How to make JComboBoxes transparent

Question

For some reason "setOpaque(false)" doesn't seem to work for JComboBoxes. I'm using java 1.4
Below is some code. If you compile and run it you should get a window with a button and a combo box, the button is transparent, the combo box is not.
import javax.swing.*;
import java.awt.*;
import java.awt.geom.Area;
* test transparent swing components
class TransTest
public static void main(String args[])
     JFrame theFrame = new JFrame("transparency test");
     JComponent contentPane = new JComponent()
          public void paint(Graphics g)
          Graphics2D g2 = (Graphics2D)g;
          Area s = new Area((Shape)new Rectangle(0, 0, getWidth(), getHeight()));
          g2.setPaint(new GradientPaint(0f, 0f, Color.GRAY, 150f, 100f, Color.WHITE, true));
          g2.fill(s);
          super.paintChildren(g);
     theFrame.setContentPane(contentPane);
     theFrame.setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
     JButton button = new JButton("button");
     button.setOpaque(false);          // this works
     JComboBox combo = new JComboBox();
     combo.addItem("combo");
     combo.setOpaque(false);               // this doesn't seem to work
     JPanel pnl = new JPanel();
     pnl.setOpaque(false);
     pnl.add(button);
     pnl.add(combo);
     contentPane.setLayout(new BorderLayout());
     contentPane.add(pnl, BorderLayout.CENTER);
     theFrame.pack();
     theFrame.setVisible(true);

Answer 1

Try setOpaque(true) and setBackground(new Color(r,g,b,a)) where r, g and b are respectively the values of red, green and blue and a the alpha value (0 means totaly transparent, 255 totaly opaque). Sometimes a component should be opaque with a transparent background to make the transcparency works.
But I have to ask you a question : have you bugs with the display ? Indeed, I made an app that doesn't use a gradient paint like you, but the JFrame have a ContentPane that display a picture. I made every JComponent transparent in order to see the background picture. The display is very dirty with JButton, JTextArea, JTextField and JList. Indeed, when moving cursor over JButton, the JButton displays the top lecft corner of the JFrame instead of the underlaying part of the picture as expected with transparency. With JTextField and JTextArea, when typing text the same problem occured. With JList, the problem appear when scrolling. only a full update(Graphics g) can clean the display, that is I must call it with every mouseMoved, mouseClicked, etc. events. The problem appears only with JDK > 1.2.2 (JDK1.3 and later)