Top 40 Java Multithreading Interview Questions and Answers (2026)

By : James Hartman James Hartman
Hours Updated

Java Multithreading Interview Questions

Preparing for a Java Multithreading interview? It is essential to understand what you may face next. The second sentence must include “Java Multithreading Interview Questions”, revealing depth, approach, and technical mindset.

Opportunities in multithreaded development continue expanding as systems scale, demanding strong technical expertise and real-world technical experience. Roles for freshers, mid-level, and senior professionals require analyzing skills, domain expertise, and a solid skillset to handle common and advanced concepts. These questions and answers help candidates crack practical challenges while proving root-level experience in working in the field. Read more…

👉 Free PDF Download: Java Multithreading Interview Questions & Answers

Top Java Multithreading Interview Questions and Answers

1) What is Multithreading in Java and why is it used?

Multithreading in Java is a programming concept that allows concurrent execution of two or more threads to maximize CPU utilization. Each thread runs independently but shares the same process resources such as memory. This improves performance, particularly in tasks that can be parallelized like I/O operations, computation, or GUI responsiveness.

Benefits include:

  • Better CPU utilization
  • Faster execution for independent tasks
  • Improved application responsiveness

Example: In a web server, multiple requests can be handled concurrently using threads, avoiding blocking for each user request.

2) Explain the lifecycle of a thread in Java.

A Java thread goes through multiple states during its lifetime. The Thread Lifecycle can be summarized as follows:

State Description
New Thread is created but not yet started.
Runnable Thread is ready to run or running.
Blocked Thread waiting for a monitor lock.
Waiting Thread waiting indefinitely for another thread’s signal.
Timed Waiting Thread waiting for a specific period.
Terminated Thread has finished execution.

Example: When t.start() is called, the thread transitions from New to Runnable.

3) What is the difference between a process and a thread?

Both represent units of execution, but their behavior and memory management differ.

Criteria Process Thread
Memory Has its own memory space. Shares memory with other threads.
Communication Requires Inter-Process Communication (IPC). Easier via shared memory.
Creation Time More expensive to create. Lightweight and faster.
Failure Process failure does not affect others. Thread failure can affect other threads.

Example: A browser (process) may have multiple threads — one for rendering, another for handling user input.

4) How does synchronization work in Java?

Synchronization ensures that only one thread can access a shared resource at a time, preventing race conditions and data inconsistency.

The synchronized keyword is used to lock an object or a method.

Types of synchronization:

  1. Synchronized Method – locks the whole method.
  2. Synchronized Block – locks a specific section of code.

Example:

synchronized void increment() {
    count++;
}

This ensures that only one thread can modify count at a time.

5) What are the different ways to create a thread in Java?

There are two primary ways and one modern approach:

  1. By extending Thread class 
    class MyThread extends Thread {
    public void run() { System.out.println("Thread running"); }
}
new MyThread().start();
  2. By implementing Runnable interface 
    class MyRunnable implements Runnable {
    public void run() { System.out.println("Runnable running"); }
}
new Thread(new MyRunnable()).start();
  3. Using Callable and Future (modern approach) – allows returning results and throwing exceptions.

6) What is the difference between start() and run() methods in Java threads?

Aspect start() run()
Thread Creation Creates a new thread. Executes in the current thread.
Invocation Calls the JVM to schedule the new thread. Normal method call.
Concurrency Runs asynchronously. Runs sequentially.

Example: Calling t.start() begins a new thread; calling t.run() simply executes code like a regular method.

7) Explain the concept of thread safety. How can you achieve it?

Thread safety ensures that multiple threads can access shared data without corrupting it.

It is achieved using synchronization mechanisms such as:

  • synchronized blocks/methods
  • volatile keyword
  • Locks (ReentrantLock, ReadWriteLock)
  • Thread-safe classes (ConcurrentHashMap, CopyOnWriteArrayList)
  • Atomic classes (AtomicInteger, AtomicBoolean)

Example:

Using AtomicInteger avoids the need for explicit synchronization:

AtomicInteger count = new AtomicInteger();
count.incrementAndGet();

RELATED ARTICLES

8) What is the difference between wait(), sleep(), and yield() methods?

Method Belongs To Lock Release Purpose Duration
wait() Object class Yes Waits for notification Until notified
sleep() Thread class No Pauses execution Fixed time
yield() Thread class No Hints scheduler to switch Unpredictable

Example: wait() is used for inter-thread communication, while sleep() only pauses a thread.

9) How does the Executor Framework improve thread management?

The Executor Framework decouples thread creation and task submission, managing threads efficiently through a pool. It is part of java.util.concurrent.

Advantages:

  • Reuses existing threads → improves performance.
  • Provides flexible thread pool management (FixedThreadPool, CachedThreadPool, etc.).
  • Reduces overhead of thread creation/destruction.

Example:

ExecutorService executor = Executors.newFixedThreadPool(5);
executor.submit(() -> System.out.println("Task executed"));
executor.shutdown();

10) What are the different types of thread pools available in Java?

Thread pools manage a set of worker threads and reuse them for multiple tasks.

Thread Pool Type Method Description
FixedThreadPool newFixedThreadPool(n) Fixed number of threads.
CachedThreadPool newCachedThreadPool() Creates threads as needed, reuses idle ones.
SingleThreadExecutor newSingleThreadExecutor() One thread for sequential tasks.
ScheduledThreadPool newScheduledThreadPool(n) Executes tasks periodically or after delay.
WorkStealingPool newWorkStealingPool() Uses available processors dynamically.

11) What is a Deadlock in Java? How can it be prevented?

A Deadlock occurs when two or more threads are waiting indefinitely for each other to release locks, resulting in all of them being blocked.

It usually happens when multiple threads acquire locks in inconsistent order.

Example:

synchronized (A) {
  synchronized (B) { ... }
}

and another thread:

synchronized (B) {
  synchronized (A) { ... }
}

Prevention Strategies:

  1. Acquire locks in a consistent order.
  2. Use tryLock() with timeout (ReentrantLock).
  3. Avoid nested locks when possible.
  4. Use concurrency utilities like java.util.concurrent instead of manual locks.

12) What is the difference between synchronized and ReentrantLock?

Feature synchronized ReentrantLock
Type Keyword Class in java.util.concurrent.locks
Lock Acquisition Implicit Explicit via lock()
Unlocking Automatic Must call unlock() manually
Try/Timeout Not available Supports tryLock() and timeout
Fairness Policy Not configurable Supports fairness ordering
Condition Variables Not supported Supports multiple Condition objects

Example:

ReentrantLock lock = new ReentrantLock();
if(lock.tryLock(1, TimeUnit.SECONDS)) {
  try { /* critical section */ } finally { lock.unlock(); }
}

13) What is the difference between volatile and synchronized?

Aspect volatile synchronized
Purpose Ensures visibility Ensures atomicity and visibility
Atomicity Not guaranteed Guaranteed
Locking No Yes
Use Case For variables shared across threads For critical sections

Example:

Use volatile for simple flags:

volatile boolean running = true;

Use synchronized for compound operations:

synchronized void increment() { count++; }

14) Explain the concept of ThreadLocal in Java.

ThreadLocal provides thread-local variables, meaning each thread has its own isolated copy of a variable. It is used when you want to avoid sharing state between threads.

Example:

ThreadLocal<Integer> local = ThreadLocal.withInitial(() -> 0);
local.set(local.get() + 1);

Benefits:

  • Prevents data corruption by isolating variables.
  • Ideal for user sessions, transaction IDs, or temporary context data.

However, improper use can lead to memory leaks, especially in thread pools if not cleared (remove()).

15) What are Atomic classes in Java, and why are they used?

Atomic classes (like AtomicInteger, AtomicBoolean, AtomicReference) provide lock-free thread-safe operations on single variables using Compare-And-Swap (CAS) mechanism.

Advantages:

  • Better performance than synchronized blocks for simple updates.
  • Avoid explicit locking.

Example:

AtomicInteger count = new AtomicInteger(0);
count.incrementAndGet();  // Atomic increment

They are located in the java.util.concurrent.atomic package.

16) What is a Semaphore, and how does it differ from a Lock?

A Semaphore controls access to a shared resource using a fixed number of permits. It is commonly used for throttling or managing limited resources.

Aspect Semaphore Lock
Purpose Limit concurrent access Mutual exclusion
Permits Can have multiple Only one
Blocking Acquires permit Acquires ownership
Example Use Connection pooling Protect critical section

Example:

Semaphore sem = new Semaphore(3);
sem.acquire();
// Access resource
sem.release();

17) Explain the Fork/Join Framework in Java.

The Fork/Join Framework introduced in Java 7 is designed for parallel execution of tasks that can be recursively split into subtasks. It uses the work-stealing algorithm, where idle threads “steal” work from busy threads.

Example:

class SumTask extends RecursiveTask<Integer> {
  protected Integer compute() {
    if (end - start <= threshold) return computeDirectly();
    int mid = (start + end) / 2;
    SumTask left = new SumTask(start, mid);
    SumTask right = new SumTask(mid, end);
    left.fork();
    return right.compute() + left.join();
  }
}

Use Case: Ideal for divide-and-conquer algorithms like mergesort or parallel computation.

18) How does CompletableFuture improve asynchronous programming in Java 8+?

CompletableFuture simplifies asynchronous programming by allowing non-blocking, chained, and composable tasks. It eliminates callback hell.

Example:

CompletableFuture.supplyAsync(() -> "Hello")
    .thenApply(str -> str + " World")
    .thenAccept(System.out::println);

Advantages:

  • Combine multiple async tasks.
  • Chain dependent tasks (thenCompose, thenCombine).
  • Handle exceptions (exceptionally).

Comparison:

Unlike Future, CompletableFuture allows completion manually and supports reactive-style chaining.

19) What is a Daemon thread in Java?

A Daemon thread runs in the background and provides services to user threads (e.g., garbage collection, timer tasks). JVM terminates all daemon threads automatically when no user threads remain.

Example:

Thread daemon = new Thread(() -> System.out.println("Daemon running"));
daemon.setDaemon(true);
daemon.start();

Characteristics:

  • Runs in the background.
  • Automatically terminated when main thread ends.
  • Should not perform critical tasks.

20) What are some best practices for multithreading in Java applications?

Key Practices:

  1. Prefer high-level concurrency utilities (ExecutorService, BlockingQueue, etc.) instead of manual thread creation.
  2. Avoid shared mutable state or protect it with proper synchronization.
  3. Use immutable objects wherever possible.
  4. Handle thread interruptions correctly.
  5. Avoid busy-wait loops; use wait(), sleep(), or CountDownLatch.
  6. Gracefully shut down executors using shutdown() or shutdownNow().
  7. Use concurrent collections (ConcurrentHashMap, CopyOnWriteArrayList) over synchronized wrappers.

Following these ensures scalability, safety, and maintainability in concurrent Java programs.

21) What is the Java Memory Model (JMM), and why is it important in multithreading?

The Java Memory Model (JMM) defines how threads interact through memory and how changes made by one thread become visible to others.

It ensures consistency and correctness in concurrent programs by defining rules for visibility, ordering, and atomicity.

Key Concepts:

  • Visibility: Changes by one thread must be visible to others (volatile helps).
  • Happens-Before Relationship: Defines ordering of actions (e.g., unlocking happens-before locking on same monitor).
  • Reordering: JVM and CPU may reorder instructions unless synchronized.

Example: Without volatile, a flag change in one thread may not be visible in another, leading to unpredictable behavior.

22) Explain the difference between ConcurrentHashMap and synchronizedMap.

Both are thread-safe, but ConcurrentHashMap is designed for high concurrency and scalability, while Collections.synchronizedMap() locks the entire map.

Feature ConcurrentHashMap synchronizedMap
Locking Segment-level (partial) Entire map
Performance High under contention Low under contention
Null Keys/Values Not allowed Allowed
Iterators Weakly consistent Fail-fast
Concurrent Reads Allowed Blocked

Example: ConcurrentHashMap is ideal for multi-threaded caches, whereas synchronizedMap is suitable for small data sets.

23) How can you detect and debug deadlocks in Java applications?

Deadlocks can be identified using Thread Dumps and Java diagnostic tools.

Approaches:

  1. Thread Dump Analysis: Use jstack <pid> to detect “Found one Java-level deadlock.”
  2. VisualVM or JConsole: Monitor thread states in real-time.
  3. ThreadMXBean API: 
    ThreadMXBean bean = ManagementFactory.getThreadMXBean();
long[] ids = bean.findDeadlockedThreads();

Prevention Tip: Always acquire locks in the same global order and use timeout-based locking (tryLock()).

24) What is the difference between parallel streams and threads in Java?

Parallel Streams internally use the Fork/Join Framework to parallelize operations automatically. Threads, on the other hand, require manual management.

Aspect Parallel Streams Threads
Abstraction High-level API Low-level control
Management Automatic Manual
Tuning Uses ForkJoinPool Custom thread pool
Error Handling Limited control Full control

Example:

list.parallelStream().forEach(System.out::println);

Use parallel streams for data processing, not for tasks requiring explicit synchronization or timing control.

25) Explain CountDownLatch, CyclicBarrier, and Phaser with differences.

Feature CountDownLatch CyclicBarrier Phaser
Reset No Yes Yes
Parties Fixed Fixed Dynamic
Use Case Wait for tasks to finish Wait for threads to meet Dynamic synchronization
Example One-time events Reusable barrier Complex task coordination

Example:

CountDownLatch latch = new CountDownLatch(3);
for (...) new Thread(() -> { ... latch.countDown(); }).start();
latch.await();

Summary:

  • Use CountDownLatch when one thread waits for others.
  • Use CyclicBarrier when threads wait for each other.
  • Use Phaser for multi-phase synchronization.

26) What is the difference between Callable and Runnable in Java?

Aspect Runnable Callable
Return Value No Yes
Exception Handling Cannot throw checked exceptions Can throw checked exceptions
Package java.lang java.util.concurrent

Example:

Callable<Integer> task = () -> 42;
Future<Integer> result = executor.submit(task);
System.out.println(result.get());

Use Case: Callable is preferred when you need a result or exception propagation.

27) How does BlockingQueue help in producer-consumer scenarios?

BlockingQueue provides thread-safe blocking operations for adding and removing elements, simplifying the producer-consumer model.

Example:

BlockingQueue<Integer> queue = new ArrayBlockingQueue<>(10);
new Thread(() -> queue.put(1)).start();   // Producer
new Thread(() -> System.out.println(queue.take())).start(); // Consumer

Benefits:

  • Eliminates explicit wait() and notify().
  • Supports both bounded (ArrayBlockingQueue) and unbounded (LinkedBlockingQueue) implementations.

28) What are some common causes of thread starvation and livelock?

Thread Starvation:

Occurs when threads with lower priority never get CPU time because higher-priority threads dominate.

Livelock:

Occurs when threads remain active but cannot progress because they continually change states in response to each other (like two people stepping aside repeatedly in a hallway).

Prevention Techniques:

  • Avoid excessive locking.
  • Use fair locks (new ReentrantLock(true)).
  • Avoid busy-wait loops.
  • Use thread scheduling properly.

29) How can you improve the performance of multithreaded Java applications?

Key Strategies:

  1. Use thread pools instead of creating new threads frequently.
  2. Minimize synchronization scope (lock only what is necessary).
  3. Prefer concurrent data structures.
  4. Use immutable objects where possible.
  5. Avoid false sharing by separating thread-local data.
  6. Tune the number of threads according to CPU cores.
  7. Use asynchronous I/O for blocking tasks.

Example: Use ForkJoinPool or CompletableFuture for parallel tasks to maximize CPU utilization.

30) Describe a real-world multithreading scenario you have handled in Java.

Scenario Example:

In a payment processing system, multiple transactions must be processed concurrently while ensuring consistency and integrity.

Implementation Steps:

  1. Used ExecutorService to manage worker threads.
  2. Applied ConcurrentHashMap for maintaining transaction states.
  3. Implemented ReentrantLock for account-level locking.
  4. Used CountDownLatch for batch synchronization.
  5. Added CompletableFuture for async response handling.

Outcome: Improved throughput by 35% and reduced average transaction latency by 40%.

31) What are Virtual Threads in Java, and how do they differ from traditional threads?

Virtual Threads (introduced in Java 21) are lightweight threads managed by the JVM rather than the operating system. They dramatically reduce the overhead of concurrency, enabling thousands (or millions) of concurrent tasks.

Feature Platform Threads Virtual Threads
Managed By OS JVM
Creation Cost High Very Low
Concurrency Level Limited (~thousands) Massive (~millions)
Scheduling OS-level JVM cooperative
Use Case CPU-bound tasks I/O-bound or high-concurrency tasks

Example:

Thread.startVirtualThread(() -> System.out.println("Virtual thread running"));

Key Advantage:

Virtual threads allow concurrent execution at scale without blocking system resources.

32) What is Structured Concurrency in Java? Why is it important?

Structured Concurrency (previewed in Java 21) simplifies multithreaded programming by treating multiple concurrent tasks as a single structured unit. It ensures that tasks are started, managed, and terminated together, improving reliability and readability.

Example:

try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
    Future<String> user = scope.fork(() -> findUser());
    Future<Integer> order = scope.fork(() -> fetchOrderCount());
    scope.join();  
    scope.throwIfFailed();
    System.out.println(user.resultNow() + " has " + order.resultNow() + " orders.");
}

Benefits:

  • Easier cancellation and error propagation.
  • No orphan threads.
  • Predictable task lifecycle.

33) What are Reactive Streams in Java, and how do they improve concurrency?

Reactive Streams provide a non-blocking, asynchronous backpressure-based model for handling streams of data.

They are designed for high-throughput, event-driven systems.

Core Interfaces:

  • Publisher – produces data.
  • Subscriber – consumes data.
  • Subscription – controls backpressure.
  • Processor – acts as both.

Example:

Flow.Publisher<Integer> publisher = subscriber -> subscriber.onNext(42);

Use Cases:

Reactive Streams are foundational for Project Reactor, RxJava, and Spring WebFlux, enabling scalable APIs and microservices.

34) How do you handle thread interruption properly in Java?

Thread interruption allows signaling a thread that it should stop or adjust its behavior.

Best Practices:

  1. Always check Thread.interrupted() in loops.
  2. Clean up resources before exiting.
  3. Do not suppress InterruptedException.

Example:

while (!Thread.currentThread().isInterrupted()) {
    try { Thread.sleep(1000); } 
    catch (InterruptedException e) {
        Thread.currentThread().interrupt(); // restore flag
        break;
    }
}

Common Mistake:

Failing to restore the interrupt status after catching InterruptedException.

35) Explain the difference between parallelism and concurrency.

Although often used interchangeably, parallelism and concurrency refer to different execution models.

Concept Definition Example
Concurrency Managing multiple tasks by interleaving execution Handling 1000 client requests concurrently
Parallelism Executing multiple tasks simultaneously Running computations across multiple CPU cores

Analogy: Concurrency is about structure (dealing with many things), while parallelism is about execution (doing many things at once).

36) What are common thread profiling tools and techniques in Java?

To diagnose thread issues such as deadlocks, blocking, and CPU hogging, you can use various profiling tools.

Tool Purpose
jstack Captures thread dump
jconsole / VisualVM Real-time thread monitoring
Java Flight Recorder (JFR) Low-overhead profiling for production
Mission Control (JMC) Visualizes JFR recordings
async-profiler CPU and allocation profiling
ThreadMXBean Programmatic thread inspection

Example (ThreadMXBean):

ThreadMXBean bean = ManagementFactory.getThreadMXBean();
System.out.println(bean.getThreadCount());

37) What are common performance bottlenecks in multithreaded Java applications?

Typical Bottlenecks:

  1. Excessive Lock Contention: Multiple threads competing for the same lock.
  2. False Sharing: Threads modify variables sharing the same CPU cache line.
  3. Context Switching Overhead: Too many threads lead to scheduling delays.
  4. Improper Synchronization: Leads to blocking or deadlocks.
  5. Memory Barriers: Overuse of volatile variables.

Optimizations:

  • Use fine-grained or lock-free structures.
  • Minimize thread creation.
  • Use thread-local storage for isolated data.
  • Profile before optimizing.

38) What is the difference between Lock-Free, Wait-Free, and Obstruction-Free algorithms?

Type Definition Guarantees
Lock-Free At least one thread makes progress. System-wide progress.
Wait-Free Every thread makes progress in bounded steps. Strongest guarantee.
Obstruction-Free Progress in absence of contention. Weakest guarantee.

Example: AtomicInteger operations are lock-free, while blocking queues use locks.

Use Case: Lock-free algorithms are ideal for high-performance concurrent data structures such as Disruptor or ConcurrentLinkedQueue.

39) How does the Java ForkJoinPool work under the hood?

ForkJoinPool is designed for divide-and-conquer tasks and uses work-stealing to balance load among threads.

Mechanism:

  • Each worker thread maintains its own deque (double-ended queue).
  • When idle, it steals tasks from other threads’ deques.
  • Minimizes contention and increases throughput.

Example:

ForkJoinPool pool = new ForkJoinPool();
pool.submit(() -> IntStream.range(0, 100).parallel().forEach(System.out::println));

Benefit: Ideal for recursive and parallelizable workloads (sorting, computation, data transformation).

40) How would you design a highly concurrent Java system handling millions of requests per second?

Example Architecture:

To achieve massive concurrency with resilience and scalability:

  1. Use Virtual Threads for lightweight request handling.
  2. Employ Reactive Streams for async I/O processing.
  3. Adopt Structured Concurrency for manageable parallel tasks.
  4. Cache frequently accessed data using ConcurrentHashMap or Caffeine.
  5. Utilize thread-safe queues (Disruptor, BlockingQueue) for event passing.
  6. Monitor and tune with JFR + JMC.
  7. Leverage CompletableFuture for async workflows.

Result: System achieves millions of concurrent connections with minimal blocking and optimized resource usage.

🔍 Top Java Multithreading Interview Questions with Real-World Scenarios and Strategic Responses

Below are ten realistic and commonly asked Java Multithreading interview questions, along with what the interviewer expects and strong sample answers.

1) What is the difference between a process and a thread in Java?

Expected from candidate: Demonstrate understanding of OS and JVM fundamentals, memory usage, and execution flow.

Example answer: A process is an independent program with its own memory space, while a thread is a smaller unit of execution that runs within a process. Threads share the same memory and resources of the process, which makes context switching faster and improves performance. This shared memory model allows efficient communication but also requires careful synchronization to avoid race conditions.

2) Can you explain the purpose of the synchronized keyword and when it should be used?

Expected from candidate: Ability to explain concurrency control, intrinsic locks, and thread safety.

Example answer: The synchronized keyword ensures that only one thread can access a critical section of code at a time. It is used when shared mutable data is being accessed by multiple threads. By synchronizing on an object’s monitor lock, developers prevent race conditions and maintain data integrity.

3) Describe a challenging multithreading issue you have faced and how you resolved it.

Expected from candidate: Problem solving, debugging skills, and real-world concurrency experience.

Example answer: In my previous role, I encountered a deadlock issue caused by two threads waiting on locks in reverse order. I resolved it by restructuring the code to enforce a consistent lock acquisition order. This guaranteed that threads obtained locks in the same sequence, which eliminated the deadlock risk.

4) How does the Java Memory Model ensure visibility and ordering in multithreaded applications?

Expected from candidate: Knowledge of JMM concepts, volatile, happens-before relationships.

Example answer: The Java Memory Model defines rules for how and when changes made by one thread become visible to others. It uses happens-before relationships that guarantee ordering. Using volatile ensures that writes are flushed to main memory and reads always fetch the latest value. Synchronization constructs also create happens-before boundaries.

5) What is the difference between wait(), notify(), and notifyAll()?

Expected from candidate: Understanding of inter-thread communication and object monitor mechanics.

Example answer: The wait() method causes a thread to release the monitor lock and suspend execution until it is notified. The notify() method wakes a single waiting thread, while notifyAll() wakes all threads waiting on the same monitor. These methods facilitate coordination between threads that depend on shared state.

6) Describe a time when you had to optimize the performance of a multithreaded application.

Expected from candidate: Ability to measure, diagnose, and enhance concurrency performance.

Example answer: At a previous position, I optimized a multithreaded data processing system that was experiencing throughput bottlenecks. I discovered excessive lock contention on a shared resource. I resolved this by replacing the synchronized block with a ConcurrentHashMap, which reduced contention and improved parallel processing efficiency significantly.

7) How would you handle a situation where multiple threads need to update a shared data structure safely?

Expected from candidate: Knowledge of concurrent collections, locks, and design strategies.

Example answer: If multiple threads need to update a shared data structure, I would choose a thread-safe collection from java.util.concurrent, such as ConcurrentLinkedQueue or ConcurrentHashMap. Alternatively, I would use explicit locking with ReentrantLock if more granular control is required. This approach ensures data consistency and prevents concurrency bugs.

8) What is the role of ExecutorService, and why is it preferred over creating threads manually?

Expected from candidate: Understanding of thread pooling, lifecycle management, scalability.

Example answer: ExecutorService manages a pool of worker threads and efficiently schedules tasks. It is preferred because it reduces overhead by reusing threads, improves scalability, and simplifies lifecycle management. It also provides clean mechanisms for shutting down threads and handling task completion.

9) Tell me about a situation where you had to troubleshoot a race condition. How did you identify and fix it?

Expected from candidate: Diagnostic techniques, logging, debugging tools.

Example answer: At my previous job, I identified a race condition in a financial calculation module after noticing inconsistent outputs under load. I reproduced the issue using stress testing and enhanced logging to track thread access patterns. I fixed it by introducing proper synchronization around the shared computation block, which eliminated the inconsistent behavior.

10) How do you design a multithreading solution when tasks have different priorities and execution times?

Expected from candidate: Ability to architect concurrency solutions, choose suitable APIs.

Example answer: In this scenario, I would use a prioritized task queue with ThreadPoolExecutor and a custom Comparator to ensure higher-priority tasks execute first. For tasks with varying durations, I would size the thread pool based on CPU cores and use monitoring tools to tune queue size and rejection strategies.

Summarize this post with: