Question

Alright, I’ve been getting a NPE that I can’t figure out and it’s driving me absolutely batty. I’ve got a linked list of Reservation objects and a 2d array of booleans to keep track of the available seats:

class ResList
{
    private Reservation head;
    private boolean [][] seats;

    ResList()
    {
        head = null; //empty list
        boolean[][] seats = new boolean[5][25];
    }

I’ve also got a method isAvailable() to determine whether a seat is available:

boolean isAvailable(int f, int s)
    {        
        if(f<0 || f>4 || s < 0 || s > 24)
            return false;
        else
            return !seats[f][s]; // this line throws the NPE
    }

But when I do this:

jcbSeat = new JComboBox();        
        for(int i = start; i <= stop; i++)
        {        
            if(list.isAvailable(selectedFlight, i))
                jcbSeat.addItem(i+1);
        }

I get a NPE where noted. I added some debugging lines to the ResList constructor, and can access seats[][] there just fine, but when I execute the method, ka-boom: NPE. What is going on here?

Answer 1

Your constructor is initializing a local variable seats, not the instance variable seats.
Try with :

ResList()
{
   head = null; //empty list
   seats = new boolean[5][25];
}