Hi all, i really need help to overcome this error.

I'm going to paste the interesting parts of my code:

TEST BEAN ( we are inside a method called by the java test client ):

     .

     .

     .

      try {

                                        System.out.println("lowering the amount of the selected book");

                                        Book abook = (Book) bookop.findByISBN(80002).get(0);  // Book is an entity bean --- bookop is the istance of a session bean that performs                                                                                                 // operations on Book

            bookop.updateMin(abook.getISBN(), 2); // tot - 2

             // OPERATION updateMin DONE SUCCESSFULLY

      } catch (Exception e) {

                                        e.printStackTrace();

      }

      Users an_user = (Users) user.findByUser("mouse").get(0);  // Users is an entity bean --- user is the istance of a session bean that performs                                                                                                   // operations on Users

      cart.checkoutCart(an_user); // cart is the istance of a session statefull bean that performs classic operations like in a web site ( bookshop )

                                                 // to see what this method go down

-end

CART BEAN ( the called method ):

     public void checkoutCart(Users customer) {

            for (int i = 0; i < books.size(); i++)   // where books is: private List<CartBook> books , a simple class made up of a Book and the quantity                                                                    // to purchase of that book

                              {

                                             Book abook = books.get(i).getBook();

                                             Integer quantity = books.get(i).getQuantity();

                 try {

                         System.out.println("lowering the amount of the selected book");

                        DBOperation.updateMin(abook.getISBN(), quantity); // DBOperation is the same as bookop seen before

                         // ERROR [STDERR] java.lang.NullPointerException

                                             } catch (Exception e) {

                         e.printStackTrace();

                                             }

                              }

     }

-end

so basically the code is the same; the difference is that going through the cart bean i can lower the quantity of a list of books ( that are the books that a customer puts in his cart: when he performs a checkout operation the method updateMin lowers the amount of every books purchased in the store ) instead of only one.

the problem is that through the cart i get always the ERROR [STDERR] java.lang.NullPointerException in the DBOperation.updateMin methods that is exactly the same of bookop.updateMin.

Every cicle of the for( ) command i get this error.

I can assure that my list is not empty and abook & quantity are not null.

I really don't get the problem!

Thank you in advance!!