Java Notes

Multithreading Interview Questions

35+ Java Multithreading interview questions covering threads, synchronization, locks, executor framework, concurrent collections, and thread communication.

1. What is a Thread? How to create threads in Java?

A thread is the smallest unit of execution. Two ways to create:

java example

// Way 1: Extend Thread class
class MyThread extends Thread {
    public void run() { System.out.println("Thread running: " + getName()); }
}
new MyThread().start();

// Way 2: Implement Runnable (preferred)
class MyTask implements Runnable {
    public void run() { System.out.println("Task running"); }
}
new Thread(new MyTask()).start();

// Way 3: Lambda (Java 8+)
new Thread(() -> System.out.println("Lambda thread")).start();

// Way 4: Callable (returns result)
Callable<Integer> task = () -> { return 42; };
ExecutorService executor = Executors.newSingleThreadExecutor();
Future<Integer> future = executor.submit(task);
int result = future.get();  // Blocks until done

3. What is synchronization? Why is it needed?

Synchronization prevents multiple threads from accessing shared resources simultaneously (race condition):

Example

// ❌ Race condition class Counter { int count = 0; void increment() { count++; } // Not atomic: read → modify → write } // ✅ Synchronized class Counter { int count = 0; synchronized void increment() { count++; } // Only one thread at a time }

4. synchronized method vs synchronized block?

java example

// Synchronized method — locks entire object (this)
public synchronized void method() { /* ... */ }

// Synchronized block — locks specific object (fine-grained)
public void method() {
    // Non-synchronized code here...
    synchronized (this) {
        // Only this part is locked
    }
    // Can also lock on specific object:
    synchronized (sharedList) {
        sharedList.add(item);
    }
}

Block is preferred — locks only what's needed, better performance.

5. What is a deadlock? How to prevent it?

Deadlock occurs when two threads hold locks and wait for each other:

java example

// Thread 1: locks A, waits for B
synchronized(lockA) {
    synchronized(lockB) { /* work */ }
}
// Thread 2: locks B, waits for A
synchronized(lockB) {
    synchronized(lockA) { /* work */ }
}

Prevention:

Lock ordering — always acquire locks in same order
Lock timeout — use tryLock() with timeout
Avoid nested locks
Use concurrent utilities instead of manual synchronization

6. wait(), notify(), notifyAll() — explain.

These are inter-thread communication methods (called inside synchronized block):

java example

class SharedQueue {
    private Queue<Integer> queue = new LinkedList<>();
    private int capacity = 5;

    public synchronized void produce(int item) throws InterruptedException {
        while (queue.size() == capacity) {
            wait();  // Release lock and wait
        }
        queue.add(item);
        System.out.println("Produced: " + item);
        notifyAll();  // Wake up waiting consumers
    }

    public synchronized int consume() throws InterruptedException {
        while (queue.isEmpty()) {
            wait();  // Release lock and wait
        }
        int item = queue.poll();
        System.out.println("Consumed: " + item);
        notifyAll();  // Wake up waiting producers
        return item;
    }
}

7. sleep() vs wait()

Feature	sleep()	wait()
Class	Thread	Object
Lock	Does NOT release	Releases lock
Wake up	After time expires	notify()/notifyAll()
Called in	Anywhere	synchronized block only
Purpose	Pause execution	Inter-thread communication

8. What is volatile keyword?

Ensures visibility — all threads see the latest value:

java example

private volatile boolean running = true;

// Thread 1
public void run() {
    while (running) { /* work */ }  // Always reads from main memory
}

// Thread 2
public void stop() {
    running = false;  // Immediately visible to Thread 1
}

volatile provides visibility, NOT atomicity. For atomic operations, use AtomicInteger.

9. What is the Executor Framework?

java example

// Fixed thread pool
ExecutorService executor = Executors.newFixedThreadPool(5);

// Submit tasks
executor.submit(() -> System.out.println("Task 1"));
executor.submit(() -> System.out.println("Task 2"));

// Submit with result
Future<String> future = executor.submit(() -> "Hello");
String result = future.get();  // Blocks until complete

// Shutdown
executor.shutdown();
executor.awaitTermination(60, TimeUnit.SECONDS);

Executor Type	Description
`newFixedThreadPool(n)`	Fixed number of threads
`newCachedThreadPool()`	Creates threads as needed, reuses
`newSingleThreadExecutor()`	Single thread, tasks queued
`newScheduledThreadPool(n)`	For scheduled/periodic tasks

10. What is CompletableFuture?

java example

CompletableFuture<String> future = CompletableFuture
    .supplyAsync(() -> fetchFromDatabase())      // Async
    .thenApply(data -> transform(data))          // Transform
    .thenCompose(result -> saveAsync(result))    // Chain async
    .exceptionally(ex -> "Error: " + ex.getMessage());

// Combine multiple futures
CompletableFuture<Void> all = CompletableFuture.allOf(future1, future2, future3);
CompletableFuture<Object> any = CompletableFuture.anyOf(future1, future2);

11. What are Atomic classes?

java example

AtomicInteger counter = new AtomicInteger(0);
counter.incrementAndGet();  // Thread-safe increment
counter.compareAndSet(5, 10);  // CAS operation
counter.getAndAdd(5);  // Get current, add 5

Uses CPU-level CAS (Compare-And-Swap) — no locks needed.

12. What is ThreadLocal?

Provides thread-specific storage — each thread has its own copy:

java example

private static ThreadLocal<SimpleDateFormat> dateFormatter = 
    ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyy-MM-dd"));

// Each thread gets its own SimpleDateFormat instance
String date = dateFormatter.get().format(new Date());

13-35. Additional Questions

13. What is a daemon thread? Background thread (GC, signal handler). JVM exits when only daemon threads remain. Set via thread.setDaemon(true).

14. join() method? Makes calling thread wait until target thread finishes.

15. What is thread starvation? A thread never gets CPU time because higher-priority threads dominate.

16. What is livelock? Threads keep responding to each other without making progress (neither blocked nor making progress).

17. ReentrantLock vs synchronized?

ReentrantLock: tryLock(), timeout, fairness, multiple conditions
synchronized: simpler syntax, automatic unlock

18. What is ReadWriteLock? Multiple readers OR one writer. ReentrantReadWriteLock — improves read-heavy scenarios.

19. What is CountDownLatch? Allows threads to wait until a set of operations complete. Count down from N to 0.

20. What is CyclicBarrier? All threads wait at barrier until all arrive. Can be reused (cyclic).

21. What is Semaphore? Controls access to a resource with limited permits. acquire() and release().

22. What is a Fork/Join framework? Divide task into subtasks recursively, execute in parallel, join results. Uses work-stealing algorithm.

23. Thread priority? 1 (MIN) to 10 (MAX), default 5. Hint to scheduler, not guaranteed.

24. Can we restart a dead thread? No. Once TERMINATED, cannot be started again. Create a new thread.

25. What is thread-safe? Code that works correctly with multiple concurrent threads accessing shared state.

26. What is an immutable object? Object whose state cannot change — inherently thread-safe without synchronization.

27. What is a race condition? When program behavior depends on relative timing of threads.

28. What is the happens-before relationship? Guarantees that memory writes by one thread are visible to another.

29. What is false sharing? Multiple threads modify variables on the same cache line, causing unnecessary cache invalidation.

30. What is a Phaser? Flexible synchronization barrier for multiple phases. More advanced than CyclicBarrier/CountDownLatch.

31. BlockingQueue implementations? ArrayBlockingQueue (bounded), LinkedBlockingQueue (optionally bounded), PriorityBlockingQueue, SynchronousQueue.

32. What is a thread dump? Snapshot of all thread states. Get via jstack, kill -3, or Thread.dumpStack().

33. How to handle exceptions in threads? Thread.setUncaughtExceptionHandler() or catch within run().

34. What is StampedLock? Optimistic reading lock. Even faster than ReadWriteLock for read-heavy workloads.

35. Virtual threads (Java 21)? Lightweight threads (Project Loom). Millions of virtual threads possible. Thread.startVirtualThread(() -> task()).

13. What is a Thread Pool? How to use ExecutorService?

Answer: A Thread Pool is a collection of pre-created threads that execute tasks. Instead of creating a new thread for each task, threads are reused from the pool. The ExecutorService interface manages thread pools.

Example

ExecutorService executor = Executors.newFixedThreadPool(5); for (int i = 0; i < 10; i++) { final int taskId = i; executor.submit(() -> { System.out.println("Task " + taskId + " - Thread: " + Thread.currentThread().getName()); }); } executor.shutdown();

Output

Task 0 - Thread: pool-1-thread-1
Task 1 - Thread: pool-1-thread-2
Task 2 - Thread: pool-1-thread-3
Task 3 - Thread: pool-1-thread-4
Task 4 - Thread: pool-1-thread-5
Task 5 - Thread: pool-1-thread-1

14. What does the volatile keyword do?

Answer: A volatile variable's value is always read/written from main memory (not CPU cache). It provides a visibility guarantee — one thread's changes are immediately visible to other threads. But it does not guarantee atomicity.

java example

private volatile boolean running = true;
// Thread 1
public void stop() { running = false; } // Visible to Thread 2 immediately
// Thread 2  
while (running) { /* work */ } // Always reads latest value

15. How do wait(), notify(), notifyAll() work?

Answer: These are Object class methods that must be called inside a synchronized block. wait() puts the thread into a waiting state and releases the lock. notify() wakes up one waiting thread, notifyAll() wakes up all.

16. What is the difference between Callable and Runnable?

Answer:

Feature	Runnable	Callable
Method	run()	call()
Return	void	Generic type
Exception	Cannot throw checked	Can throw checked
Result	No	Future<V>

java example

Callable<Integer> task = () -> {
    Thread.sleep(1000);
    return 42;
};
Future<Integer> future = executor.submit(task);
System.out.println("Result: " + future.get()); // Blocks until done

Output

Result: 42

17. What is the difference between ReentrantLock and synchronized?

Answer: ReentrantLock: tryLock() (non-blocking), lockInterruptibly(), fairness policy, multiple conditions. synchronized: simpler syntax, auto-release, but less flexible. ReentrantLock is better for complex scenarios.

18. What is CountDownLatch?

Answer: Used to make one thread wait for multiple threads to complete. The waiting thread proceeds when the countdown reaches zero. One-time use — cannot be reset.

Example

CountDownLatch latch = new CountDownLatch(3); // 3 worker threads for (int i = 0; i < 3; i++) { new Thread(() -> { // do work latch.countDown(); // Decrement count }).start(); } latch.await(); // Wait until count reaches 0 System.out.println("All tasks completed!");

Output

All tasks completed!

19. CyclicBarrier vs CountDownLatch?

Answer: CountDownLatch: one-time use, one thread waits for others. CyclicBarrier: reusable, multiple threads wait for each other (meet at the barrier), then all proceed together.

20. What is CompletableFuture?

Answer: Introduced in Java 8 — for asynchronous programming. Non-blocking, chainable operations, exception handling. An advanced version of Future that supports callbacks.

java example

CompletableFuture<String> future = CompletableFuture
    .supplyAsync(() -> "Hello")
    .thenApply(s -> s + " World")
    .thenApply(String::toUpperCase);
System.out.println(future.get());

Output

HELLO WORLD

21. How to create a Thread-safe Singleton?

Answer: Use double-checked locking with volatile, enum singleton (best), or static inner class (Bill Pugh) approach.

22. What is a Semaphore?

Answer: Controls access to limited resources. Maintains a count of permits. acquire() takes a permit (or waits), release() returns the permit. Example: DB connection pool limit.

23. What is ThreadLocal?

Answer: Maintains a separate variable copy for each thread. Thread-safe without synchronization. Common use: per-request context (user info, transaction), SimpleDateFormat (not thread-safe).

24. How do Atomic classes (AtomicInteger, etc.) work?

Answer: They use CAS (Compare-And-Swap) operation — lock-free thread safety. incrementAndGet(), compareAndSet() etc. Faster than synchronized for simple counters.

25. What is the Fork/Join Framework?

Answer: Splits large tasks into smaller subtasks and executes them in parallel. Uses work-stealing algorithm. Extend RecursiveTask (returns value) and RecursiveAction (void).

26. What is ReadWriteLock?

Answer: Multiple readers can read simultaneously (shared lock), but a writer takes exclusive access. Provides better performance than synchronized in read-heavy scenarios.

27. What is Thread starvation?

Answer: A low-priority thread never gets to execute because high-priority threads always occupy the CPU. Solution: fair locks, priority adjustment, proper thread pool configuration.

28. What is the Happens-before relationship?

Answer: A concept of the Java Memory Model — guarantees that one action's results will be visible to another action. Examples: synchronized block exit happens-before next entry, volatile write happens-before subsequent read.

29. What is Phaser?

Answer: An advanced version of CountDownLatch + CyclicBarrier. Supports dynamic party registration, multiple phases, and flexible advancement. Useful in complex synchronization scenarios.

30. What is BlockingQueue? Name its types.

Answer: A thread-safe queue where the producer waits when the queue is full, and the consumer waits when the queue is empty. Types: ArrayBlockingQueue (bounded), LinkedBlockingQueue (optionally bounded), PriorityBlockingQueue (priority-based), SynchronousQueue (zero capacity).

Summary

In this chapter, we learned about Multithreading Interview Questions in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

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Java Notes

Multithreading Interview Questions

35+ Java Multithreading interview questions covering threads, synchronization, locks, executor framework, concurrent collections, and thread communication.

1. What is a Thread? How to create threads in Java?

A thread is the smallest unit of execution. Two ways to create:

java example

// Way 1: Extend Thread class
class MyThread extends Thread {
    public void run() { System.out.println("Thread running: " + getName()); }
}
new MyThread().start();

// Way 2: Implement Runnable (preferred)
class MyTask implements Runnable {
    public void run() { System.out.println("Task running"); }
}
new Thread(new MyTask()).start();

// Way 3: Lambda (Java 8+)
new Thread(() -> System.out.println("Lambda thread")).start();

// Way 4: Callable (returns result)
Callable<Integer> task = () -> { return 42; };
ExecutorService executor = Executors.newSingleThreadExecutor();
Future<Integer> future = executor.submit(task);
int result = future.get();  // Blocks until done

3. What is synchronization? Why is it needed?

Synchronization prevents multiple threads from accessing shared resources simultaneously (race condition):

Example

4. synchronized method vs synchronized block?

java example

// Synchronized method — locks entire object (this)
public synchronized void method() { /* ... */ }

// Synchronized block — locks specific object (fine-grained)
public void method() {
    // Non-synchronized code here...
    synchronized (this) {
        // Only this part is locked
    }
    // Can also lock on specific object:
    synchronized (sharedList) {
        sharedList.add(item);
    }
}

Block is preferred — locks only what's needed, better performance.

5. What is a deadlock? How to prevent it?

Deadlock occurs when two threads hold locks and wait for each other:

java example

// Thread 1: locks A, waits for B
synchronized(lockA) {
    synchronized(lockB) { /* work */ }
}
// Thread 2: locks B, waits for A
synchronized(lockB) {
    synchronized(lockA) { /* work */ }
}

Prevention:

Lock ordering — always acquire locks in same order
Lock timeout — use tryLock() with timeout
Avoid nested locks
Use concurrent utilities instead of manual synchronization

6. wait(), notify(), notifyAll() — explain.

These are inter-thread communication methods (called inside synchronized block):

java example

class SharedQueue {
    private Queue<Integer> queue = new LinkedList<>();
    private int capacity = 5;

    public synchronized void produce(int item) throws InterruptedException {
        while (queue.size() == capacity) {
            wait();  // Release lock and wait
        }
        queue.add(item);
        System.out.println("Produced: " + item);
        notifyAll();  // Wake up waiting consumers
    }

    public synchronized int consume() throws InterruptedException {
        while (queue.isEmpty()) {
            wait();  // Release lock and wait
        }
        int item = queue.poll();
        System.out.println("Consumed: " + item);
        notifyAll();  // Wake up waiting producers
        return item;
    }
}

7. sleep() vs wait()

Feature	sleep()	wait()
Class	Thread	Object
Lock	Does NOT release	Releases lock
Wake up	After time expires	notify()/notifyAll()
Called in	Anywhere	synchronized block only
Purpose	Pause execution	Inter-thread communication

8. What is volatile keyword?

Ensures visibility — all threads see the latest value:

java example

private volatile boolean running = true;

// Thread 1
public void run() {
    while (running) { /* work */ }  // Always reads from main memory
}

// Thread 2
public void stop() {
    running = false;  // Immediately visible to Thread 1
}

volatile provides visibility, NOT atomicity. For atomic operations, use AtomicInteger.

9. What is the Executor Framework?

java example

// Fixed thread pool
ExecutorService executor = Executors.newFixedThreadPool(5);

// Submit tasks
executor.submit(() -> System.out.println("Task 1"));
executor.submit(() -> System.out.println("Task 2"));

// Submit with result
Future<String> future = executor.submit(() -> "Hello");
String result = future.get();  // Blocks until complete

// Shutdown
executor.shutdown();
executor.awaitTermination(60, TimeUnit.SECONDS);

Executor Type	Description
`newFixedThreadPool(n)`	Fixed number of threads
`newCachedThreadPool()`	Creates threads as needed, reuses
`newSingleThreadExecutor()`	Single thread, tasks queued
`newScheduledThreadPool(n)`	For scheduled/periodic tasks

10. What is CompletableFuture?

java example

CompletableFuture<String> future = CompletableFuture
    .supplyAsync(() -> fetchFromDatabase())      // Async
    .thenApply(data -> transform(data))          // Transform
    .thenCompose(result -> saveAsync(result))    // Chain async
    .exceptionally(ex -> "Error: " + ex.getMessage());

// Combine multiple futures
CompletableFuture<Void> all = CompletableFuture.allOf(future1, future2, future3);
CompletableFuture<Object> any = CompletableFuture.anyOf(future1, future2);

11. What are Atomic classes?

java example

AtomicInteger counter = new AtomicInteger(0);
counter.incrementAndGet();  // Thread-safe increment
counter.compareAndSet(5, 10);  // CAS operation
counter.getAndAdd(5);  // Get current, add 5

Uses CPU-level CAS (Compare-And-Swap) — no locks needed.

12. What is ThreadLocal?

Provides thread-specific storage — each thread has its own copy:

java example

private static ThreadLocal<SimpleDateFormat> dateFormatter = 
    ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyy-MM-dd"));

// Each thread gets its own SimpleDateFormat instance
String date = dateFormatter.get().format(new Date());

13-35. Additional Questions

13. What is a daemon thread? Background thread (GC, signal handler). JVM exits when only daemon threads remain. Set via thread.setDaemon(true).

14. join() method? Makes calling thread wait until target thread finishes.

15. What is thread starvation? A thread never gets CPU time because higher-priority threads dominate.

16. What is livelock? Threads keep responding to each other without making progress (neither blocked nor making progress).

17. ReentrantLock vs synchronized?

ReentrantLock: tryLock(), timeout, fairness, multiple conditions
synchronized: simpler syntax, automatic unlock