String rNum1;
    String afm1;
    Date rentDate, retDate;
    rNum1 = reader.readString("Vehicle Registration Number: ");
    afm1 = reader.readString("Client AFM: ");
    rentDate = reader.readDate("Rent Date: ");
    retDate = reader.readDate("Return Date: ");

    /* Using the object cc of a constructor i call the method findRentals that takes the input variables of the user.*/

    cc.findRentals(rNum1, afm1, rentDate, retDate);
    /* Method findRentals */
    public Rental findRentals(String rNum, String afm, Date rentDate, Date retDate) {

        /* I initialized data into the object rental */

        rentals.insert(new RentalItem(new Rental(100, cc.findClient("123456789"), cc.findVehicle("XNK5544"), new Date("3/4/2019"), new Date("22/4/2019"), 950)));
        rentals.insert(new RentalItem(new Rental(101, cc.findClient("987456321"), cc.findVehicle("XNA1204"), new Date("4/5/2019"), new Date("4/8/2019"), 210)));
        rentals.insert(new RentalItem(new Rental(102, cc.findClient("741258963"), cc.findVehicle("XNO1706"), new Date("6/5/2019"), new Date("6/15/2019"), 405)));
        rentals.insert(new RentalItem(new Rental(103, cc.findClient("258963147"), cc.findVehicle("XNX9901"), new Date("6/5/2019"), new Date("6/15/2019"), 240)));
        rentals.insert(new RentalItem(new Rental(104, cc.findClient("123456789"), cc.findVehicle("XNA1207"), new Date("6/5/2019"), new Date("6/14/2019"), 2250)));
        rentals.insert(new RentalItem(new Rental(105, cc.findClient("987654321"), cc.findVehicle("XNA1208"), new Date("6/7/2019"), new Date("6/15/2019"), 2400)));
        rentals.insert(new RentalItem(new Rental(106, cc.findClient("741258963"), cc.findVehicle("XNK5544"), new Date("6/5/2019"), new Date("6/15/2019"), 450)));
        rentals.insert(new RentalItem(new Rental(107, cc.findClient("258963147"), cc.findVehicle("XNM1345"), new Date("8/5/2019"), new Date("8/15/2019"), 800)));

        /* using a constructor i called the method searchForRental from  an other class into my main class. The searchForRental method , basically prints out the Rental according to the input variables that the user wrote.*/
        public Rental findRentals(String rNum, String afm, Date rentDate, Date retDate) {
            Rental rental = corporation.searchForRental(rNum, afm, rentDate, retDate);
            if(rental == null) {
                System.out.println("Rental not Found.");
            }else {
                rental.print();
            }
                return rental;
            }

        public Rental searchForRental(String rNum, String afm, Date rentDate, Date retDate) {
        /* Here is the problem */
Why does the Exception thread comes here;
And, what does NullPointerException means?
I thought the problem was that the input variables weren't initialized and the enviroment automatically changed them to null???

            RentalItem rItem = (RentalItem)rentals.search(rNum, afm, rentDate, retDate); /* This is where the thread error begins*/
            if(rItem != null) {
                return (Rental)rItem.getData();
            }else {
                return null;
        }

    }