java: Printing odd even numbers using 2 threads

Question

I am trying to print odd and even numbers using 2 different threads alternately. I was able to achieve it using wait, notify and synchronize block but now i want to evaluate if we can achieve it without using wait, notify and synchronize.

Following is the code i have but its not working:

public class OddEvenUsingAtomic {

AtomicInteger nm = new AtomicInteger(0);
AtomicBoolean chk = new AtomicBoolean(true);

public static void main(String args[]) {
    final OddEvenUsingAtomic pc = new OddEvenUsingAtomic();

    new Thread(new Runnable() {

        @Override
        public void run() {
            while (true) {

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

                if (pc.chk.compareAndSet(true, false)) {

                    System.out.println("Odd: " + pc.nm.incrementAndGet());
                }
            }

        }

    }).start();

    new Thread(new Runnable() {

        @Override
        public void run() {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            while (true) {
                if (pc.chk.compareAndSet(false, true)) {

                    System.out.println("Even: " + pc.nm.incrementAndGet());
                }
            }

        }

    }).start();
}

}

Any ideas?

I created another version after suggestions from Bruno which seems to be working better:

import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;

public class OddEvenUsingAtomic {

    AtomicInteger nm = new AtomicInteger(0);
    AtomicBoolean chk = new AtomicBoolean(true);

    public static void main(String args[]) {
        final OddEvenUsingAtomic pc = new OddEvenUsingAtomic();

        new Thread(new Runnable() {

            @Override
            public void run() {
                while (true) {

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

                    if (pc.chk.get() == Boolean.TRUE) {

                        System.out.println("Odd: " + pc.nm.incrementAndGet());
                        pc.chk.compareAndSet(true, false);
                    }
                }

            }

        }).start();

        new Thread(new Runnable() {

            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                while (true) {
                    if (pc.chk.get() == Boolean.FALSE) {

                        System.out.println("Even: " + pc.nm.incrementAndGet());
                        pc.chk.compareAndSet(false, true);
                    }
                }

            }

        }).start();
    }
}

Answer 1

The code is not correctly synchronized, that's the problem.

The following execution order is allowed in your code:

1. First thread sees chk == true , sets it to false and enters the if block.
2. Second thread sees chk == false , sets it to true and enters the if block.

Now, you have 2 threads both inside their if blocks, getting ready to:

1. incrementAndGet() the number
2. Print it.

Therefore, you have absolutely no control on what is going to happen.

• You can have any of the threads call incrementAndGet() , therefore you can have thread "Odd" printing, first, an odd number, and later, an even number.
• You can have the first thread print the number, loop, see that the condition is satisfied again (since the second thread has set chk to true again, print, all of this before the second thread had the chance to print).

As you can see, to achieve the result you want, you must have the following operations done atomically:

• compareAndSet() the boolean
• incrementAndGet() the number
• print it

If the 3 operations are not atomic, then you can have the threads being scheduled to run the operations in any possible order, you have no control on the output. The easiest way to achieve this is to use a synchronized block:

public static void main(final String... args) {
  final Object o = new Object();
  // ... thread 1 ...
    synchronized(o) {
      if (boolean is in the expected state) { change boolean, get number, increment, print }
    }
  // ... thread 2 ...
    synchronized(o) {
      if (boolean is in the expected state) { change boolean, get number, increment, print }
    }
}

Answer 2

Here are two threads printing odds and evens with no wait, notify, or synchronized (at least not in the code you can see):

import java.util.concurrent.*;

public class ThreadSignaling {
    public static void main(String[] args) {
        BlockingQueue<Integer> evens = new LinkedBlockingQueue<>();
        BlockingQueue<Integer> odds = new LinkedBlockingQueue<>();
        ExecutorService executorService = Executors.newFixedThreadPool(2);
        executorService.submit(() -> takeAndOfferNext(evens, odds));
        executorService.submit(() -> takeAndOfferNext(odds, evens));
        evens.offer(0);
    }

    private static void takeAndOfferNext(BlockingQueue<Integer> takeFrom,
                                         BlockingQueue<Integer> offerTo) {
        while (true) {
            try {
                int i = takeFrom.take();
                System.out.println(i);
                offerTo.offer(i + 1);
            } catch (InterruptedException e) {
                throw new IllegalStateException("Unexpected interrupt", e);
            }
        }
    }
}

Answer 3

class MultiThreading {

    Integer counter = 0;
    Thread even;
    Thread odd;
    boolean flagEven = true;
    boolean flagOdd;

    class ThreadEven implements Runnable {
        @Override
        public void run() {
            try {
                while (counter < 100) {
                    if (flagEven) {
                        System.out.println(counter);
                        counter++;
                        flagEven = false;
                        flagOdd = true;
                    }
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    class ThreadOdd implements Runnable {
        @Override
        public void run() {
            try {
                synchronized (even) {
                    while (counter < 100) {
                        if (flagOdd) {
                            System.out.println(counter);
                            counter++;
                            flagOdd = false;
                            flagEven = true;
                        }
                    }
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    public void start() {
        even = new Thread(new ThreadEven());
        odd = new Thread(new ThreadOdd());
        even.start();
        odd.start();
    }

}

}

call in the main method

new MultiThreading().start();