Are you preparing for a job interview and worried about Java multithreading questions? We can understand why! Many Java developers find multithreading and concurrency topics challenging, especially during interviews. So, we have prepared this guide to cover the top 20+ most commonly asked Java multithreading interview questions, along with clear and concise answers.
No matter what your experience level is, these questions will help you understand key concepts and confidently tackle your next interview.
Let’s boost your chances of landing that dream job!
Java Multithreading Interview Questions for Freshers
Here are some commonly asked interview questions on multithreading Java and their answers.
- What is multithreading in Java?
Multithreading in Java is a feature that allows concurrent execution of two or more parts of a program. Each part is called a thread, and it helps in performing multiple tasks simultaneously, improving the performance of applications.
- What is the difference between a process and a thread?
A process is an independent program in execution, having its own memory space. A thread is a subset of a process and shares the same memory space as other threads within the same process, making it more lightweight.
- What is the “synchronized” keyword used for?
The “synchronized” keyword in Java is used to control the access of multiple threads to a shared resource. It ensures that only one thread can access the synchronized method or block at a time, preventing thread interference and consistency problems.
- What is a deadlock?
A deadlock is a situation where two or more threads are blocked forever, waiting for each other. This usually happens when two threads have a circular dependency on a pair of synchronized resources.
- What are the different states of a thread in Java?
A thread in Java can be in one of the following states:
- New: The thread has been created but has not yet started.
- Runnable: The thread is ready to run but waiting for CPU time.
- Blocked: The thread is waiting for a monitor lock to enter a synchronized block/method.
- Waiting: The thread is waiting indefinitely for another thread to perform a specific action.
- Timed Waiting: The thread is waiting for a specified amount of time.
- Terminated: The thread has finished its execution.
- What is the difference between wait() and sleep() methods in Java?
The wait() method is used for inter-thread communication and releases the lock held by the thread, allowing other threads to acquire the lock. The sleep() method pauses the execution of the current thread for a specified time but does not release any locks.
- What is the purpose of the “volatile” keyword in Java?
You may also come across Java concurrency interview questions like this one during the interview. Here’s how you should answer it.
The “volatile” keyword in Java is used to indicate that a variable will be changed by different threads. It ensures that the variable’s value is always read from and written to the main memory, making changes visible to all threads.
However, it doesn’t guarantee atomicity. For that, synchronized methods or locks are needed.
Java Multithreading Interview Questions for Experienced
Here are some important interview questions for multithreading in Java for experienced candidates.
- What is the Callable interface, and how is it different from Runnable?
The Callable interface, introduced in Java 5, is similar to Runnable but can return a result and throw a checked exception. It has a single method, call(), which returns a value of a generic type.
In contrast, Runnable has a run() method that returns void and cannot throw checked exceptions.
- Explain the concept of a thread pool and its benefits.
A thread pool is a collection of reusable threads that can be used to execute multiple tasks concurrently. The benefits of using a thread pool include,
- Improved performance through reduced overhead of thread creation and termination.
- Better resource management by limiting the number of active threads.
- The ability to manage and control task execution.
- What are Future and FutureTask in Java?
Future represents the result of an asynchronous computation. It provides methods to check if the computation is complete, wait for its completion, and retrieve the result.
FutureTask is an implementation of Future that also implements Runnable, allowing it to be executed by a thread or a thread pool. It can be used to submit a task for execution and obtain a Future to monitor its progress and result.
- What is the ForkJoinPool, and how does it work?
ForkJoinPool is a special type of thread pool designed for parallel processing of tasks that can be broken down into smaller sub-tasks.
It uses a work-stealing algorithm where idle threads can “steal” tasks from busy threads to ensure efficient use of resources. It is particularly useful for divide-and-conquer algorithms.
- How do you handle thread safety in Java collections?
Thread safety in Java collections can be handled using synchronized collections from the Collections class (e.g., Collections.synchronizedList()), concurrent collections from the java.util.concurrent package (e.g., ConcurrentHashMap, CopyOnWriteArrayList), and by using explicit synchronization blocks or locks around critical sections.
- What are the key features of Java 8 that enhance multithreading?
The interviewer may also ask multithreading interview questions Java 8. You can expect this question.
Java 8 introduced several features to enhance multithreading, including lambda expressions and the java.util.concurrent package enhancements, and the CompletableFuture class.
Lambda expressions allow for more concise and expressive code when working with threads. The enhancements in the java.util.concurrent package provide more efficient ways to manage and control concurrent tasks.
The CompletableFuture class enables asynchronous programming and simplifies handling asynchronous results.
- Can you share an experience where you had to resolve a difficult multithreading issue in a project?
This is one of the most commonly-asked Java multithreading interview questions for 10 years experienced candidates.
“Once, in a project, multiple threads were accessing a shared resource simultaneously, causing data inconsistencies. To fix it, I applied synchronization techniques and used concurrent data structures like ConcurrentHashMap. After debugging and testing, I resolved the issue, ensuring the application’s reliability.”
Also Read - Top 20+ HashMap Interview Questions With Answers
Multithreading Scenario-Based Questions
You may also encounter a few scenario-based multithreading questions on Java. Here are some common questions and their answers.
- Suppose you need to perform a task asynchronously in your application. How would you achieve this using Java?
“I would use the Thread class or Runnable interface to create a separate thread for the asynchronous task. Alternatively, I could use the ExecutorService from the java.util.concurrent package for more control over thread management.”
- What would you do if you wanted to wait for a specific event to occur in one thread before proceeding with another thread?
“I would use synchronization tools such as CountDownLatch or CyclicBarrier to coordinate the threads. These tools allow one or more threads to wait until a specific event or condition occurs before proceeding.”
- You have a long-running task that might block other operations. How would you prevent this from affecting the overall performance of the application?
“I would execute the long-running task asynchronously using a separate thread to prevent blocking other operations. This can be achieved using techniques such as thread pooling or the CompletableFuture class in Java 8.”
- You need to perform a set of independent tasks concurrently and then combine their results. How would you approach this problem?
“I would use the ExecutorService framework to execute the independent tasks concurrently and then use Future objects to obtain their results. Once all tasks are completed, I would combine their results as needed.”
- You have a producer-consumer scenario where multiple producer threads are producing data, and multiple consumer threads are consuming it. How would you implement this scenario?
“I would use a thread-safe data structure such as BlockingQueue to store the data produced by the producer threads. The consumer threads would then retrieve and process the data from the queue, ensuring thread safety and proper synchronization.”
Java Multithreading Coding Interview Questions
Here are some important coding Java thread interview questions and their answers.
- Write a Java program to create a simple thread by extending the Thread class.
public class MyThread extends Thread {
public void run() {
System.out.println(“MyThread is running…”);
}
public static void main(String[] args) {
MyThread thread = new MyThread();
thread.start();
}
}
- Write a Java program to create a thread by implementing the Runnable interface.
public class MyRunnable implements Runnable {
public void run() {
System.out.println(“MyRunnable is running…”);
}
public static void main(String[] args) {
Thread thread = new Thread(new MyRunnable());
thread.start();
}
}
- Write a Java program to demonstrate synchronization using the synchronized keyword.
public class SynchronizationExample {
private int count = 0;
public synchronized void increment() {
count++;
}
public static void main(String[] args) throws InterruptedException {
SynchronizationExample obj = new SynchronizationExample();
Thread thread1 = new Thread(() -> {
for (int i = 0; i < 1000; i++) {
obj.increment();
}
});
Thread thread2 = new Thread(() -> {
for (int i = 0; i < 1000; i++) {
obj.increment();
}
});
thread1.start();
thread2.start();
thread1.join();
thread2.join();
System.out.println(“Count: ” + obj.count);
}
}
- Write a Java program to demonstrate how to use the CompletableFuture class for asynchronous programming.
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
public class CompletableFutureExample {
public static void main(String[] args) throws ExecutionException, InterruptedException {
CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> “Hello”);
CompletableFuture<String> completedFuture = future.thenApply(result -> result + ” World”);
System.out.println(completedFuture.get());
}
}
Also Read - Top 25 Exception Handling Questions In Java Interview
Wrapping Up
With these 15+ Java multithreading interview questions and answers, you now have a strong foundation to tackle multithreading-related queries confidently. Looking for exciting opportunities in the IT sector? Visit Hirist to find the latest job openings at top IT companies in India. Simply install the Hirist app on your phone and apply for jobs on the go.