How to make sure two threads printing even odd numbers maintain even first then odd order for this implementation?

Question

I have created two runnable jobs: PrintEvenNumbersJob and PrintOddNumbersJob and spawned two threads to execute these jobs. This seems to work perfectly fine! But I smell so

Answer 1

I believe you will need a referee:

public class Referee {
    private boolean evensTurn = true;

    public void waitMyTurn(boolean even) {
        synchronized(this) {
            while (even != evensTurn) {
                try {
                    wait();
                } finally {
                }
            }
        }
    }

    public void done() {
        synchronized(this) {
            evensTurn = !evensTurn;
            notify();
        }
    }
}

public class PrintEvenNumbersJob implements Runnable {

    private Referee referee;

    public PrintEvenNumbersJob(Referee referee) {
        this.referee = referee;
    }

    @Override
    public void run() {
        for (int i = 0; i <= 10; i += 2) {

            referee.waitMyTurn(true);

            System.out.println(i);

            referee.done();
        }
    }
}

public class PrintOddNumbersJob implements Runnable {

    private Referee referee;

    public PrintOddNumbersJob(Referee referee) {
        this.referee = referee;
    }

    @Override
    public void run() {
        for (int i = 0; i <= 10; i += 2) {

            referee.waitMyTurn(false);

            System.out.println(i);

            referee.done();
        }
    }
}