JCombo Box problem

I have three combo boxes

First combo box display the year

Second combo box display the month for the selected year.

Third combo box display number of week in a selected month for the year.

I am select year, month and the third combo box display the number of week. If i select any one week then some exception thrown.

Ex.

Exception in thread "AWT - EventQueue-0" java.lang.NullPointerException atLabour_weekly. itemStateChanged.

My Sample code.

public void itemStateChanged(ItemEvent ie)
{
    if(ie.getSource() == cmbyear)
    {
        year=cmbyear.getSelectedItem().toString();
        //cmbmonth.setEnabled(true);
    }
    else if(ie.getSource() == cmbmonth)
    {
        if(!(cmbmonth.getSelectedItem().equals("Select")))
        {
            month=cmbmonth.getSelectedItem().toString();
            Calendar ca1 = Calendar.getInstance();
            int y=Integer.parseInt(year);
            int m = Integer.parseInt(month)-1;
            ca1.set(y,m,1);
            int days = ca1.getActualMaximum(Calendar.DAY_OF_MONTH);
            cmbweek.removeAllItems();
            cmbweek.addItem("Select");
            java.util.Date date;
            int j=1;
            for(int i=1;i<=days;i++)
            {   
                date=(new GregorianCalendar(y,m,i)).getTime();  
                SimpleDateFormat f = new SimpleDateFormat("EEEE");
                    String day=f.format(date);
                    if(day.equals("Saturday"))
                {
                        cmbweek.addItem(j+" Week");
                    j++;
                    }
                }
        }       
    }
    else if(ie.getSource() == cmbweek)
    {
        System.out.println("Hai");

        if(!(cmbweek.getSelectedItem().equals("Select")))
        {
            System.out.println(cmbweek.getSelectedItem().toString());
        }

    }

}

Please Any one help me as soon as possible.