Wierd JComboBox

Question

Hello,
In my application, I open a JComboBox inside a JPanel.
But the combo box is wierd : the top line is badly drawn. In non editable mode, the current text is drawn, but the arrow is not. In editable mode, nothing is drawn, just a gray line. But the combo box is active : when I click on the top line, the list appears, properly drawn.
The JPanel is set to opaque. It has a layout manager set to null, and I use setbounds to set the position and size of the combo box. But I have tried also with a layout manager, and the result is the same.
The paint function of the JPanel draws an image in the back every 1/50 second. I have tried to remove this regular paint, and the result is the same.
For your information, Labels and ListBox work perfectly in this application.
I do not understand what is going wrong. Does anyone has an idea?
Thanks, Francois

Answer 1

Hello,
I have made a simple application that deponstrates the problem. Here is the main class :
* ComboBox problems demonstration
package combotry;
import java.awt.*;
import javax.swing.*;
public class Main
    JFrame window;
    CPanel panel;
    JComboBox combo;
    public Main()
    public void run()
     // Creates a JFrame without layout manager
     window = new JFrame() ;
     window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     window.getContentPane().setLayout(null);
     // Creates panel
     panel=new CPanel();
     panel.setPreferredSize(new Dimension(640, 480));
     panel.setLayout(null);
     window.getContentPane().add(panel);
     panel.setOpaque(true);
     // Position window and panel
     window.setVisible(true);
     Insets insets=window.getInsets();
     int width=640+insets.left+insets.right;
     int height=480+insets.top+insets.bottom;
     window.setSize(width, height);
     panel.setBounds(0, 0, 640, 480);
     // Creates combobox
     String[] petStrings = { "Bird", "Cat", "Dog", "Rabbit", "Pig" };
     combo=new JComboBox(petStrings);
     combo.setSelectedIndex(4);
     Dimension d=combo.getPreferredSize();
     combo.setEditable(true);
     combo.setVisible(true);     
     panel.add(combo);
     combo.setBounds(100, 100, 250, d.height);
        // Main loop
     do
         try
          Thread t=Thread.currentThread();
          t.sleep(20);
         catch(InterruptedException e)
     }while(true);     
    public static void main(String[] args)
     new Main().run();
}And here is the Panel class :
* CPanel.java
package combotry;
import javax.swing.*;
import java.awt.*;
public class CPanel extends JPanel
    public CPanel()
    protected void paintComponent(Graphics g)
     Graphics2D g2=(Graphics2D)g;
     Color c=new Color(0xFFFFFF);
     g2.setColor(c);
     g2.fillRect(0, 0, 640, 480);          
}If you run this application, you will see that the combo box top line is gray. When you click on it, the list opens. But no arrow, just a gray line. Same behavior (a little better) if you set editable to false.
Any idea anyone?