e) {
    bgColour = (bgColour == Color.YELLOW) ? Color.GREEN : Color.YELLOW;
    System.out.println("Setting colour " +
      (bgColour == Color.YELLOW ? "yellow" : "green"));
    panel.setColour(bgColour);
   * Semiopaque panel.
  private class TransparentPanel extends JPanel {
    private Color panelColour = Color.YELLOW;
    private float panelOpacity = 0.2f;
    public TransparentPanel() {
      super();
    public void setColour(Color transparentColour) {
      this.panelColour = transparentColour;
      validate();
      repaint();
    public void setOpacity(float opacity) {
      this.panelOpacity = opacity;
      validate();
      repaint();
    public void paintComponent(Graphics g) {
      Graphics2D g2d = (Graphics2D)g.create();
      g2d.setComposite(
          AlphaComposite.getInstance(AlphaComposite.SRC_OVER, panelOpacity));
      g2d.setColor(panelColour);
      g2d.fillRect(0, 0, getWidth(), getHeight());
}

Answer 1

your panel reports being fully opaque while in fact it isn't, thuss confusing the painting mechanism
CU
Jeanette