void Invoice::print(InvoiceFormatter& formatter)
{
   print_header("I N V O I C E");
   print_string("Description", true);
   print_string("Unit Price", false);
   print_string("Qty", false);
   print_string("Total Price", false);

   double amount_due = 0;
   for (ItemIterator iter = inv.create_iterator();
      !iter.is_done(); iter.next())
   {
      Item* it = iter.get();
      print_string(it->get_description(), true);
      print_number(it->get_unit_price(), 2);
      print_number(it->get_quantity(), 0);
      print_number(it->get_total_price(), 2);
      amount_due = amount_due + it->get_total_price();
   }

   print_footer("AMOUNT DUE:", amount_due);
}