Updating volatile boolean array using Threads

petegoast :

I'm a CS student currently learning about concurrent programming so my knowledge on threads is still, uh, tentative.

I'm just a bit stuck at the logic of updating a shared array with threads. I'm creating a program that allows a potentially infinite number of threads to constantly update a boolean array of size 10 to simulate the idea of a seating area where people can go in, sit down for a random amount of time, and then leave. Here is my code:

class Viewer extends Thread{
    private String name;
    private int index;
    volatile boolean[] seats;


    Viewer(boolean[] st, String n){
        seats = st;
        name = n;
    }

    public void run() {
        ViewingStand vs = new ViewingStand(seats);
        this.index = vs.findSeat(name, seats);
            try {
                Thread.sleep((long)(Math.random() * 1000));
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            seats = vs.leaveSeat(name, seats, index);

    }
}

class ViewingStand{
    private volatile boolean[] area; //the array used by threads
    private int seatNo; //index of whatever area is being taken or left.
    Random rand = new Random();
    boolean found = false;

    public ViewingStand(boolean st[]){
    this.area = st;
    }

    public int findSeat(String s, boolean[] seats){
        this.area = seats;
        while(found == false) {
            for(int i=0; i < area.length; i++) {
                if(area[i] == true) {
                    found = true;
                    this.seatNo = i; 
                    area[seatNo] = false;
                    System.out.println(s + " has found a seat.");
                    return this.seatNo;
                }
            }
            System.out.println(s + " has started searching again.");
        }
        return -1; //should never reach this
    }

    public boolean[] leaveSeat(String s, boolean[] area, int n){
        this.area = area;
        this.area[n] = false;
        System.out.println(s + " has left their seat.");
        return this.area;
    }

The result of this program is the array initially getting filled with 10 elements (the size of the array I passed from the main program), those threads then leave 'an' array but clearly not the same one I'm passing back and forth between both ViewingStand methods, as every subsequent thread after the 10th gets stuck looking for a seat. Would love some input to point me in the right direction. Thank you!

Krease :

I'll ignore the concurrency issues at first and go straight for what seems like the logic error you're asking about - leaveSeat is setting this.area[n] = false - which seems to indicate that the seat is taken (your findSeat method assumes a seat is empty if the value is true).

On concurrency issues: You're likely to have issues with your loop checking the seats - it's possible for multiple threads to determine a seat is empty (and go into the if block), and all "claim" the same seat. You should construct one instance of ViewingStand and have it manage access to the seats - using concurrency controls like synchronized or locking to ensure multiple threads don't modify the state of the seats at the same time.