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Top Java Concurrency Interview Questions and Answers

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Java Concurrency interview questions are the most challenging ones to answer in a technical interview. Concurrency in Java refers to running multiple programs or applications parallelly. This capability boosts program throughput and interaction. Java Concurrency interview questions focus on multithreading and key concepts to assess your subject knowledge.

Read on to learn about the types of Java Concurrency interview questions asked in coding interviews and how to answer them.

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Here's what we'll discuss in the article:

  • Java Concurrency Interview Questions and Answers
  • Sample Java Concurrency Interview Questions for Practice
  • FAQs on Java Concurrency Interview Questions

Java Concurrency Interview Questions and Answers

Here are some frequently asked Java Concurrency interview questions and answers for experienced professionals. Whether you have applied for the position of software engineer, software developer, engineering manager, or tech lead, these Java Concurrency interview questions will help you self-assess your skills and clear any FAANG interview with ease.

Q1. What is Concurrency in Java?

This one is amongst the first basic Java Concurrency interview questions. Concurrency is a significant feature of any object-oriented programming language. It provides us with the quality to perform multiple processes side-by-side. In addition, threading helps to execute complex tasks quickly.

Q2. What is a thread, and how many minimum threads does a Java program have?

Concurrency in Java interview questions goes hand in hand with multithreading. A thread is the most compact component of programmed instructions that a scheduler can execute independently. Threads help to accomplish complex tasks in the background. All the Java programs have at least one thread in Java, known as the main thread.

Q3. What utility classes are there in the concurrent package in Java?

Concurrent packages hold utility classes that are useful in concurrent programming. In Java Concurrency interview questions, the hiring manager can ask about each one of them. This package contains the following concurrent utilities:

  • Executors
  • Queues
  • Timing
  • Synchronizers
  • Concurrent Collections.

Q4. What is Java Concurrency API?

APIs form an important part of Java Concurrency interview questions. The concurrent collection APIs are a set of collections APIs designed and optimized specifically for synchronized multithreaded access. They are grouped under Java.util. Concurrent package. The Concurrency API introduces the concept of an ExecutorService. It allows a higher-level replacement for working with threads directly. Executors can run asynchronous tasks and typically manage a pool of threads. So, you don't have to create new threads manually.

Q5. How to create a thread instance and execute it?

There are two options to create an instance of a thread. The first option is to pass a runnable example to the constructor and then call start(). As runnable is a functional interface, we can give it as a lambda expression:

Thread thread1 = new Thread(() ->

 System.out.println("Hello World from Runnable!"));

thread1.start();

As threads also perform runnable, the second option for creating a thread is to create an anonymous subclass, override its run() method, and then call start():

Thread thread2 = new Thread() {

    @Override

    public void run() {

     System.out.println("Hello World from subclass!");

    }

};

thread2.start();

Q6. What is an exchanger in Java Concurrency?

This is one of the most popular advanced Java Concurrency interview questions asked in a technical interview. Two threads can easily exchange data between themselves through an exchanger. An exchanger ensures a smooth procedure of sharing data between two threads. It provides a point known as a synchronization point, where two threads can swap or pair elements. Whenever two threads call the exchange() method, the exchanger swaps the objects presented by those threads.

Q7. Could one of these threads block if two threads simultaneously call a synchronized method on different object instances? What will happen if the method is static?

When the method is an instance method, the instance tends to function as a monitor for the method. In that case, the two threads simultaneously calling the method on different instances will get other monitors, so none of the threads will get blocked. When the method is static, the class object plays the role of a monitor for both threads. So probably one of them will block and wait for other threads to exit the synchronized method.

Q8. What is a CountDownLatch in Java Concurrency?

Concurrency interview questions in Java make up a major segment in a technical interview. Ensure that you get all the concepts right. Furthermore, learn to draw comparisons between the key concepts. The CountDownLatch acts as a synchronizer that allows one thread to wait for the other threads before starting the processing. As a result, the CountDownLatch plays a significant role in server-side Java applications. 

For example, when the other thread uses one or more events, and we need to use those events in the currently executing thread, we can force that thread to wait by using CountDownLatch until we implement the threads that need to use those events. To excel in Java tech interviews, you must master these Java Concurrency interview questions.

Q9. What do you mean by the blocking method In Java?

Blocking methods perform the assigned tasks without handing over the control to other threads. An appropriate example of the blocking method is the Read() method of theInputStream. It blocks until it receives input data, detects the end of the stream, or throws out an exception.

Q10. What is a ReadWriteLock in Java Concurrency?

ReadWriteLock is one of the vital aspects of interview questions on Concurrency in Java. In Java multithreading applications, the ReadWriteLock plays a significant role. Multiple read and write can occur simultaneously for a shared resource in multithreading applications. A ReadWriteLock is mainly required when two multiple writes occur or 'read and write' co-occur. There will be a chance of writing and reading the wrong value in such a case. Here, the ReadWriteLock enhances the performance by locking either read or write operations.

Q11. What is the best way to create the Executors class?

This is one of the most popular Java Concurrency interview questions asked by hiring managers in a coding interview. There are several methods for the classes of the executor framework in the executors class, including ExecutorService, ScheduledExecutorService, Executor, Callable, etc. 

Directly making use of the ThreadPoolExecutor and ScheduledThreadPoolExecutor is not the best way. Instead, using the static factory methods of the utility class is the best way of getting an executor. The static factory methods of utility class include newCachedThreadPool(), newSingleThreadExecutor(), newFixedThreadPool(int numThreads), and newScheduledThreadPool(int numThreads) method.

Q12. What is the thread's interrupt flag? How can you set it?

The interrupt flag is an internal Thread flag set when the thread is interrupted. To set an interrupt flag, call thread.interrupt() on the thread object. When a thread is currently inside one of the methods that throw InterruptedException (wait, join, sleep ), this method immediately throws InterruptedException. The thread is free to process this Exception as per its logic.

Q13. What are Callable and Future?

In the Concurrency package, the java.util.concurrent. The callable interface is similar to the Runnable interface. However, it can return any object and can throw Exceptions. The callable interface uses generics to define the return type of object. 

The Executors class provides methods to execute callable in a thread pool. Since callable tasks run parallel, you have to wait for the returned object. Callable tasks return java.util.concurrent.Future object. Using Future, you can find out the status of the callable task and get the returned object. It provides the get() method to wait for the callable to finish and then return the result. You must master this question as it is one of the most important Java Concurrency interview questions.

Q14. Can we force run the garbage collection at any time? How can we monitor the GC activities?

GC is one of the most common advanced Java Concurrency interview questions asked in software developer tech interviews. No, we cannot force garbage collection in Java. Still we can request it by calling either Runtime.getRuntime().gc() method or the System.GC() method. However, there is no guarantee that GC will immediately start running after calling these methods.

To monitor GC activities in real-time, we can use tools like VisualVM with its VisualGC plug-in for JConsole to monitor GC activities and the memory status of JVM. For offline monitoring, we can redirect GC output to a log file by using this JVM parameter - -XlogGC=<PATH>

Q15. What are Concurrent Collection Classes?

Hiring managers might ask about concurrent collection classes in Java Concurrency interview questions, and for that, you must have the correct answer to create a strong impression on the recruiter. Concurrent collection classes support complete concurrency of retrievals and adjustable expected concurrency for updates. The major classes are:

  • ConcurrentHashMap
  • CopyOnWriteArrayList 
  • CopyOnWriteArraySet
  • BlockingQueue
  • ConcurrentSkipListMap

Q16. Mention the available implementations of Executorservice in the standard library.

Executor service and its implementations are often a part of Java Concurrency interview questions. The ExecutorService has three standard implementations:

  • ThreadPoolExecutor for executing tasks using a pool of threads. Once a thread finishes executing its task, it goes back into the pool. When all threads are busy, the task has to await its turn.
  • ScheduledThreadPoolExecutor allows to schedule task execution. It can also schedule tasks with fixed-rate/ delay.
  • ForkJoinPool deals with recursive algorithms tasks. It implements the work-stealing algorithm that allows it to use available threads efficiently.

Sample Java Concurrency Interview Questions for Practice

This section has additional Java Concurrency interview questions to help you with the tech interview prep. 

  1. What is the difference between a thread and a process?
  2. What is a Daemon thread? State its use cases.
  3. How can you create a Daemon thread?
  4. What is a scheduler?
  5. What makes Executor different from Executorservice?
  6. What is JMM?
  7. What is the meaning of a synchronized keyword before a block?
  8. State the most common problems one can face in a multithreading environment. 
  9. Explain the conditions of Deadlock, Livelock, and Starvation.
  10. Explain the purpose and the use-cases of the Fork/Join framework

This completes the list of common interview questions on Concurrency in Java. Also, read our Top Advanced Java Interview Questions for your coding interview.

FAQs on Java Concurrency Interview Questions

Q1. What should I prepare for Java Concurrency interview questions? 

For Concurrency interview questions in Java, you must first know the basic concepts of Java. Next, learn about the different methods to implement concurrent programs. You can opt for self-learning or enroll in a Java Programming Course for additional support.

Q2. Are Java Concurrency interview questions asked at Java fresher interviews?

As a fresher, you must know the primary abilities of Java. Concurrency is one of those abilities. Thus, Java Concurrency interview questions are important for freshers and experienced developers.

Q3. How should you prepare for Java Concurrency interview questions?

You should practice the crucial topics and watch mock interviews to learn about the most anticipated Java Concurrency interview questions. You must also pay special attention to multithread problems, ReadWriteLock, and Deadlock interview questions.

Q4. How are Java Concurrency interview questions important?

Concurrency is a very important feature of any OOP’s language. Java Concurrency interview questions form an integral part of many tech interviews. The most advanced Java interviews for varied positions such as front-end developers and back-end developers include Concurrency.

Q5. What are the most advanced topics covered in Java Concurrency interview questions?

Multithreading and Java Concurrency interview questions are hiring managers' favorites. Knowing these topics offer you a competitive edge over others in Java Concurrency interview questions.

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