Concurrency

5. What are the different ways to create a thread?

   1. Extends Thread class.
   2. Implements Runnable interface.
   3. ExecutorService

6. What are the states of a thread?

   1. NEW: Thread created but not started (start() not called).
   2. RUNNABLE: Thread is executing or ready to execute.
   3. BLOCKED: Waiting for a monitor lock (e.g., in a synchronized block).
   4. WAITING: Waiting indefinitely for another thread (e.g., wait(), join()).
   5. TIMED_WAITING: Waiting with a timeout (e.g., sleep(), wait(timeout)).
   6. TERMINATED: Thread has completed execution.

7. What are the ways to create a thread pool?

   1. Using Executors Factory Methods:

      ExecutorService pool = Executors.newFixedThreadPool(4); // Fixed-size pool
      ExecutorService single = Executors.newSingleThreadExecutor(); // Single thread
      ExecutorService cached = Executors.newCachedThreadPool(); // Dynamic pool
   2. Using ThreadPoolExecutor:

      ThreadPoolExecutor pool = new ThreadPoolExecutor(
          2, // core pool size
          4, // max pool size
          60, TimeUnit.SECONDS, // keep-alive time
          new LinkedBlockingQueue<>() // work queue
      );

8. What are the states of a thread pool?

9. What is the difference between the submit() and execute() methods in a thread pool?

• need example

13. How can you ensure thread safety in a Java program?

14. Use synchronized methods and blocks.

15. Use concurrent collections.

16. Use atomic variables for lock-free operations.

17. Use ReentrantLock for fine-grained control.

18. Use volatile for shared variables.

19. Use immutable object.

20. What is the principle behind synchronized lock upgrading in multithreading?

21. What is a deadlock?

• A deadlock occurs when two or more threads are blocked forever, each waiting for a resource that another holds, forming a circular dependency.
• Four conditions.

16. What is the difference between deadlock and starvation?

• Deadlock: Threads are blocked forever.
• Starvation: Threads unable to access resource because of low priority or unfair scheduling.

17. How can deadlocks be prevented?

18. Acquire all required locks at once.

    synchronized (lock1) {
        synchronized (lock2) { /* work */ }
    }

19. Always lock in the same order.

20. Use tryLock() with ReentrantLock to avoid indefinite waiting.

21. What is ThreadLocal? What are its use cases?

• ThreadLocal provides thread-local variables.
• Storing per-thread context (e.g., user ID, transaction ID).

19. Explain the underlying implementation of synchronized.

20. What is the difference between synchronized and volatile?

21. What are the communication methods between threads?

• wait(), notify(), notifyAll()
• Volatile keyword
• Locks and Conditions?

22. What are the communication methods between processes?

23. What is thread safety? Is Vector a thread-safe class?

24. What is a FutureTask?

25. What is the difference between the heap and the stack in Java?

• Heap:
  1. Share across threads
  2. Store objects and class instances
• Stack
  1. Thread-specific
  2. Store method local variables
  3. Tied to method scope

26. What is a thread pool, and why is it used?

• Reusable threads managed by ExecutorService

27. What is the difference between livelock and deadlock in Java?

• Livelock: Threads actively change states but cannot progress.
• Deadlock: Threads blocked forever waiting for resources.