Custom painting of JPanel

Question

I'm not very good at this and I hope to get some help from people who understands the issue a lot more that I do.

So here's the deal. In my application there is background JPanel with Image drawn over it. Then there is a small JPanel which I'm trying to create custom painting for. I wanted to have JPanel with rounded corners and semi-transparent background so I modified paintComponent method to fill semi-transparent rounded rectangle. But when I place components inside like say JComboBox, the list of items appears and I click somewhere else to close it JPanel paints itself in original way making it semitransparent all around but with small rectangle painted with original grey background color. I see that it has to do something with invoking paintComponent on its parrent or paintChildren but I don't know how to organize those methods or where to put them. I also have proble with transparent colors overlaping each other.

Here is an example source code:

public class RoundedPanel extends JPanel {

   private final int radius;


   public RoundedPanel(int cornerRadius) {
      radius=cornerRadius;
   }

   public void paintComponent(Graphics g) {
        Color bg = getBackground();
        g.setColor(new Color(bg.getRed(),bg.getGreen(),bg.getBlue(),40));
        g.fillRoundRect(0,0, getWidth()-1, getHeight()-1, radius, radius);
        g.setColor(new Color(0,0,0,70));
        g.drawRoundRect(0,0, getWidth()-1, getHeight()-1, radius, radius);
   }

   public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setSize(400, 300);
        frame.setLocation(400, 300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel content = new JPanel();
        JPanel wl = new JPanel();
        JPanel el = new JPanel();
        JPanel sl = new JPanel();
        JPanel nl = new JPanel();
        RoundedPanel rp = new RoundedPanel(50);
        JComboBox combobox = new JComboBox();

        frame.setContentPane(content);
        content.setBackground(Color.red);
        content.setLayout(new BorderLayout());
        wl.add(new JButton("west"));
        el.add(new JButton("east"));
        sl.add(new JButton("south"));
        nl.add(new JButton("north"));
        content.add(wl,BorderLayout.WEST);
        content.add(el,BorderLayout.EAST);
        content.add(nl,BorderLayout.NORTH);
        content.add(sl,BorderLayout.SOUTH);

        content.add(rp,BorderLayout.CENTER);
        rp.setBackground(Color.BLACK);

        combobox.setModel(new javax.swing.DefaultComboBoxModel(new String[] { "Třída 1.B", "Třída 1.C", "Třída 2.C" }));
        rp.add(combobox);
        frame.setVisible(true);
    }
}

I hope some of will help me out :-) thanks

EDIT: I found out that JComboBox (and its pop-up menu) draws correctly if pop-up menu overlaps outside the JPanel that contains JComboBox and has the custom paintComponent method.

Answer 1

Try this:

RoundedPanel rp = new RoundedPanel(50);
rp.setOpaque(false);