Threads related program not working

Question

I am trying to run the following program for threads and expecting that the variable object should be updated by two different threads. Very strangely, it is just showing values 'addition -1' and 'multiply -2'. But when I am using debugger and debugging it step by step, then 'addition' and multiplication is happening as expected. If I put 'Thread.sleep' in 'run' method of 'P' for 1000 ms. It works fine. I have commented out the code of 'Thread.sleep', but if you uncomment it and run, you will find expected results. Can anyone clarify this to me, please?

public class TestingThreads {   

    public static void main(String[] args) {

        Variable b = new Variable();
        Thread tp = new Thread(new P(b));
        Thread tq = new Thread(new Q(b));
        tp.start();
        tq.start();
        try {
            tp.join();
            tq.join();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

}

class Variable {
    int i = 0;
    boolean on = true;

    public synchronized void addition() {
        if (!on) {
            try {
                wait();
            } catch (InterruptedException e) {

                e.printStackTrace();
            }

        }
        i = i + 1;
        on=false;
        notify();
        System.out.println("addition " + i);



    }

    public synchronized void multiply() {

        if (on) {
            try {
                wait();
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

        }
        i = i * 2;
        on=true;
        notify();
        System.out.println("multiply " + i);

    }

}

class P implements Runnable {

    Variable b;

    P(Variable b) {
        this.b = b;
    }


    @Override
    public void run() {


        while (true) {
            /*uncomment it and it will work fine.*/

            /*try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }*/

            b.addition();

        }
    }

}

class Q implements Runnable {

    Variable b;

    Q(Variable b) {
        this.b = b;
    }

    @Override
    public void run() {

        while (true) {

            b.multiply();
        }
    }
}

Threads related program not working

Answers (1)

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