Java Multithreading Interview Questions and Answers 2026

Multithreading is a favorite topic for testing a developer's understanding of concurrency. Here are commonly asked questions with answers.

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

A process is an independent program with its own memory space, while a thread is a lightweight unit of execution within a process that shares memory with other threads in the same process.

2. What are the ways to create a thread in Java?

By extending the Thread class or implementing the Runnable interface and passing it to a Thread object.

3. What is the difference between start() and run()?

start() creates a new thread and calls run() on it asynchronously; calling run() directly just executes the code on the current thread, without starting a new one.

4. What is synchronization, and why is it needed?

Synchronization controls access to shared resources by multiple threads, preventing race conditions and ensuring data consistency.

synchronized void increment() {
    count++;
}

5. What is a deadlock?

A deadlock occurs when two or more threads are blocked forever, each waiting for a resource held by the other.

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

wait() releases the object's lock and waits for notification, while sleep() pauses the thread without releasing any locks it holds.

7. What is the Executor framework?

A higher-level API (java.util.concurrent) for managing thread pools, allowing tasks to be submitted without manually creating and managing individual threads.

ExecutorService executor = Executors.newFixedThreadPool(4);
executor.submit(() -> System.out.println("Task running"));

8. What is the volatile keyword?

volatile ensures that a variable's value is always read from and written to main memory, making updates visible across threads immediately.

9. What is a race condition?

A race condition occurs when multiple threads access and modify shared data concurrently, leading to unpredictable results depending on execution order.

10. What are some common concurrency utilities in java.util.concurrent?

ConcurrentHashMap, CountDownLatch, Semaphore, CyclicBarrier, and various thread-safe queues, all designed to simplify concurrent programming.