Printing Even and Odd numbers using 2 different classes

Question

This was a question asked in one of my job interviews:

You have 2 different classes (that implements Runnable) say EvenThread & OddThread. As the name suggests, the EvenThread prints only even numbers and the odd thread prints only odd numbers, consider a range of 0-100.

class EvenThread implements Runnable {

    @Override
    public void run() {
        for (int i = 0; i <= 10; i += 2) {
            System.out.println(i);
        }
    }
}

class OddThread implements Runnable {

    @Override
    public void run() {
        for (int i = 1; i < 10; i += 2) {
            System.out.println(i);
        }
    }
}

public class EvenOdd {

    public static void main(String args[]) {
        Thread tEven = new Thread(new EvenThread());
        Thread tOdd = new Thread(new OddThread());

        tEven.start();
        tOdd.start();
    }
}

Now we need to enforce a mechanism in such a way that, the numbers are printed in sequence (i.e. 0, 1, 2, 3, 4,.... and so on).

I have seen many similar questions in Stack Overflow, but they have just one class to print number and 2 synchronized methods are invoked in it.

Could any of the experts please suggest?

Answer 1

Here's an ugly example with a low-level-ish wait/notify mechanism:

public class Main {
    static boolean turn = false; // false is even, true is odd

    public static void main(String[] args) {
        Object o = new Object();
        Thread tEven = new Thread(new EvenThread(o));
        Thread tOdd = new Thread(new OddThread(o));

        tEven.start();
        tOdd.start();
    }

    // TODO some inheritance with [Even/Odd]Thread

    static class EvenThread implements Runnable {
        Object o;

        EvenThread(Object o) {
            this.o = o;
        }

        @Override
        public void run() {
            for (int i = 0; i <= 10; i += 2) {
                synchronized (o) {
                    try {
                        while (turn) {
                            o.wait();
                        }
                    }
                    catch (InterruptedException ie) {
                        ie.printStackTrace();
                    }
                    finally {
                        System.out.println(i);
                        turn = !turn;
                        o.notifyAll();
                    }
                }
            }
        }
    }

    static class OddThread implements Runnable {
        Object o;

        OddThread(Object o) {
            this.o = o;
        }

        @Override
        public void run() {
            for (int i = 1; i < 10; i += 2) {
                synchronized (o) {
                    try {
                        while (!turn) {
                            o.wait();
                        }
                    }
                    catch (InterruptedException ie) {
                        ie.printStackTrace();
                    }
                    finally {
                        System.out.println(i);
                        turn = !turn;
                        o.notifyAll();
                    }
                }
            }
        }
    }
}

Output

0
1
2
3
4
5
6
7
8
9
10