Question

I have main class where I define a 2d array of type Seat and then use a method to randomly select a seat element and set it as “booked”

However if it has to randomly select more than 1 (Sometimes even if it only needs to randomly select one) seat element it returns this error:

"Exception in thread "main" java.lang.NullPointerException
    at cinemasystem.Seat.bookSeat(Seat.java:173)
    at cinemasystem.CinemaSystem.main(CinemaSystem.java:70)
Java Result: 1"

I have no clue what I am doing wrong, probably something with my random generating method, anyhow any help would be appreciated.

Main class:

public static void main(String[] args) {


        Seat cinemaactual = new Seat("Cinema");
        Seat[][] cinema = new Seat[12][23];
        Seat bookedSeat = new Seat("");

        Ticket ticket = new Ticket();
        Scanner scan = new Scanner(System.in);
        String answer, contiune, list;
        cinemaactual.CreateTheatre(cinema);
        int number, check = 0, category, id;

        do {

            System.out.print("Welcome to the Theatre Booking System. (QUIT to exit)"
                    + "\nWould you like to purchase tickets or list available seats?"
                    + "(/Purchase/List/Help)");
            answer = scan.nextLine();

            if (answer.equalsIgnoreCase("purchase")) {
                do {

                    System.out.println("Please choose the cateogry of ticket "
                            + "you would like, followed by who the ticket is for"
                            + "and the amount required. (separated by a space)\n"
                            + "1. Gold\n2. Silver\n3. Bronze\n\n1. Adult\n2."
                            + " Child\n3. Concession");
                    category = scan.nextInt();
                    check = category;
                    id = scan.nextInt();
                    number = scan.nextInt();
                    if (category == 1 || category == 2 || category == 3 && id == 1 || id == 2 || id == 3) {

                        ticket.SetType(category);
                        if (category == 1) {
                            ticket.SetName("Gold");
                        } else if (category == 2) {
                            ticket.SetName("Siler");
                        } else {
                            ticket.SetName("Bronze");
                        }
                        ticket.SetNumber(number);
                        ticket.SetID(id);
                        if (id == 1) {
                            ticket.SetCategory("Adult");
                        } else if (id == 2) {
                            ticket.SetCategory("Child");
                        } else {
                            ticket.SetCategory("Bronze");
                        }
                        System.out.print("You have selected "
                                + ticket.GetNumber() + " " + ticket.GetName()
                                + " ticket(s) at the " + ticket.GetCategory() + " price.");
                        System.out.println();
                        ticket.BuyTicket(category, id);
                        bookedSeat = Seat.bookSeat(cinema, number);


                    } else {

                        System.out.print("Sorry, incorrect input, please enter an apropriate value.");
                        check = scan.nextInt();
                    }
                } while (check == 0 || check > 3);

                do {
                    System.out.print("Would you like to perchase more tickets? (Yes/No)");
                    contiune = scan.nextLine();


                    if (contiune.equalsIgnoreCase("Yes")) {
                        System.out.print("Would you like to list available seats? (Yes/No) (A/G/S/B)");
                        list = scan.nextLine();
                        cinemaactual.DisplayTheatre(cinema);
                        if (list.equalsIgnoreCase("Yes")) {
                            cinemaactual.DisplayTheatre(cinema);

                        }
                    }

The method that randomly selects a seat, in my seat class:

 public static Seat bookSeat(Seat[][] x, int number){
        int count = 0;
        Seat book = new Seat("");
        Random rand = new Random();
        while(count < number){
    if (x != null) {

        do {
        book = x[rand.nextInt(x.length)][rand.nextInt(x.length)];
        book.bookSeat(true);}

        while (book.isBooked());
    }   count++; }
    return book;
}

Line 172 and 173 of Seat Class

172: book = x[rand.nextInt(x.length)][rand.nextInt(x.length)];

173: book.bookSeat(true);}

CreateTheatre Method in class Seat:

 public Seat[][] CreateTheatre(Seat[][] x) {

        for (int row = 0; row < 8; row++) {
            for (int col = 0; col < 4; col++) {
                x[row][col] = new Seat("B");
            }
        }
        for (int row = 8; row < 12; row++) {
            for (int col = 0; col < 4; col++) {
                x[row][col] = new Seat("S");
            }
        }

        for (int row = 0; row < 8; row++) {
            for (int col = 19; col < 23; col++) {
                x[row][col] = new Seat("B");
            }
        }
        for (int row = 8; row < 12; row++) {
            for (int col = 19; col < 23; col++) {
                x[row][col] = new Seat("S");
            }
        }
        for (int row = 3; row < 5; row++) {
            for (int col = 4; col < 9; col++) {
                x[row][col] = new Seat("B");
            }
        }
        for (int row = 3; row < 5; row++) {
            for (int col = 14; col < 19; col++) {
                x[row][col] = new Seat("S");
            }
        }
        for (int row = 9; row < 12; row++) {
            for (int col = 7; col < 4; col++) {
                x[row][col] = new Seat("S");
            }
        }
        for (int row = 3; row < 5; row++) {
            for (int col = 14; col < 20; col++) {
                x[row][col] = new Seat("B");
            }
        }

        for (int row = 5; row < 9; row++) {
            for (int col = 4; col < 9; col++) {
                x[row][col] = new Seat("S");
            }
        }

        for (int row = 5; row < 9; row++) {
            for (int col = 14; col < 20; col++) {
                x[row][col] = new Seat("S");
            }
        }
        for (int row = 6; row < 9; row++) {
            for (int col = 9; col < 14; col++) {
                x[row][col] = new Seat("G");
            }
        }
        for (int row = 9; row < 12; row++) {
            for (int col = 7; col < 16; col++) {
                x[row][col] = new Seat("G");
            }
        }

        for (int row = 9; row < 12; row++){
            for (int col = 4; col < 7; col++){
                x[row][col] = new Seat("S");
            }
        }

        for (int row = 9; row < 12; row++){
            for (int col = 16; col < 19; col++){
                x[row][col] = new Seat("S");
            }
        }
        return x;
            }

Answer 1

If you checked CreateTheatre logic there are possibilities that values inside cinema can be null. Put default values inside array by having one loop.

Take a example of row 0 you only fill values from 0 to 4 and then 19 to 23. Apart from that I do not see you filling values.

for(int i = 0; i<cinema.length;i++)
    for(int j=0; j<cinema[i].length;j++)
      cinema[i][j] = new Seat("");

Update:
If don’t want to put empty seats then you can just add null check before calling book.bookSeat(true)

if(book != null){
 book.bookSeat(true);
}

---

You can print multidimentional arrays using Arrays.deepToString(cinema).

As pointed out in other thread use book = x[rand.nextInt(12)][rand.nextInt(23)]