Why is there a NullPointerException in the output?

Asked Mar 14 '18 at 07:59

Active Mar 14 '18 at 07:59

Viewed 33 times

I was learning Threads , and I'm stuck up on this program, why is this resulting in NPE? why do the threads not run?

here's my code:

class Chicks {
        synchronized void yack(long id) {
        for(int x = 1; x < 3; x++) {
            System.out.print(id + " ");
            Thread.yield();
        }
    }
}
class ChicksYack implements Runnable {
    Chicks c;
    public static void main(String[] args) {
        new ChicksYack().go();
    }
    void go() {
        c = new Chicks();
        new Thread(new ChicksYack()).start();
        new Thread(new ChicksYack()).start();
    }
    public void run() {
        c.yack(Thread.currentThread().getId());
    }
}

asked Mar 14 '18 at 07:59

Codeninja

You should initiate `Chicks c` in every new created `ChicksYack` – xingbin Mar 14 '18 at 08:04
when you start a new thread then `Chicks c` is a new instance and it is not initialize on that thread – TuyenNTA Mar 14 '18 at 08:06
create the instance in the run method, eg: public void run() { c = new Chicks(); c.yack(Thread.currentThread().getId()); } – Amit Upadhyay Mar 14 '18 at 08:07
Does this mean , c=new Chick(); inside go() method creates Chick instance only for main thread and not for other two threads? – Codeninja Mar 14 '18 at 08:43
@Codeninja yes. – lscoughlin Mar 14 '18 at 13:49
Thanks for the reply @Iscoughlin – Codeninja Mar 15 '18 at 14:15

Why is there a NullPointerException in the output?

0 Answers0