Java Machine Coding: Print Sequence with Multiple Threads

Java Machine Coding: Print Sequence with Multiple Threads

Printing a strictly ordered sequence using multiple threads is a classic FAANG concurrency screen because it instantly exposes whether you grasp inter-thread coordination. Candidates who struggle typically fall back on brittle busy-waiting loops or create deadlocks under high thread contention.

The Mistake Most Candidates Make

• Spinning on volatile variables: Relying on shared flags inside tight while loops, burning 100% CPU core capacity through busy-waiting.
• Overusing notifyAll() on single intrinsic locks: Waking up every dormant thread simultaneously, causing a thundering herd problem where $N-1$ threads immediately go back to sleep.
• Missing spurious wakeup guards: Using if instead of while around wait checks, leading to corrupt execution order when the JVM awakens a thread unpredictably.

The Right Approach

• Core mental model: Model execution as an explicit turn-based state machine where each worker blocks on its own condition variable until specifically signaled by its predecessor.
• Key entities/classes: ReentrantLock, Condition[], and a shared currentTurn state integer.
• Why it beats the naive approach: Target-signaling dedicated condition queues yields $O(1)$ wakeups and completely prevents CPU burn and contention overhead.

The Key Insight (Code)

lock.lock();
try {
    // 1. Guard against spurious wakeups
    while (currentTurn != threadId) {
        conditions[threadId].await();
    }
    // 2. Perform work and advance state
    System.out.print(number + " ");
    currentTurn = (currentTurn + 1) % totalThreads;
    // 3. Wake ONLY the next thread in sequence
    conditions[currentTurn].signal();
} finally {
    lock.unlock();
}

Key Takeaways

• Always wrap .await() calls in a while loop check to preserve invariants against JVM spurious wakeups.
• Array-based Condition references allow deterministic, point-to-point signaling between arbitrary thread topologies.
• Update sequencing state strictly within the critical section prior to firing .signal() to ensure atomicity.

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