Question

Recently I have gone through with one simple threading program, which leads me some issues for the related concepts… My sample program code looks like :

class NewThread implements Runnable {
    Thread t;
    NewThread() {
        t = new Thread(this, "Demo Thread");
        System.out.println("Child thread: " + t);
        t.start(); // Start the thread
    }
    public void run() {
        try {
            for (int i = 5; i > 0; i--) {
                System.out.println("Child Thread: " + i);
                Thread.sleep(500);
            }
        } catch (InterruptedException e) {
            System.out.println("Child interrupted.");
        }
        System.out.println("Exiting child thread.");
    }
}
class ThreadDemo {
    public static void main(String args[]) {
        new NewThread(); // create a new thread
        try {
            for (int i = 5; i > 0; i--) {
                System.out.println("Main Thread: " + i);
                Thread.sleep(1000);
            }
        } catch (InterruptedException e) {
            System.out.println("Main thread interrupted.");
        }
        System.out.println("Main thread exiting.");
    }
}

Now this program giving me the output as follows :

Child thread: Thread[Demo Thread,5,main]
Main Thread: 5
Child Thread: 5
Child Thread: 4
Main Thread: 4
Child Thread: 3
Child Thread: 2
Main Thread: 3
Child Thread: 1
Exiting child thread.
Main Thread: 2
Main Thread: 1
Main thread exiting.

So, that’s very much clear to me. But as soon as I am replacing the object creation code (calling of a NewThread class constructor) to as follows :

NewThread nt = new NewThread(); // create a new thread

the output becomes a bit varied like as follows :

Child thread: Thread[Demo Thread,5,main]
Main Thread: 5
Child Thread: 5
Child Thread: 4
Child Thread: 3
Main Thread: 4
Child Thread: 2
Child Thread: 1
Main Thread: 3
Exiting child thread.
Main Thread: 2
Main Thread: 1
Main thread exiting.

And some times it’s giving me same output in both the cases. So, i am not getting the exact change in both the scenario.

I would like to know that you the variation in the output is coming here ?

Thanks in advance…

Answer 1

The changed output is due to nature of both OS process scheduling and JVM thread scheduling. Even if you take out the second thread there is no guarantee that your thread will wake-up exactly after 500ms.