public class BookRental { //該類描述出租記錄
        String id; 
        String customerName; 

        ... 

    } 
    public class BookRentals { 
       PRivate Vector rentals; 
       public String getCustomerName(String rentalId) { 根據出租id取出客戶姓名
           for (int i = 0; i < rentals.size(); i++) { 
              BookRental rental = (BookRental) rentals.elementAt(i); 
              if (rental.getId().equals(rentalId)) { 
                  return rental.getCustomerName(); 
               } 
           } 
           throw new RentalNotFoundException(); 
        } 
    } 
    public class RentalNotFoundException extends Exception { 

        ... 

    } 

    public class BookRentals { 
        private Vector rentals; 
       public String getCustomerName(String rentalId) { 
           for (int i = 0; i < rentals.size(); i++) { 
              BookRental rental = (BookRental) rentals.elementAt(i); 
              if (rental.getId().equals(rentalId)) { 
                  return rental.getCustomerName(); 
               } 
           } 
           throw new RentalNotFoundException(); 
        } 
        public void deleteRental(String rentalId) { 
             for (int i = 0; i < rentals.size(); i++) { 
                 BookRental rental = (BookRental) rentals.elementAt(i); 
                 if (rental.getId().equals(rentalId)) { 
                     rentals.remove(i); 
                     return; 
                  } 
              } 
             throw new RentalNotFoundException(); 
         } 
     } 

    public class BookRentals { 
        private Vector rentals; 
        public String getCustomerName(String rentalId) { 
           int rentalIdx = getRentalIdxById(rentalId); 
           return ((BookRental) rentals.elementAt(rentalIdx)).getCustomerName(); 
        } 
        public void deleteRental(String rentalId) { 
           rentals.remove(getRentalIdxById(rentalId)); 
        } 
        private int getRentalIdxById(String rentalId) { //新增加的一個方法
            for (int i = 0; i < rentals.size(); i++) { 
               BookRental rental = (BookRental) rentals.elementAt(i); 
               if (rental.getId().equals(rentalId)) { 
                   return i; 
               } 
            } 
           throw new RentalNotFoundException(); 
        } 
     }