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🛠 Java Concurrency Practice Sheet

A structured set of hands-on exercises to practice concurrency in Java.
Organized from easy → advanced with hints and stretch goals.

🟢 Level 1: Warm-up

1. Race Condition Demo

  • Goal: Show how multiple threads updating a shared counter cause incorrect results.
  • Hint: Use a normal int and increment inside multiple threads without synchronized.
  • Stretch: Fix using synchronized, then AtomicInteger.

2. Thread States Logger

  • Goal: Observe thread lifecycle (NEW, RUNNABLE, TERMINATED).
  • Hint: Use Thread.getState().
  • Stretch: Write code that pushes a thread into WAITING using wait() and notify().

🟡 Level 2: Synchronization & Communication

3. Thread-Safe Counter

  • Goal: Implement counter with synchronized, ReentrantLock, and AtomicInteger.
  • Stretch: Compare performance of the three.

4. Producer-Consumer (Classic)

  • Goal: Implement using wait()/notify().
  • Hint: Shared buffer (list/queue), producer adds, consumer removes.
  • Stretch: Replace with BlockingQueue.

5. Deadlock Simulation

  • Goal: Write two threads holding different locks and waiting for each other.
  • Stretch: Fix deadlock using lock ordering or tryLock().

🟠 Level 3: Executor Framework & Locks

6. Simple Thread Pool

  • Goal: Implement mini ExecutorService:
    • Task queue
    • Worker threads consuming tasks
  • Hint: Use BlockingQueue<Runnable>.
  • Stretch: Add graceful shutdown logic.

7. Scheduled Tasks

  • Goal: Print "Hello" every 2 seconds using ScheduledExecutorService.
  • Stretch: Cancel after 10 seconds.

8. Reader-Writer Lock Example

  • Goal: Simulate shared cache → multiple readers, fewer writers.
  • Hint: Use ReentrantReadWriteLock.
  • Stretch: Add cache invalidation with ScheduledExecutorService.

🔴 Level 4: Advanced Concurrency

9. Countdown Latch Simulation

  • Goal: Worker threads start after all are ready.
  • Hint: Use CountDownLatch.
  • Stretch: Convert to CyclicBarrier (threads meet at barrier repeatedly).

10. Bounded Blocking Queue

  • Goal: Implement your own BlockingQueue with wait()/notify().
  • Stretch: Add timeout support for offer() and poll().

11. CompletableFuture Demo

  • Goal: Call three async tasks, combine results.
  • Hint: Use CompletableFuture.supplyAsync(), then thenCombine().
  • Stretch: Handle exceptions with exceptionally().

🔵 Level 5: Expert-Level

12. Fork/Join Recursive Sum

  • Goal: Sum an array of integers using RecursiveTask.
  • Stretch: Compare runtime with sequential loop.

13. Dining Philosophers Problem

  • Goal: Solve using ReentrantLock.
  • Stretch: Avoid deadlock using resource hierarchy or tryLock.

14. Thread Dump Analysis

  • Goal: Write code that deadlocks → run jstack to analyze.
  • Stretch: Fix by ordering locks.

15. Custom Rate Limiter

  • Goal: Allow max 5 requests per second.
  • Hint: Use Semaphore.
  • Stretch: Implement token bucket algorithm.

✅ How to Use

  1. Start with Level 1–2 → basics and synchronization.
  2. Move to Level 3–4 → practical tools (ExecutorService, locks, latches).
  3. Attempt Level 5 → system design & real-world patterns.
  4. Always code small demos — don’t just read.

Happy coding 🚀