How to properly synchronize two threads

Question

This is a problem I have always heard about in school but never had a reason to mess with until I was asked for an interview.

Prompt: Using 2 threads print "Thread i: The number is 'j'" in order where j = 1:100 and i is the thread number. Thread 1 can only print odd j's and Thread 2 can only print even j's.

EDIT the output of j must be ordered

This was my attempt but I did not move on in the interview process. Is there any fundamental part I am missing? Are there any optimizations?

import java.util.concurrent.Semaphore;

public class ThreadSynchronization implements Runnable {

  private int start;
  private Semaphore semaphore;

  private ThreadSynchronization(int start, Semaphore semaphore) {
      this.start = start;
      this.semaphore = semaphore;
  }

  public static void main(String[] args) {
      Semaphore semaphore = new Semaphore(1, true);
      semaphore.acquireUninterruptibly();

      start(1, semaphore);
      start(2, semaphore);

      semaphore.release();
  }

  private static void start(int start, Semaphore semaphore) {
      ThreadSynchronization ts = new ThreadSynchronization(start, semaphore);
      Thread thread = new Thread(ts);
      thread.start();
      while (thread.getState() != Thread.State.WAITING) ;
  }

  @Override
  public void run() {
      for (int i = start; i <= 100; i += 2) {
          semaphore.acquireUninterruptibly();
          System.out.println("Thread " + start + ": The number is '" + i + "'");
          semaphore.release();
      }
  }
}

Answer 1

One thread can keep aquiring and releasing the Semaphore, while the other thread starves.

You can do this with wait and notify, try this:

import java.util.concurrent.atomic.AtomicInteger;

class Odd implements Runnable {

    private AtomicInteger integer;
    private final Object lock;

    public Odd(AtomicInteger integer, Object lock) {
        this.integer = integer;
        this.lock = lock;
    }

    @Override
    public void run() {
        synchronized (lock) {
            try {
                while (integer.get() <= 100) {
                    while (integer.get() % 2 == 0) {
                        lock.notify();
                        lock.wait();
                    }
                    if (integer.get() <= 100) {
                        System.out.println("Thread " +
                                Thread.currentThread().getName() + ": The number is '" + integer.get() + "'");
                    }
                    integer.getAndIncrement();
                    lock.notify();
                }
            } catch (Exception e) {

            }
        }
    }
}

class Even implements Runnable {

    private AtomicInteger integer;
    private final Object lock;

    public Even(AtomicInteger integer, Object lock) {
        this.integer = integer;
        this.lock = lock;
    }

    @Override
    public void run() {
        synchronized (lock) {
            try {
                while (integer.get() <= 100) {
                    while (integer.get() % 2 != 0) {
                        lock.notify();
                        lock.wait();
                    }
                    if (integer.get() <= 100) {
                        System.out.println("Thread " +
                                Thread.currentThread().getName() + ": The number is '" + integer.get() + "'");
                    }

                    integer.getAndIncrement();
                    lock.notify();
                }
            } catch (Exception e) {

            }
        }
    }
}

public class ThreadSynchronization {

    public static void main(String[] args) throws Exception{
        Object lock = new Object();
        AtomicInteger integer = new AtomicInteger(1);
        Odd odd = new Odd(integer, lock);
        Even even = new Even(integer, lock);

        Thread thread1 = new Thread(odd, "1");
        Thread thread2 = new Thread(even, "2");

        thread1.start();
        thread2.start();

        thread1.join();
        thread2.join();
    }

}

Answer 2

Use an object to arbiter:

public class Switch {
    private boolean expected;

    public Switch(boolean init) {
        expected = init;
    }

    public void waitFor(boolean value) {
        synchronized(this) {
            while (value != expected) {
                try {
                    wait();
                } catch (InterruptedException ex) {
                    // deal with it
                }
            }
            expected = !expected;
            notifyAll();
        }
    }
}

Then:

public class ThreadSynchronization implements Runnable {
    private static Switch arbiter = new Switch(true);

    private int start;

    private ThreadSynchronization(int start) {
        this.start = start;
    }

    public static void main(String[] args) {
        start(1);
        start(2);
    }

    private static void start(int start) {
        ThreadSynchronization ts = new ThreadSynchronization(start);
        Thread thread = new Thread(ts);
        thread.start();
    }

    @Override
    public void run() {
        boolean odd = start%2 != 0;
        for (int i = start; i <= 100; i += 2) {
            arbiter.waitFor(odd);
            System.out.println("Thread " + start + ": The number is '" + i + "'");
        }
    }
}

Answer 3

You was very close to the right solution, but the task requires 2 semaphores:

public class ThreadSynchronization implements Runnable {

    private int start;
    private Semaphore semaphore1;
    private Semaphore semaphore2;

    private ThreadSynchronization(int start, Semaphore semaphore1, Semaphore semaphore2) {
        this.start = start;
        this.semaphore1 = semaphore1;
        this.semaphore2 = semaphore2;
    }

    private static void start(int start, Semaphore semaphore1, Semaphore semaphore2) {
        ThreadSynchronization ts = new ThreadSynchronization(start, semaphore1, semaphore2);
        Thread thread = new Thread(ts);
        thread.start();
    }

    @Override
    public void run() {
        for (int i = start; i <= 100; i += 2) {
            semaphore1.acquireUninterruptibly();
            System.out.println("Thread " + start + ": The number is '" + i + "'");
            semaphore2.release();
        }
    }

    public static void main(String[] args) {
        Semaphore semaphore1 = new Semaphore(1);
        Semaphore semaphore2 = new Semaphore(0);

        start(1, semaphore1, semaphore2);
        start(2, semaphore2, semaphore1); // in reverse order
    }
}