import java.awt.*; 
import java.awt.event.*; 
import javax.swing.*; 

public class ComboBox extends JPanel 
{ 
    public ComboBox() 
    { 
        String[] x1 = { "a", "b", "c", "d", "e", "f", "g", "h", "i", "j" }; 
        String[] x2 = { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9" }; 
        String[] x3 = { "k", "l", "m", "n", "o", "p", "q", "r", "s", "t" }; 
  
        JComboBox cb1 = new JComboBox(x1); 
        JComboBox cb2 = new JComboBox(x2); 
        JComboBox cb3 = new JComboBox(x3); 
  
        add(cb1); 
        add(cb2); 
        add(cb3); 

        setBorder(BorderFactory.createEmptyBorder(20,20,20,20)); 
    } 
  
    public static void main(String[] args) 
    { 
        SwingUtilities.invokeLater(new Runnable() 
        { 
            public void run() 
            { 
                System.out.println(UIManager.getLookAndFeel()); 
                JFrame frame = new JFrame("ComboBox"); 
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
                JComponent newContentPane = new ComboBox(); 
                newContentPane.setOpaque(true); 
                frame.setContentPane(newContentPane); 
                frame.pack(); 
                frame.setVisible(true); 
            } 
        }); 
    } 
}