Java Multithreading Interview Questions and Answers

Java multithreading interview questions are critical for software engineers. Multithreading helps a program run several parts or threads concurrently. A thread is a light unit of execution that shares memory with other process threads, but runs independently.

Multithreading interview questions in Java are on the advantages. The benefits are efficient CPU utilization, improved responsiveness, handling multiple web services, running background tasks, parallel execution, and context switching speed.

You will be asked to code during the Java multithreading interview questions. This blog presents questions and answers on Java multithreading, thread creation and execution, Java synchronization, thread communication, and other important multithreading interview questions in Java.

The blog will guide candidates, answer Java multithreading interview questions, and help developers who want to enhance their knowledge of Java.

Key Takeaways

Multithreading in Java: Quick Overview

Java multithreading helps several threads, which are small units of a program, to run at the same time in a single process. It is used in parallel processing, background operations, and managing multiple users simultaneously. Threads are created with the Thread class or by implementing the Runnable interface.

Multithreading matters since application performance is enhanced when several threads run simultaneously and not one after another. Responsiveness increases, and CPU resource usage is enhanced. The following figure presents a block diagram of a Java process with three threads. You are asked to draw and explain such diagrams in Java multithreading interview questions.

The heap has all object instances and is accessible by the threads. The method area has the static fields, class metadata, and constants, and shared access makes synchronization essential.

Each thread has a program counter that tracks instructions, a stack to hold variables and call frames for method invocation, and native JNI/ OS calls. Thread states are actively executing, waiting to secure a monitor lock.

Core Java Multithreading Interview Questions

This section presents commonly asked Java multithreading interview questions and answers.

Q1. What is Multithreading in Java?

Multithreading in Java is the functionality to run multiple threads or tasks concurrently in a single program. An analogy is of a cook preparing several dishes at the same time rather than doing them one after another.

Q2. What is a Thread in Java?

A thread in Java is the smallest discrete unit that runs independently in a program. It allows the application to carry out several tasks in the same process.

Q3. What is the Difference Between a Process and a Thread?

A process is an independent program that has a memory space. Tread is an execution in a process and shares memory with other threads.

The following table gives a comparison of processes and threads for key aspects.

Q4. What are the Different States of a Thread in Java?

A thread has different states in its lifecycle, which are part of the Thread.State enumeration. The states are NEW, RUNNABLE, BLOCKED, WAITING, TIMED_WAITING, AND Simple Lifecycle Flow.

A simple life cycle state of a thread is given below.

NEW
│
▼
RUNNABLE ────────────────► TERMINATED
│ ▲
│ │
▼ │
BLOCKED / WAITING / TIMED_WAITING
│
└──────────────► (back to RUNNABLE)

Q5. How Do You Create a Thread in Java?

Threads are created in two ways. The first method involves creating a subclass that extends the Thread class and overrides the run() method. The second method that is often preferred is to define a class to implement the Runnable interface and its run() method.

Q6. What is the Difference Between start() and run()?

start() is to create a new thread and calls run() internally. The class run() performs as a standard method in the current thread. The following table compares the two.

Q7. What are the Advantages of Multithreading in Java?

Multithreading has multiple threads running simultaneously in a program. Benefits are:

• Program performance: Has improved since tasks run in parallel, taking less execution time.
• CPU use: CPU runs continuously and does not wait for instructions since it processes multiple threads
• Responsiveness: Is improved since the applications and UI are smooth while background tasks run.
• Resources: These are shared as threads share memory, allowing faster processes.

Q8. What is the difference between a User Thread and a Daemon Thread?

The user thread is the main thread that runs an application. Daemon thread runs in the background and supports user threads.

The following table compares a user thread and a daemon thread.

Q9. How Do You Create a Daemon Thread in Java?

A daemon thread is created by calling the setDaemon(true) function before starting the thread, but an error appears.

The following code shows the creation of a daemon thread.

class MyDaemon extends Thread {
    public void run() {
        while (true) {
            System.out.println("Daemon thread running...");
        }
    }
}

public class Main {
    public static void main(String[] args) {
        MyDaemon t = new MyDaemon();
        
        t.setDaemon(true);  // Set as daemon thread
        t.start();          // Start the thread
        
        System.out.println("Main thread ends");
    }
}

Q10. What is Thread Priority in Java?

Thread priority is a value from 1 to 10 to show thread importance, and it helps the CPU scheduler to decide which thread to run first. Code for the thread priority is:

Thread t1 = new Thread();
t1.setPriority(Thread.MAX_PRIORITY); // 10

Q11. Can You Start a Thread Twice?

No, you cannot start a thread twice. This is a common trap question in multithreading interview questions in Java.

The reason why you cannot start a thread twice in Java is that the start() method is called, it moves from NEW to RUNNABLE to TERMINATED. If you call the start() again after termination on the same thread, a IllegalThreadStateException is thrown.

Q12. What Happens If You Call run() Directly Instead of start()?

If you call run() directly, then a new thread is not created, and it runs as a normal method in the current thread. This is a common trap question in multithreading interview questions in Java.

The reason is start() creates a new thread and then calls run() internally, and multithreading occurs. run() executes in the same thread without concurrency.

The code snippet is:

Thread t = new Thread(() -> {
    System.out.println("Running in thread");
});

t.run();   // Runs in main thread
t.start(); // Runs in new thread.

Thread Creation and Execution in Java

A Java application begins with one main thread that uses the main() method. Additional threads can be created and run from this thread.

Q13. Creating Threads Using the Thread Class

A thread is created by extending the thread class and overriding its run() method. Java interview questions on multithreading are on this topic.

Code example of creating the thread with the thread class is:

// Step 1: Create a class that extends Thread
class MyThread extends Thread {

    // Step 2: Override the run() method
    public void run() {
        // Task that the thread will perform
        for (int i = 1; i <= 5; i++) {
            System.out.println("Child Thread: " + i);
            try {
                Thread.sleep(500); // pause for 500ms
            } catch (InterruptedException e) {
                System.out.println("Thread interrupted");
            }
        }
    }
}

// Main class
public class ThreadExample {
    public static void main(String[] args) {

        // Step 3: Create thread object
        MyThread t1 = new MyThread();

        // Step 4: Start the thread
        t1.start();

        // Main thread work
        for (int i = 1; i <= 5; i++) {
            System.out.println("Main Thread: " + i);
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                System.out.println("Main thread interrupted");
            }
        }
    }
}

// Output:
// Main Thread: 1
// Child Thread: 1
// Main Thread: 2
// Child Thread: 2

Explanation of code for creating the thread with the thread class is given below. Java multithreading interview questions ask you to explain the code.

1. Extend the Thread class: class MyThread extends the thread and allows the class to behave like a thread.
2. Override the run() method: It is the code to run in the new thread. It prints numbers from 1 to 5.
3. Create an object of the thread: MyThread t1 = new MyThread();. This only creates the thread object, the execution is not started.
4. Call start() method t1.start(); This creates a new thread and internally calls run().
5. Main thread execution: The main() method runs in the main thread with the main thread and child thread running concurrently.
6. Thread.sleep() usage: The run execution is paused for 500 milliseconds. It shows concurrent execution clearly.

Q14. Creating Threads Using the Runnable Interface

The Runnable interface is the best way to create threads as it allows better flexibility, and it is also possible to extend another class. Java multithreading interview questions are on this topic.

Code sample to create threads with the Runnable interface is:

// Step 1: Create a class that implements Runnable
class MyRunnable implements Runnable {

    // Step 2: Override the run() method
    public void run() {
        // Task that the thread will perform
        for (int i = 1; i <= 5; i++) {
            System.out.println("Child Thread: " + i);
            try {
                Thread.sleep(500); // pause for 500ms
            } catch (InterruptedException e) {
                System.out.println("Thread interrupted");
            }
        }
    }
}

// Main class
public class RunnableExample {
    public static void main(String[] args) {

        // Step 3: Create Runnable object
        MyRunnable r1 = new MyRunnable();

        // Step 4: Pass Runnable object to Thread
        Thread t1 = new Thread(r1);

        // Step 5: Start the thread
        t1.start();

        // Main thread work
        for (int i = 1; i <= 5; i++) {
            System.out.println("Main Thread: " + i);
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
                System.out.println("Main thread interrupted");
            }
        }
    }
}

// Output (Example)
// Main Thread: 1
// Child Thread: 1
// Main Thread: 2
// Child Thread: 2

Explanation of the steps of the code to create threads with the Runnable interface is:

1. Implement the Runnable interface: The class MyRunnable implements Runnable and defines a task for the thread to execute.
2. Override the run() method: It has the logic in the new thread. In this case, it prints numbers from 1 to 5.
3. Create a Runnable object: The code MyRunnable r1 = new MyRunnable(); defines the task and not the thread.
4. Create a Thread object: Thread t1 = new Thread(r1); The thread is created and given the task (Runnable).
5. Call start() method t1.start(); It starts a new thread and internally calls run().
6. Main thread execution: The main() method runs separately, and both threads execute concurrently.
7. Thread.sleep() usage: It adds a delay to make thread execution visible.

Q15. What is the Difference Between Runnable and Callable?

While Runnable and Callable define tasks to run in a separate thread, they have some differences. Runnable does not return a result and cannot give checked exceptions. Callable returns a result and gives checked exceptions.

The following table compares Runnable and Callable.

Q16. Why is Runnable Preferred Over Extending the Thread Class?

Runnable is preferred over extending the thread class since it is a better design choice. Runnable supports multiple inheritance, provides separation of concerns, leading to cleaner and maintainable code. Runnable code is reusable, and multiple threads can use the runnable object. It fits modern Java and the Executor Framework, and it is flexible. It is frequently asked multithreading interview questions in Java.

Q17. What Happens If We Don’t Override the run() Method?

If you create a thread but do not override the run() method, then nothing is executed by the thread. The reason is that the thread class already has a default run() method, and the default implementation is empty, doing nothing. This is a common trap in Java multithreading interview questions.

Also Read: 100+ Java Algorithms Interview Questions for All Experience Levels

Java Synchronization and Thread Safety Interview Questions

Thread safety is a common topic in Java multithreading interview questions. Thread safety helps a method or class to perform as required when multiple threads are run. If it is not present, then shared data concurrent access can lead to race conditions.

In race conditions, the result depends on whether you are forced to rely on unpredictable thread timing causing data corruption. Multithreading in Java interview questions focus on several aspects of thread safety and synchronization.

Q18. What is Synchronization in Java?

Synchronization mechanisms control access to shared resources in a multithreaded environment. It allows only one thread to access a critical section of code at a time. The process prevents race conditions and resulting data inconsistency.

Code snippet with a synchronized method is given below:

class Counter {
    private int count = 0;

    // synchronized method
    public synchronized void increment() {
        count++;
    }

    public int getCount() {
        return count;
    }
}

class MyThread extends Thread {
    Counter counter;

    MyThread(Counter counter) {
        this.counter = counter;
    }

    public void run() {
        for (int i = 0; i < 1000; i++) {
            counter.increment(); // shared resource
        }
    }
}

public class Main {
    public static void main(String[] args) throws InterruptedException {
        Counter counter = new Counter();

        MyThread t1 = new MyThread(counter);
        MyThread t2 = new MyThread(counter);

        t1.start();
        t2.start();

        t1.join();
        t2.join();

        System.out.println("Final Count: " + counter.getCount());
    }
}

Q19. What is a Race Condition in Java?

A race condition happens when several threads simultaneously access and modify shared data at the same time. The program result relies on unpredictable timing and the order of thread execution.

Code snippet of a race condition without Synchronization

class Counter {
    int count = 0;

    // NOT synchronized
    public void increment() {
        count++;  // not atomic (read → modify → write)
    }
}

class MyThread extends Thread {
    Counter counter;

    MyThread(Counter counter) {
        this.counter = counter;
    }

    public void run() {
        for (int i = 0; i < 1000; i++) {

Q20. What is the Difference Between Synchronized Method and Synchronized Block?

The synchronized method is used to make the whole method thread-safe. A synchronized block is used for better performance and control.

Code for synchronized method:

class Counter {
    int count = 0;

    // NOT synchronized
    public void increment() {
        count++;  // not atomic (read → modify → write)
    }
}

class MyThread extends Thread {
    Counter counter;

    MyThread(Counter counter) {
        this.counter = counter;
    }

    public void run() {
        for (int i = 0; i < 1000; i++) {

The following table compares the synchronized method and synchronized block.

Q21. What is Class-Level Lock vs Object-Level Lock

Object-Level Lock synchronizes blocks or non-static methods. Only one thread can access a synchronized code instance. Class-Level Lock manages static data, allowing a single thread to execute a static synchronized block for all instances of that class.

The following table compares Class-Level Lock and Object-Level Lock.

Q22. What is the Volatile Keyword in Java?

The volatile keyword allows visibility of changes to variables across threads. Each thread obtains updated values from the main memory. The volatile keyword stops variable caching in thread value memory, allowing visibility and stopping atomicity. Java multithreading interview questions are on this topic.

Code snippet for volatile keywords is:

class SharedFlag {
    boolean running = true;
}

public class Main {
    public static void main(String[] args) {
        SharedFlag flag = new SharedFlag();

        Thread t1 = new Thread(() -> {
            while (flag.running) {
                // Infinite loop (may never stop)
            }
            System.out.println("Stopped");
        });

        Thread t2 = new Thread(() -> {
            try { Thread.sleep(1000); } catch (Exception e) {}
            flag.running = false;
            System.out.println("Flag changed to false");
        });

        t1.start();
        t2.start();
    }
}

Q23. What is the Difference Between volatile and synchronized?

While volatile and synchronized are used in Java for multithreading to handle data, they serve different purposes. Volatile gives visibility of variable changes over several threads. Synchronization provides properties of mutual exclusion, visibility, and atomicity.

This is a common topic of multithreading in Java interview questions. The following table compares volatile and synchronized in Java.

Q24. What is the Java Memory Model (JMM)?

Java Memory Model (JMM) explains the process of thread interaction with memory. It specifies the method of variable storage, read, and update between thread-local and main memory.

JMM lays down the shared data rules for visibility, ordering, and consistency. Multithreading interview questions in Java are on JMM.

Q25. What is the happens-before relationship in Java?

The happens-before relationship is from the Java Memory Model. It specified that an action’s result is visible to another and runs in a specific order. It removes outdated values, prevents out-of-order execution, and gives consistent output.

Multithreading in Java interview questions are on this subject. Code snippet for happens-before relationships in Java is:

class SharedData {
    int value = 0;
    volatile boolean flag = false;
}

public class Main {
    public static void main(String[] args) {
        SharedData data = new SharedData();

        Thread writer = new Thread(() -> {
            data.value = 42;     // (1)
            data.flag = true;    // (2) happens-before
        });

        Thread reader = new Thread(() -> {
            if (data.flag) {     // (3)
                System.out.println(data.value); // (4)
            }
        });

        writer.start();
        reader.start();
    }
}

Q26. What is Thread Safety? How Do You Achieve It?

Thread safety ensures that the code or a class behaves as required when it is accessed by multiple threads simultaneously. It stops race conditions and data inconsistency. Java interview questions on multithreading are frequently on thread safety.

The ways to achieve thread safety are by using synchronized, volatile, atomic classes, locks, immutable objects, thread local variables, and concurrent collections.

Q29. What is the Lock Interface? Why is it Better Than a Synchronized Block?

Lock interface is available in java.util.concurrent.locks, and is a flexible and powerful means to control thread synchronization in comparison to synchronized. One of the common Java multithreading interview questions permits explicit unlocking and locking with a common implementation using the ReentrantLock class.

Also Read: Advanced Java Interview Questions and Answers for Coding Interview

Thread Communication and Coordination Interview Questions

Thread communication and coordination is the process for multiple threads to work, share data safely, and coordinate execution order together. Java multithreading interview questions commonly focus on thread communication and coordination.

Q30. What are wait(), notify(), and notifyAll() Methods?

Object classes wait(), notify(), and notifyAll() carry out inter-thread communication. They make threads to coordinate execution in shared resources.

Code snippet for wait() method is:

// wait()
synchronized (obj) {
    obj.wait(); // releases lock and waits
}

// notify()
synchronized (obj) {
    obj.notify(); // wakes up one waiting thread
}

// notifyAll()
synchronized (obj) {
    obj.notifyAll(); // wakes all waiting threads
}

Q31. What is the Difference Between Wait() and Sleep()?

Methods wait() and sleep() help to pause thread execution. However, they have differences in locking and synchronization. This is a common trap question in core Java multithreading interview questions.

The following table compares wait() and sleep().

Q32. Why are wait(), notify(), and notifyAll() Defined in the Object Class?

wait() and sleep() methods pause thread execution, but they are different in locking and synchronization. wait() is used for thread coordination, and releases the lock and waits for notification. sleep() is used for timing delays, and it keeps the lock and waits for the time. This is a common trap question in core Java multithreading interview questions.

Q33. What is the Difference Between notify() and notifyAll()?

A common trap question in core Java multithreading interview questions is notify(), which wakes one thread and is used when only one thread must be run. notifyAll() wakes all threads, and is used when multiple threads have to proceed.

Q34. How Do Threads Communicate with Each Other in Java?

Threads communicate with shared objects and coordination methods such as wait(), notify(), and notifyAll(). They pause a thread and instruct when the work is ready.

Multithreading in Java interview questions are on this topic. Code snippet of how threads communicate with each other in Java is:

class SharedResource {
    private int data;
    private boolean hasData = false;

    public synchronized void produce(int value) throws InterruptedException {
        while (hasData) {
            wait(); // wait until data is consumed
        }
        data = value;
        System.out.println("Produced: " + value);
        hasData = true;
        notify(); // notify consumer
    }

    public synchronized void consume() throws InterruptedException {
        while (!hasData) {
            wait(); // wait until data is produced
        }
        System.out.println("Consumed: " + data);
        hasData = false;
        notify(); // notify producer
    }
}

public class Main {
    public static void main(String[] args) {
        SharedResource resource = new SharedResource();

        Thread producer = new Thread(() -> {
            try {
                for (int i = 1; i <= 5; i++) {
                    resource.produce(i);
                }
            } catch (InterruptedException e) {}
        });

        Thread consumer = new Thread(() -> {
            try {
                for (int i = 1; i <= 5; i++) {
                    resource.consume();
                }
            } catch (InterruptedException e) {}
        });

        producer.start();
        consumer.start();
    }
}

Q35. What is the join() Method in Java?

The join() method pauses the current thread until another thread completes its execution. Execution order and sequence between threads is maintained, and it is implemented for dependency handling.

Multithreading interview questions in Java are on coding with the join() method. A code snippet for the join() method is:

class Example {
    public static void main(String[] args) throws InterruptedException {

        Thread t1 = new Thread(() -> {
            for (int i = 1; i <= 3; i++) {
                System.out.println("Thread t1: " + i);
                try { Thread.sleep(500); } catch (Exception e) {}
            }
        });

        Thread t2 = new Thread(() -> {
            System.out.println("Thread t2 starts after t1 finishes");
        });

        t1.start();
        t1.join(); // main thread waits for t1 to finish

        t2.start();
    }
}

Q36. What is Context Switching in Java?

Context switching process managed by the OS scheduler, where the CPU pauses one thread and switches to another. The execution state is saved and restored. It helps in concurrency and multitasking.

Q37. What is Thread Scheduler and Time Slicing?

Managed by JVM/OS, the thread scheduler selects the thread that accesses CPU time at any moment. Time slicing is a scheduling technique where threads receive a quantum or a time slot to execute before the CPU switches to another thread. Multithreading interview questions in Java are frequently on this topic.

Java Concurrency Utilities and Thread Pool Interview Questions

Java has several sets of utilities for easier multithreading and to avoid low-level thread handling. The Java multithreading interview questions in this section are on Java concurrency utilities and the thread pool.

Q38. What is a Thread Pool in Java?

A thread pool in Java is a collection of pre-created threads that are reused to execute multiple tasks, instead of creating a new thread every time.

An analogy is preparing common gravy and using the same gravy for different dishes, adding the appropriate ingredients to make them taste different.

Q39. What is ExecutorService in Java?

ExecutorService is a high-level interface from java.util.concurrent that separates task submission from thread management. It manages a thread pool and handles task execution. Stability and performance are improved, and it runs asynchronous tasks.

Java multithreading interview questions are on writing the code for ExecutorService. A code example of ExecutorService is:

import java.util.concurrent.*;

public class ExecutorServiceExample {
    public static void main(String[] args) {

        // Create a thread pool with 2 threads
        ExecutorService executor = Executors.newFixedThreadPool(2);

        // Submit tasks
        for (int i = 1; i <= 3; i++) {
            int taskId = i;
            executor.submit(() -> {
                System.out.println(
                    "Executing Task " + taskId +
                    " by " + Thread.currentThread().getName()
                );
            });
        }

        // Shutdown the executor
        executor.shutdown();
    }
}

Q40. What is the Difference Between ConcurrentHashMap and Hashtable?

ConcurrentHashMap is a scalable and fast class used for full synchronization. It has fine-grained locking and is better than Hashtable which is older and slower. Java multithreading interview questions are on comparisons and use cases of the two classes.

The following table compares ConcurrentHashMap and Hashtable.

Q41. What is the Fork/Join Framework in Java?

The Fork/Join Framework belongs to java.util.concurrent, and efficiently uses several CPU cores. It is used for parallel processing and splits a large task into smaller subtasks called forks. Forks are processed concurrently, and then the results are joined.

Java concurrency and multithreading interview questions are on this topic. Analogy is servicing a large truck. A single mechanic takes more time. When multiple mechanics service different areas at the same time, the work is much faster.

Q42. What is BlockingQueue in Java?

BlockingQueue from java.util.concurrent is a thread-safe queue to automatically manage synchronization between threads. It blocks the producer thread when the queue is full, and the consumer thread when the queue is empty. Java multithreading interview questions are on BlockingQueue.

The two main types of BlockingQueue are Bounded BlockingQueue, and Unbounded BlockingQueue. The Bounded BlockingQueue queue with a fixed capacity blocks when it is full. An example is ArrayBlockingQueue. The Unbounded BlockingQueue does not have a fixed limit and rarely blocks on insert. An example is LinkedBlockingQueue.

Q43. What is CountDownLatch in Java?

Frequently asked in Java multithreading interview questions, CountDownLatch available in java.util.concurrent, is a synchronization utility. It permits threads to wait for the sets of operations to be completed.

CountDownLatch has a counter, and threads call countDown() to reduce the count, and waiting threads call await() to wait until the count becomes 0.

Code snippet for CountDownLatch is:

import java.util.concurrent.CountDownLatch;

public class CountDownLatchExample {
    public static void main(String[] args) throws InterruptedException {

        CountDownLatch latch = new CountDownLatch(3);

        Runnable task = () -> {
            System.out.println(
                Thread.currentThread().getName() + 
                " completed work"
            );
            latch.countDown(); // reduce count
        };

        new Thread(task).start();
        new Thread(task).start();
        new Thread(task).start();

        latch.await(); // main thread waits
        System.out.println(
            "All tasks finished. Main thread continues."
        );
    }
}

Q44. What is CyclicBarrier? How is it different from CountDownLatch?

CyclicBarrier in java.util.concurrent is a synchronization utility. It permits a thread group to wait for each other at a common barrier point before continuing. They are released together after they reach the barrier.

CountDownLatch waits for threads to finish while others finish. A table of comparison of CyclicBarrier and CountDownLatch is given below.

Q45. What is Semaphore in Java?

A Semaphore from java.util.concurrent is a synchronization tool to allow only a fixed number of threads to access a resource at the same time. Permits control access to shared resources. acquire() obtains a permit and waits if it is not available, and release() returns a permit.

Java multithreading interview questions ask you to give a real- world example, such as: A hotel has 10 rooms and only 10 guests can stay at a time. When more guests arrive, they have to wait until a room is vacant.

Q46. What is CompletableFuture in Java?

The CompletableFuture class helps in asynchronous programming. It runs tasks in the background, chains multiple tasks, and manages results without blocking. It supports non-blocking execution, task chaining with thenApply and thenAccept, and combines several tasks.

Multithreading interview questions in Java will be on coding CompletableFuture. A sample code for CompletableFuture is:

import java.util.concurrent.CompletableFuture;

public class CompletableFutureExample {
public static void main(String[] args) {

CompletableFuture.supplyAsync(() -> {
return "Hello";
}).thenApply(result -> {
return result + " World";
}).thenAccept(finalResult -> {
System.out.println(finalResult);
});

// Prevent main from exiting immediately (for demo)
try { Thread.sleep(1000); } catch (Exception e) {}
}
}

Also Read: Top Java Concurrency Interview Questions and Answers

Advanced Java Multithreading Interview Questions

Senior developers with 5-8 years’ experience face advanced Java multithreading interview questions. Advanced multithreading interview questions in Java are on starvation, Livelock, AtomicInteger, ReentrantLock, ReadWriteLock, Busy Spinning, and Shutdown Hook.

Q47. What is Deadlock in Java? How Do You Prevent It?

Deadlock happens in Java programs when multiple threads are permanently blocked because a thread waits for a resource held by another thread. The problems occur due to improper lock handling, mutual exclusion, hold and wait, no preemption, and circular wait.

Core Java multithreading interview questions are on coding for deadlock. AN example code for deadlock is:

import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.TimeUnit;

public class TryLockExample {
    private static final ReentrantLock lock1 = new ReentrantLock();
    private static final ReentrantLock lock2 = new ReentrantLock();

    public static void main(String[] args) {

        Thread t1 = new Thread(() -> {
            try {
                if (lock1.tryLock(1, TimeUnit.SECONDS)) {
                    System.out.println("Thread 1 acquired lock1");

                    if (lock2.tryLock(1, TimeUnit.SECONDS)) {
                        System.out.println("Thread 1 acquired lock2");
                        lock2.unlock();
                    }
                    lock1.unlock();
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });

        t1.start();
    }
}

Q48. What is Thread Starvation?

Thread starvation occurs when a thread cannot access CPU time or resources for a long period, since other threads keep using the resources. The thread is not deadlocked or blocked, but ignored.

Multithreading in Java interview questions ask you to explain thread starvation with an analogy. An example is: During a banquet, the table of food is surrounded by VIPs or high-priority guests who keep on eating. Other guests of low-priority threads never reach the table.

Q49. What is Livelock in Java?

Livelock starts when multiple threads are not blocked, but they continue to respond to each other and change state, without any progress. The threads are active but stuck in a loop.

An analogy is: Two people are at the elevator door, each moving to the same side to let the other pass, left-right continues, but nothing happens.

Q50. What is ThreadLocal in Java?

The ThreadLocal class gives thread-local variables, where each thread gets an independent copy of a variable. Even when multiple threads use the same ThreadLocal object, the value is not shared.

It is used to avoid synchronization and stop data interference between threads. It is used for per-thread data such as user sessions, database connections, and date formatting. Code snippet for ThreadLocal is:

public class ThreadLocalExample {

    private static ThreadLocal<Integer> threadLocal =
        ThreadLocal.withInitial(() -> 0);

    public static void main(String[] args) {

        Runnable task = () -> {
            // Each thread sets its own value
            threadLocal.set((int) (Math.random() * 100));

            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            // Each thread gets its own value
            System.out.println(
                Thread.currentThread().getName() +
                " value: " + threadLocal.get()
            );
        };

        Thread t1 = new Thread(task);
        Thread t2 = new Thread(task);

        t1.start();
        t2.start();
    }
}

// Output:
// Thread-0 value: 42
// Thread-1 value: 87

Q51. What is AtomicInteger? When Should You Use Atomic Classes?

The AtomicInteger class from the java.util.concurrent.atomic package provides thread-safe operations on an integer without synchronization. It uses thread-safe programming and lock-free, based on CAS – Compare-And-Swap. Several threads update the value without blocking each other. It offers fast, thread-safe integer updates without locks and is used for counters in concurrent applications.

You are asked to write the code in Java concurrency and multithreading interview questions for AtomicInteger use. Sample code snippet is:

import java.util.concurrent.atomic.AtomicInteger;

class Counter {
    AtomicInteger count = new AtomicInteger(0);

    void increment() {
        count.incrementAndGet(); // thread-safe
    }
}

public class AtomicExample {
    public static void main(String[] args) throws InterruptedException {
        Counter counter = new Counter();

        Thread t1 = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                counter.increment();
            }
        });

        Thread t2 = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                counter.increment();
            }
        });

        t1.start();
        t2.start();

        t1.join();
        t2.join();

        System.out.println(
            "Final Count: " + counter.count.get()
        );
    }
}

Q52. What is ReentrantLock? How is it different from synchronized?

ReentrantLock class from java.util.concurrent.locks allows explicit, flexible locking for thread synchronization. A thread with the lock can acquire it again without getting blocked. Synchronized is used for simple locking; advanced features are not needed, and code readability is more important than flexibility.

The following table compares ReentrantLock and synchronized.

Q53. What is ReadWriteLock in Java?

ReadWriteLock allows separate locks for reading and writing. Performance is improved by allowing multiple threads to read simultaneously. It is used in the implementation of ReentrantReadWriteLock.

Multithreading interview questions in Java on ReadWriteLock ask you to give a real-world use case. An example is: An example is a library system boot catalog allowing thousands of users to search and read books. When the admin runs updates, read/ write is not allowed.

Q54. What is Busy Spinning in Java?

Busy spinning happens when a thread continuously checks a loop condition without releasing the CPU, instead of waiting or sleeping. The thread does not block but keeps running and consuming CPU cycles until the condition becomes true.

Q55. What is a Shutdown Hook in Java?

The shutdown hook thread is run by the JVM just before the program terminates. It is used for cleanup tasks such as closing files, releasing resources, saving state, and logging shutdown info. It runs when the program finishes normally, System.exit() is called, and when the JVM is terminated.

Code snippet for Shutdown Hook is:

public class ShutdownHookExample {
    public static void main(String[] args) {

        // Register shutdown hook
        Runtime.getRuntime().addShutdownHook(
            new Thread(() -> {
                System.out.println("Shutdown hook executed!");
                // cleanup code here
            })
        );

        System.out.println("Application is running...");

        // Simulate some work
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Application ending...");
    }
}

Also Read: Top 20+ Java Exception Handling Interview Questions and Answers You Need To Master To Land Your Dream Role in Top Tech Companies

Java 21 Concurrency: Virtual Threads and Modern APIs

Java 21, released in 2023, is a major version that provides many advancements and is currently used in all the latest Java applications. Java multithreading interview questions will be on Java 21 concurrency, virtual threads, and modern APIs.

Q56. What are Virtual Threads in Java?

Virtual threads are created by the JVM, not the OS. They are lightweight, and millions can be created, unlike traditional platforms. Unlike traditional platform threads, you can create millions of them.

Virtual threads are efficient, mounting and unmounting is easier, they use a simplified model, and new APIs are integrated. Traditional threads require expensive memory and OS scheduling; virtual threads are scalable, cheap, and simple. The following diagram compares platform threads vs virtual threads.

Q57. What is the Difference Between Platform Threads and Virtual Threads?

Platform threads are limited in numbers, expensive, and consume extra resources, and they are controlled by the OS. Virtual threads are unlimited, lightweight, scale massively, and they are controlled by the JVM.

Multithreading in Java interview questions ask you to compare them. The following table compares platform and virtual threads.

Q58. When Should You Use Virtual Threads?

Virtual threads are used in I/O-heavy tasks such as database calls, APIs, and file operations. They are used in web servers to manage millions of requests, and in microservices and network communication. They are used for simple blocking code for scalable performance.

Q59. What is Structured Concurrency in Java 21?

Structured concurrency is a feature in Java 21 to handle multiple threads as a single unit of work. Rather than manually track threads, they are grouped in a scope to allow them to start, execute, and finish together.

Multithreading in Java interview questions on structured concurrency ask for code to improve readability, error handling, and cancellation. Code snippet for structured concurrency is:

Future<String> user = scope.fork(() -> fetchUser());
Future<String> order = scope.fork(() -> fetchOrder());

scope.join();           // Wait for all tasks
scope.throwIfFailed();  // Throw if any task failed

String result = user.resultNow() + " " + order.resultNow();
System.out.println(result);

Also Read: Top Java Interview Questions for Software Developers in 2026: What do Interviewers Test?

Java Multithreading Interview Questions for Freshers

Java multithreading interview questions for freshers are on thread creation, lifecycle, basic synchronization, and avoiding deadlocks. Focus is on definitions and simple code snippets.

Q60. What is a thread and multithreading?

A thread is a lightweight unit of execution in a process. Multithreading is running multiple threads concurrently to improve performance and responsiveness.

Q61. What is the difference between process and thread?

A process is an independent program, while a thread is part of a process that shares memory.

Q62. What is multitasking

In multitasking, an operating system runs multiple tasks and processes at the same time by sharing CPU time. It uses time-sharing and context switching, allowing tasks to run simultaneously.

Q63. What is main thread?

The main thread is the first thread created at the start of a Java program that starts to execute the main() method. It is the starting point of any Java application created automatically by the JVM. It can create and start other threads. When the main thread finishes, the program can end.

Q64. How do you find thread names

To check if the thread is live, use the isAlive() method of the Thread class.

If the response is true, then the thread has been started and not yet finished.If the response is false, then the thread is not started OR already completed.

Q65. How to check if a thread is alive

To check if the thread is live, use the isAlive() method of the Thread class.

If the response is true, then the thread has been started and not yet finished

If the response is false, then the thread is not started OR already completed

Conclusion

The blog presented several Java multithreading interview questions and answers with code snippets. Key topics for multithreading in Java interview questions and answers were discussed. Topics covered include core thread concepts and implementation, thread creation and execution, synchronization and locks, API, JVN, and several others.

The Java interview questions on multithreading and answers will help freshers and experienced developers to crack core Java multithreading interview questions. Read and practice coding in a Java IDE, run debugging and testing, understand how the code solves real-world problems, take tests, and mock interviews.

FAQs: Java Multithreading Interview Questions

Q1. Why is multithreading important in Java applications?

Multithreading permits applications to perform multiple tasks in less time, they are responsive, and scale with multi-core CPUs. Improved performance is seen in parallel execution in e-commerce apps, order processing, and payments.

Improved Responsiveness is achieved with non-blocking UI / APIs and ensures UO apps do not freeze or hang during API calls and file downloads.

Q2. What are the most common Java multithreading interview questions?

Common Java multithreading interview questions for experienced candidates are on core thread concepts and implementation, thread creation and execution, and synchronization and locks.

Other Common Java multithreading interview questions for experienced developers are on concurrency problems, thread communication, Java Memory Model, concurrent collections, APIs, and others.

Q3. How should beginners prepare for Java multithreading interviews?

Beginners can prepare for Java multithreading interview questions with the following plan. The study can be spread over 2-3 weeks.

• Step 1: Build Strong Basics
• Step 2: Learn Thread Creation
• Step 3: Master Thread Methods
• Step 4: Understand Synchronization
• Step 5: Learn Inter-Thread Communication
• Step 6: Learn Concurrency Utilities
• Step 7: Understand Problems
• Step 8: Practice Coding Questions
• Step 9: Revise with Interview Q&A
• Step 10: Mock Interviews and Debugging