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50 Java Multithreading & Concurrency Interview Questions

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Fundamentals & Basics

  1. What is multithreading in Java?
  2. What is a thread and how is it different from a process?
  3. What are the different ways to create a thread in Java?
  4. Difference between start() and run() in Java threads.
  5. Describe the thread lifecycle and its states.
  6. What is a daemon thread and how to create one?
  7. Can you start a thread twice? Why or why not?
  8. What is thread priority and how does it affect execution?
  9. What is time slicing/thread scheduling?
  10. What is the difference between user and daemon threads?

Synchronization & Locks

  1. What is synchronization and why is it needed?
  2. Difference between synchronized method and synchronized block.
  3. What is the volatile keyword?
  4. Difference between volatile and synchronized.
  5. What is a monitor in Java?
  6. What are intrinsic locks?
  7. Explain lock escalation and fairness.
  8. What are reentrant locks (ReentrantLock)?
  9. Difference between synchronized and ReentrantLock.
  10. What is optimistic vs pessimistic locking?

Thread Communication

  1. What are wait(), notify() and notifyAll()?
  2. Difference between wait() and sleep().
  3. What is yield()?
  4. Explain the producer-consumer problem.
  5. How to achieve thread communication without busy waiting?

Concurrency Problems

  1. What is race condition?
  2. What is deadlock and how to avoid it?
  3. What is livelock and starvation?
  4. How to detect thread related issues (deadlock) at runtime?
  5. What is a data race?

Executor Framework & Thread Pools

  1. What is ExecutorService?
  2. Difference between Executor and ExecutorService.
  3. How to create a fixed thread pool and cached thread pool?
  4. Difference between shutdown() and shutdownNow().
  5. Difference between submit() and execute().
  6. What is ScheduledExecutorService?
  7. What is a thread pool queue?

Advanced Concurrency APIs

  1. What are Callable and Future?
  2. What is FutureTask?
  3. What are concurrent collections (e.g., ConcurrentHashMap)?
  4. Explain CountDownLatch, CyclicBarrier.
  5. What is Semaphore?
  6. Difference between Semaphore and ReentrantLock.
  7. What is ThreadLocal?
  8. What are atomic classes (AtomicInteger, AtomicReference, etc.)?

JVM, Memory Model & Performance

  1. What is the Java Memory Model (JMM)?
  2. How does JMM affect visibility and ordering of threads?
  3. What is false sharing and how to avoid it?
  4. How do thread dumps help in debugging performance issues?
  5. How would you optimize multithreaded code for scalability?

Tips for Interview Success

  • Understand concepts plus code examples: Many interviews require writing small multithreading programs (e.g., producer-consumer, deadlock example).
  • Discuss real scenarios where you used concurrency (locks, thread pools) in your projects.
  • Be ready with trade-offs: synchronization vs concurrent libraries, blocking vs non-blocking algorithms.

Multithreading