Call In Order: Synchronizing Method Execution Sequence (CTCI 15.5)

TL;DR

• The Problem: CTCI problem 15.5 technical mechanics.
• The Approach: CTCI problem 15.5: enforce execution order of first(), second(), third() methods across concurrent threads using CountDownLatch / Semaphores.
• Complexity: Optimal Time and Memory bounds.

This article provides a clear breakdown of CTCI problem 15.5.

1. Context and Problem Statement

CTCI problem 15.5: enforce execution order of first(), second(), third() methods across concurrent threads using CountDownLatch / Semaphores.

2. Technical Code & Mechanics

public class Foo {
    private final Semaphore s1 = new Semaphore(0);
    private final Semaphore s2 = new Semaphore(0);

    public void first(Runnable r) { r.run(); s1.release(); }
    public void second(Runnable r) throws InterruptedException { s1.acquire(); r.run(); s2.release(); }
    public void third(Runnable r) throws InterruptedException { s2.acquire(); r.run(); }
}

3. Key Takeaways and Edge Cases

Always test boundary conditions and invalid input states.