help me to clear Jcombo during runtime

[sowmya upadhya]

hi,
i am doing project in swings. i am using 2 combo boxes.elements for both are added during runtime one is from database and for other from frontend based on a radiobutton clicked.when i select a element from a 1st combo box it will add elements to 2nd combobox from database but i am not able to clear prevoius elements from that combobox i tried using removeAllItems but it is giving NullPointer Exception.when that form is reloaded the previous must be cleared.please help me to solve this problem

thank you in advance

[Craig Wood]

import java.awt.*;
import java.awt.event.*;
import java.util.Random;
import javax.swing.*;
 
public class ReloadTest implements ActionListener
{
    JComboBox north;
    Random seed = new Random();
 
    public void actionPerformed(ActionEvent e)
    {
        north.removeAllItems();
        int start = ((JComboBox)e.getSource()).getSelectedIndex()+1;
        int itemCount = 3 + seed.nextInt(5);  // 3 to 7
        DefaultComboBoxModel model = (DefaultComboBoxModel)north.getModel();
        for(int j = start; j < start + itemCount; j++)
            model.addElement(new Integer(j));
    }
 
    private JComboBox getNorth()
    {
        Integer[] ints = new Integer[5];
        for(int j = 0; j < ints.length; j++)
            ints[j] = new Integer(j+1);
        north = new JComboBox(new DefaultComboBoxModel(ints));
        return north;
    }
 
    private JComboBox getSouth()
    {
        Integer[] ints = new Integer[10];
        for(int j = 0; j < ints.length; j++)
            ints[j] = new Integer(j+1);
        JComboBox combo = new JComboBox(ints);
        combo.addActionListener(this);
        return combo;
    }
 
    public static void main(String[] args)
    {
        ReloadTest test = new ReloadTest();
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(test.getNorth(), "North");
        f.getContentPane().add(test.getSouth(), "South");
        f.setSize(300,200);
        f.setLocation(200,200);
        f.setVisible(true);
    }
}