D
DevExCode
multithreading

Implement a program to print Thread1 and Thread2 from two parallel threads in sequential order.

The Problem

Two threads must alternate printing "Thread1" and "Thread2" in strict sequential order, despite running in parallel.

Solution 1: Using wait() / notify()

public class SequentialPrinter {
    private static final Object lock = new Object();
    private static boolean thread1Turn = true;

    public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
            for (int i = 0; i < 5; i++) {
                synchronized (lock) {
                    while (!thread1Turn) {
                        try { lock.wait(); } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
                    }
                    System.out.println("Thread1");
                    thread1Turn = false;
                    lock.notify();
                }
            }
        });

        Thread t2 = new Thread(() -> {
            for (int i = 0; i < 5; i++) {
                synchronized (lock) {
                    while (thread1Turn) {
                        try { lock.wait(); } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
                    }
                    System.out.println("Thread2");
                    thread1Turn = true;
                    lock.notify();
                }
            }
        });

        t1.start();
        t2.start();
    }
}

Solution 2: Using Semaphore

Semaphore sem1 = new Semaphore(1); // starts with permit
Semaphore sem2 = new Semaphore(0); // starts without permit

Thread t1 = new Thread(() -> {
    for (int i = 0; i < 5; i++) {
        try {
            sem1.acquire();
            System.out.println("Thread1");
            sem2.release();
        } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
    }
});

Thread t2 = new Thread(() -> {
    for (int i = 0; i < 5; i++) {
        try {
            sem2.acquire();
            System.out.println("Thread2");
            sem1.release();
        } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
    }
});

Why while instead of if for wait()?

  • Spurious wakeups can occur — the thread may wake up without notify() being called.
  • The while loop re-checks the condition, ensuring correctness.

Interview Insight

This tests your understanding of inter-thread communication, happens-before guarantees, and condition-based synchronization. Semaphore-based solution is cleaner and avoids the spurious wakeup pitfall explicitly.

Implement a program to print Thread1 and Thread2 from two parallel threads in sequential order. | DevExCode