85 Multithreading Interview Questions and Answers PDF 2025

Multithreading is a powerful programming concept that allows multiple threads to run concurrently within a single process, enabling faster execution and better resource utilization. A developer skilled in multithreading can optimize code to handle parallel tasks efficiently, avoid race conditions, and improve overall system throughput, making them highly valuable in today’s performance-driven software industry.

Multithreading is a must-have skill, especially for roles in Java, Python, and C++ development. Companies across domains are looking for professionals who can manage thread synchronization, deadlocks, concurrency issues, and parallel execution using language-specific tools and libraries.

This guide provides a curated list of Multithreading Interview Questions and Answers tailored for freshers, experienced candidates, and includes dedicated sections for Java, Python, and C++ multithreading.

Multithreading Interview Questions and Answers for Freshers

Que 1. What is the difference between a thread and a process?

Answer:
A process is an independent unit of execution with its own memory, while a thread is a lightweight execution unit within a process that shares the process’s memory. Threads within the same process share resources, making communication easier but requiring synchronization for shared data.

Que 2. How can you create a thread in Java?

Answer:
There are two main ways:

1. Extend the Thread class and override the run() method.
2. Implement the Runnable interface and pass it to a Thread object.
   Example:

class MyThread implements Runnable {
    public void run() {
        System.out.println("Thread is running...");
    }
}
new Thread(new MyThread()).start();

Que 3. What is the difference between start() and run() methods in threads?

Answer:

• start(): Creates a new thread and invokes the run() method in that new thread.
• run():Executes the code in the current thread if called directly, without starting a new thread.

Que 4. What are the different states of a thread?

Answer:
Threads can be in the following states:

1. New
2. Runnable
3. Blocked
4. Waiting
5. Timed Waiting
6. Terminated

Que 5. What is a daemon thread in Java?

Answer:
A daemon thread is a background thread that runs in the background to perform tasks like garbage collection. It ends automatically when all user threads have finished. You can set a thread as a daemon using thread.setDaemon(true) before starting it.

Que 6. What is thread priority and how does it affect thread execution?

Answer:
Thread priority determines the relative importance of threads. Higher-priority threads are given preference by the scheduler, but it doesn’t guarantee execution order. Priority is set using setPriority() method with values from 1 (MIN) to 10 (MAX).

Que 7. What is thread synchronization and why is it needed?

Answer:
Thread synchronization is the mechanism to control access of multiple threads to shared resources. It is needed to prevent race conditions and ensure data consistency when multiple threads modify shared data.

Que 8. What is the difference between a synchronized method and a synchronized block?

Answer:

• Synchronized method: Locks the entire method.
• Synchronized block: Locks only a specific section of code or object.
  Synchronized blocks are generally preferred for better performance as they reduce lock contention.

Que 9. What is a race condition in multithreading?

Answer:
A race condition occurs when two or more threads access shared data simultaneously, and the final result depends on the timing of thread execution. Synchronization or thread-safe constructs are used to avoid race conditions.

Que 10. How does volatile keyword help in multithreading?

Answer:
The volatile keyword ensures that a variable’s value is always read from and written to the main memory, preventing threads from using cached values. It is useful for flags and simple state variables.

Que 11. How is wait() different from sleep()?

Answer:

• Wait(): Releases the lock on the object and waits until notify() or notifyAll() is called.
• sleep(): Pauses the thread for a specified time but does not release the lock.

Que 12. What are notify() or notifyAll() methods?

Answer:

• notify(): Wakes up one waiting thread.
• notifyAll(): Wakes up all waiting threads.
  These methods are used in combination with wait() for inter-thread communication.

Que 13. What is the difference between Callable and Runnable?

Answer:

Feature	Runnable	Callable
Return value	No (void)	Yes (returns a value)
Exceptions	Cannot throw checked exceptions	Can throw checked exceptions
Execution	Used with Thread	Used with ExecutorService

Que 14. How does the join() method work in threads?

Answer:
The join() method allows one thread to wait until another thread finishes execution. Example:

Thread t = new Thread(() -> System.out.println("Task"));
t.start();
t.join(); // main thread waits for t to finish

Que 15. What are thread pools and why are they used?

Answer:
A thread pool is a collection of reusable threads managed by an Executor. It improves performance by reducing the overhead of creating new threads for each task and allows better resource management.

Que 16. How does the Executor Framework help in multithreading?

Answer:
The Executor Framework provides a higher-level API for managing threads. It separates task submission from thread management and provides built-in thread pooling through classes like ThreadPoolExecutor and ScheduledExecutorService.

Que 17. What is deadlock and how can you prevent it?

Answer:
Deadlock occurs when two or more threads are waiting for each other’s resources indefinitely. Prevention strategies include:

• Acquiring locks in a fixed order.
• Using tryLock() with a timeout.
• Minimizing synchronized code blocks.

Que 18. What is a reentrant lock in Java?

Answer:
A reentrant lock (from java.util.concurrent.locks.ReentrantLock) allows a thread to acquire the same lock multiple times without causing deadlock. It provides more advanced features than the synchronized keyword, such as try-locking and fairness policies.

Que 19. How can you handle thread safety without using synchronized?

Answer:

• Use thread-safe classes like ConcurrentHashMap or CopyOnWriteArrayList.
• Use atomic variables (AtomicInteger, AtomicLong).
• Employ immutable objects.

Que 20. What are some best practices for multithreading in Java?

Answer:

1. Keep synchronized sections as small as possible.
2. Avoid nested locks.
3. Use concurrent utilities instead of manual synchronization.
4. Handle exceptions properly in threads.
5. Test thoroughly as multithreading bugs are hard to reproduce.

Also Check: Full Stack Developer Interview Questions

Multithreading Interview Questions and Answers for Experienced

Que 21. What is the difference between user threads and daemon threads?

Answer:
User threads perform the main tasks of the application and keep the JVM alive until they finish execution. Daemon threads are background threads (like garbage collection) and the JVM terminates when all user threads finish, regardless of daemon threads running.

Que 22. How do you avoid thread starvation?

Answer:
Thread starvation happens when low-priority threads are perpetually waiting because high-priority threads monopolize CPU. To avoid it:

1. Use fair locks (e.g., new ReentrantLock(true)).
2. Avoid overly relying on thread priorities.
3. Use modern executors with fairness policies.

Que 23. How does ThreadLocal work and when would you use it?

Answer:
ThreadLocal provides thread-confined variables, where each thread has its own independent copy.
Use cases:

• Storing user session data in multi-threaded environments.
• Maintaining database connection or transaction context per thread.

Que 24. What is the difference between synchronized and ReentrantLock ?

Answer:

Feature	synchronized	ReentrantLock
Lock fairness	No	Can be fair (FIFO)
Try for lock	No	Yes (tryLock())
Interruptible	No	Yes
Condition variables	No	Yes (supports multiple)

Que 25. What is a ReadWriteLock and when is it beneficial?

Answer:
ReadWriteLock allows multiple readers or a single writer at the same time. It improves concurrency when reads are more frequent than writes.
Example:

ReadWriteLock lock = new ReentrantReadWriteLock();
lock.readLock().lock();
// perform read
lock.readLock().unlock();

Que 26. How does the Java memory model impact multithreading?

Answer:
The Java Memory Model defines how threads interact through memory, ensuring happens-before relationships. It specifies visibility guarantees of variables (e.g., volatile ensures writes are visible to all threads) and ordering of operations to avoid race conditions.

Que 27. What are the differences between submit() and execute() in ExecutorService?

Answer:

• execute(): Executes a Runnable task and does not return a value.
• submit(): Executes a Runnable or Callable task and returns a Future object for tracking completion and result retrieval.

Que 28. How do you detect and resolve deadlocks in a running application?

Answer:
Detection methods:

• Using tools like jconsole, jstack, or profilers to find threads in WAITING state.
  Resolution:

1. Acquire locks in a fixed order.
2. Use timeouts with tryLock().
3. Break circular dependencies in code design.

Que 29. What is the difference between notify() and notifyAll() and when would you prefer one over the other?

Answer:

• notify() wakes up one waiting thread, while notifyAll() wakes all waiting threads.
  Use notify() if only one thread can proceed. Use notifyAll() when multiple waiting threads might proceed, especially in complex conditions to avoid deadlocks.

Que 30. How does ForkJoinPool work in Java?

Answer:
ForkJoinPool is designed for tasks that can be split (forked) into subtasks and combined (joined) for results. It uses a work-stealing algorithm, where idle threads “steal” tasks from busy threads’ queues, maximizing CPU utilization.

Que 31. What is the difference between ConcurrentHashMap and Collections.synchronizedMap() ?

Answer:

• ConcurrentHashMap: Allows concurrent read and write operations with fine-grained locking on buckets.
• synchronizedMap(): Locks the entire map for every operation, leading to reduced concurrency.

Que 32. What is the purpose of the CountDownLatch class?

Answer:
CountDownLatch allows one or more threads to wait until a set of operations in other threads complete. Example:

CountDownLatch latch = new CountDownLatch(3);
latch.await(); // waits until count reaches 0

Que 33. How is CyclicBarrier different from CountDownLatch ?

Answer:

• CountDownLatch: One-time use; threads wait for a count to reach zero.
• CyclicBarrier: Can be reused; a fixed number of threads wait for each other to reach a common barrier point before proceeding.

Que 34. How do you handle exceptions in ExecutorService tasks?

Answer:

• Use Future.get() which wraps exceptions in ExecutionException.
• Implement a custom ThreadPoolExecutor.afterExecute() method for centralized exception handling.
• Use setUncaughtExceptionHandler() for uncaught exceptions in threads.

Que 35. What are atomic classes and why are they important in concurrency?

Answer:
Atomic classes like AtomicInteger provide thread-safe operations using low-level Compare-And-Swap (CAS). They allow lock-free, efficient updates without explicit synchronization.

Que 36. How would you prevent false sharing in multithreaded applications?

Answer:
False sharing occurs when threads modify variables close in memory, causing cache line invalidations.
Solutions:

• Padding variables with unused fields (@Contended).
• Grouping related variables together and isolating unrelated ones.

Que 37. What is the difference between fair and non-fair locks?

Answer:

• Fair locks grant access in the order threads requested it (FIFO).
• Non-fair locks allow threads to “barge in” for faster throughput, but may cause starvation.
  Example: new ReentrantLock(true) creates a fair lock.

Que 38. How do you implement thread-safe lazy initialization?

Answer:
Use the Double-Checked Locking pattern with volatile:

private volatile static Singleton instance;
public static Singleton getInstance() {
    if(instance == null) {
        synchronized(Singleton.class) {
            if(instance == null) {
                instance = new Singleton();
            }
        }
    }
    return instance;
}

Que 39. How does the CompletableFuture API simplify asynchronous programming?

Answer:
CompletableFuture allows chaining tasks (thenApply, thenCombine), combining multiple futures, handling exceptions (exceptionally), and using non-blocking callbacks, making asynchronous programming more readable and manageable.

Que 40. How would you troubleshoot a performance bottleneck caused by excessive thread context switching?

Answer:

1. Profile the application using tools like VisualVM or JFR.
2. Reduce the number of active threads (use proper thread pool sizing).
3. Minimize synchronization overhead.
4. Use lock-free algorithms or concurrent data structures to reduce contention.

Also Check: OOPs Interview Questions and Answers

Java Multithreading Interview Questions and Answers

Que 41. How can you create a thread in Java?

Answer:
There are two main approaches:

1. Extend the Thread class and override run().
2. Implement the Runnable interface and pass it to a Thread object.
   Example:

class MyTask implements Runnable {
    public void run() {
        System.out.println("Task running...");
    }
}
new Thread(new MyTask()).start();

Que 42. What is the difference between start() and run() in threads?

Answer:

• start(): Creates a new thread and calls the run() method inside it.
• run(): Executes the code in the current thread if called directly, not creating a new thread.

Que 43. What are thread states in Java?

Answer:
A thread can be in one of these states: New, Runnable, Blocked, Waiting, Timed Waiting, or Terminated. Transitions occur based on method calls like start(), sleep(), or join() .

Que 44. What is the difference between user threads and daemon threads?

Answer:
User threads keep the JVM alive until they finish. Daemon threads run in the background (e.g., garbage collector) and terminate automatically when all user threads end.

Que 45. What is thread synchronization and why is it needed?

Answer:
Thread synchronization controls concurrent access to shared resources to prevent race conditions and data inconsistency. This is done using synchronized blocks, locks, or concurrent collections.

Que 46. How is a synchronized block different from a synchronized method?

Answer:

• Synchronized method: Locks the entire method.
• Synchronized block: Locks only a specific block of code, offering finer control and better performance.

Que 47. How does volatile differ from synchronized?

Answer:

• volatile: Ensures visibility of variable updates across threads but does not guarantee atomicity.
• synchronized: Ensures both visibility and atomicity by locking.

Que 48. What is the difference between wait() and sleep()?

Answer:

• wait(): Releases the lock and waits until notify()/notifyAll() is called.
• sleep(): Pauses thread execution for a specified time but does not release the lock.

Que 49. What are notify() and notifyAll() used for?

Answer:
They are used for inter-thread communication.

• notify(): Wakes up one waiting thread.
• notifyAll(): Wakes up all waiting threads on the object’s monitor.

Que 50. How do Callable and Runnable differ?

Answer:

Feature	Runnable	Callable
Return value	void	Returns a value
Exceptions	Cannot throw checked exceptions	Can throw checked exceptions
Execution	Passed to Thread	Submitted to ExecutorService

Que 51. What is ExecutorService and why is it preferred over creating threads manually?

Answer:
ExecutorService manages a pool of threads, reuses them for tasks, and handles queueing, scheduling, and shutdown automatically. It improves performance and resource management compared to creating new threads for every task.

Que 52. How do you prevent deadlocks in multithreaded applications?

Answer:

1. Acquire locks in a consistent order.
2. Use timeouts with tryLock().
3. Minimize synchronized sections.
4. Use high-level concurrency utilities (e.g., java.util.concurrent).

Que 53. What is ThreadLocal and when would you use it?

Answer:
ThreadLocal provides thread-confined variables, meaning each thread has its own value. It’s useful for storing user sessions or transactional data in a multi-threaded environment.

Que 54. What is the difference between CountDownLatch and CyclicBarrier ?

Answer:

• CountDownLatch: One-time use; threads wait for a count to reach zero.
• CyclicBarrier: Can be reused; threads wait at the barrier until all reach it, then continue together.

Que 55. What is ForkJoinPool and how does it work?

Answer:
ForkJoinPool is designed for tasks that can be recursively split into smaller tasks using the fork-join framework. It uses a work-stealing algorithm, where idle threads steal tasks from others’ queues to maximize throughput.

Also Check: Java interview Questions and Answers

Python Multithreading Interview Questions and Answers

Que 56. How can you create a thread in Python?

Answer:
You can create a thread using the threading module:

import threading

def task():
    print("Thread running")

t = threading.Thread(target=task)
t.start()

Alternatively, you can subclass Thread and override the run() method.

Que 57. What is the difference between start() and run() in Python threads?

Answer:

• start(): Starts a new thread and calls the run() method in that thread.
• run(): Executes in the current thread if called directly, without starting a new thread.

Que 58. What are daemon threads in Python?

Answer:
Daemon threads run in the background and are terminated when all non-daemon threads finish. You can set a thread as daemon using:

t.daemon = True
t.start()

Que 59. How do you get the current thread name in Python?

Answer:
Use threading.current_thread().name:

import threading
print(threading.current_thread().name)

Que 60. What are the different states of a Python thread?

Answer:
Threads can be in the following states:

1. New (created but not started)
2. Runnable (ready to run)
3. Running (executing code)
4. Waiting (waiting for a resource or event)
5. Terminated (finished execution)

Que 61. What is the Global Interpreter Lock (GIL) and how does it affect Python multithreading?

Answer:
The GIL is a mutex that prevents multiple native threads from executing Python bytecodes simultaneously. It ensures thread safety but limits true parallel execution in CPU-bound tasks. For CPU-bound work, multiprocessing is often a better choice.

Que 62. How can you synchronize threads in Python?

Answer:
You can use the Lock class:

import threading

lock = threading.Lock()
with lock:
    # critical section

This ensures only one thread accesses the critical section at a time.

Que 63. What is the difference between Lock and RLock in Python?

Answer:

• Lock: Cannot be acquired multiple times by the same thread without deadlocking.
• RLock (Reentrant Lock): Allows the same thread to acquire it multiple times safely.

Que 64. How does join() work in Python threads?

Answer:
join() blocks the calling thread until the target thread finishes:

t = threading.Thread(target=task)
t.start()
t.join()  # waits until t is done

Que 65. How do you share data safely between threads in Python?

Answer:

1. Use synchronization primitives like Locks.
2. Use thread-safe queues (queue.Queue) for producer-consumer scenarios.
3. Use ThreadLocal storage for thread-specific data.

Que 66. What is the difference between Thread and ThreadPoolExecutor ?

Answer:

Feature	Thread	ThreadPoolExecutor
Creation	Manual for each task	Uses a pool of reusable threads
Task submission	start()	submit() or map()
Scalability	Less	More (manages threads internally)

Que 67. How do you use ThreadPoolExecutor in Python?

Answer:

from concurrent.futures import ThreadPoolExecutor

def task(n):
    print(f"Task {n} running")

with ThreadPoolExecutor(max_workers=3) as executor:
    for i in range(5):
        executor.submit(task, i)

Que 68. What is a condition variable in Python threading?

Answer:
A condition variable allows threads to wait for certain conditions to be met.

cond = threading.Condition()
with cond:
    cond.wait()      # wait for notification
    cond.notify()    # notify waiting threads

Que 69. How can you avoid race conditions in Python multithreading?

Answer:

• Use Locks or RLocks to protect shared data.
• Use thread-safe data structures (Queue, deque).
• Minimize shared state between threads.

Que 70. How would you debug a deadlock issue in Python multithreading?

Answer:

1. Use threading.enumerate() to list all active threads.
2. Check locks that are acquired but never released.
3. Use faulthandler module or logging to trace where threads are stuck.
4. Design code to avoid circular dependencies when acquiring multiple locks.

Also Check: Python Interview Questions and Answers

C++ Multithreading Interview Questions and Answers

Que 71. How can you create a thread in C++?

Answer:
You can create a thread using the std::thread class from the <thread> library:

#include <iostream>
#include <thread>
void task() {
    std::cout << "Thread running\n";
}
int main() {
    std::thread t(task);
    t.join(); // wait for thread to finish
}

Que 72. What is the difference between join() and detach() in C++ threads?

Answer:

• join(): Blocks the calling thread until the thread finishes execution.
• detach(): Separates the thread from the thread object, allowing it to run independently (as a daemon thread).

Que 73. What are the different states a C++ thread can be in?

Answer:
Threads can be:

1. Joinable: Started and can be joined.
2. Not joinable: Finished, detached, or never started.

Que 74. How do you pass arguments to a thread function?

Answer:

void task(int x) { std::cout << x; }
std::thread t(task, 5);
t.join();

Use std::ref if you need to pass arguments by reference.

Que 75. What is a race condition and how can it be avoided in C++?

Answer:
A race condition occurs when multiple threads access shared data simultaneously without synchronization. It can be avoided by using mutexes, locks, or atomic operations.

Que 76. What is the difference between std::mutex and std::recursive_mutex ?

Answer:

• std::mutex: Cannot be locked multiple times by the same thread.
• std::recursive_mutex: Allows the same thread to acquire the lock multiple times, but must release it the same number of times.

Que 77. How does std::lock_guard simplify mutex management?

Answer:
std::lock_guard automatically acquires the lock when created and releases it when it goes out of scope, preventing deadlocks caused by forgetting to unlock.

std::mutex m;
void safe() {
    std::lock_guard<std::mutex> guard(m);
    // critical section
}

Que 78. What is the difference between std::unique_lock and std::lock_guard ?

Answer:

Feature	std::lock_guard	std::unique_lock
Flexibility	Less (RAII only)	More (can lock/unlock multiple times)
Deferred locking	No	Yes
Condition variable	Cannot use	Can use with wait ()

Que 79. How do you use condition variables in C++ multithreading?

Answer:

std::mutex m;
std::condition_variable cv;
bool ready = false;

void worker() {
    std::unique_lock<std::mutex> lock(m);
    cv.wait(lock, [] { return ready; });
    // work
}

cv.notify_one() or cv.notify_all() wakes up waiting threads.

Que 80. What is a deadlock and how can it be avoided?

Answer:
A deadlock occurs when threads wait indefinitely for locks held by each other. Avoid it by:

1. Locking resources in a consistent order.
2. Using std::lock() to acquire multiple locks atomically.
3. Using try-lock techniques.

Que 81. What is std::future and how does it work with threads?

Answer:
std::future holds the result of an asynchronous operation. You can use it with std::async or std::promise to retrieve return values from threads:

auto f = std::async([] { return 42; });
std::cout << f.get();

Que 82. What is the difference between std::async and std::thread ?

Answer:

• std::thread: Starts a thread immediately and does not manage return values.
• std::async: Starts tasks lazily or eagerly and provides a std::future to access results and exceptions.

Que 83. How does std::atomic help in multithreading?

Answer:
std::atomic provides operations on variables that are atomic, meaning they can’t be interrupted. It helps avoid race conditions without using mutexes.

std::atomic<int> counter(0);
counter++;

Que 84. What is a thread-safe singleton and how can you implement it in C++?

Answer:
Use std::call_once and std::once_flag for thread-safe initialization:

class Singleton {
    static Singleton* instance;
    static std::once_flag flag;
    Singleton() {}
public:
    static Singleton* getInstance() {
        std::call_once(flag, [] { instance = new Singleton(); });
        return instance;
    }
};

Que 85. How would you detect and debug deadlocks in a C++ multithreaded application?

Answer:

1. Use logging to track lock acquisition order.
2. Use tools like Valgrind, ThreadSanitizer, or gdb for deadlock detection.
3. Enforce consistent lock order and use timeout-based lock acquisition (try_lock_for).

Multithreading Interview Questions PDF

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FAQs: Multithreading Interview Questions

Conclusion

This multithreading interview guide helps you prepare for programming job interviews with questions for both new and experienced developers. The guide covers everything from basic concepts to advanced multithreading scenarios. Use this guide to practice and master multithreading concepts for your next interview.