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Top 50 Multithreading Interview Questions and Answers

Knowledge of threads is a crucial concept for a java developer to know, therefore while facing any java interview, one is bound to get questions relating to thread, multithreading and other aspects.

For the interviewers, asking questions about the Java thread helps in understanding the skills and experiences of the interviewees as it is a very difficult field to master and is also one of the sought-after skill sets too.

One has to prepare thoroughly for the interview but to make your job easier, we have prepared the top java thread interview questions that are most frequently asked.

Benefits of using Threads in Java

Before we move into the frequently asked java thread interview questions, let us first understand the benefits of using threads in Java.

Also, you could be quite surprised to find this is one of the most common java interview questions that hiring managers ask but yet fails to get a satisfactory answer.

Therefore in order to answer the question, the interviewees need to summarize the multiple benefits provided by threads.

Threads are used in Java to achieve parallelism as it helps in making the Java application run faster by doing multiple functions at the same time.

Some of the potent benefits of using threads in Java are :

  • To achieve parallel programming. Using threads in Java helps in making a task run parallel to another one. Like for example, GUI applications JavaFX and Swing.
  • Helps in taking full advantage of CPU power. One of the biggest benefits provided by threads is the fact that helps in boosting the throughput of applications by utilizing the full power of the CPU
  • Reduces the response time.
  • By doing fast computations which divide a big problem into smaller chunks, it helps in processing the problems faster, therefore, reducing the working time.
  • Multi-threading helps in serving several clients at the same time.

However, it is important to acknowledge a fact here that multithreading comes with several benefits but it has its fair share of challenges too.

Some of the problems that are generally observed in multithreading are :

  • Increasing the throughput of the application can only reach to a certain extent. If the numbering of the thread crosses a certain threshold then the threads will start competing for CPU and context switching occurs
  • Instances occur when a thread loses all the data and the cache has to begin from the start.
  • Issues like deadlock, livelock, race conditions, starvations and memory inconsistency error occurred
  • Testing a Java program with multiple threads is very difficult too.
  • Predicting the order of execution and synchronizing becomes difficult.

50 Essential Multithreading Interview Questions and Answers

1. How will you define thread in java?

Thread is defined as an independent path of execution. It is classified as a way that helps in taking advantage of multiple CPUs available in the given machine.

When a process is spread between multiple threads, it executes faster as a result there is a marked improvement in CPU heavy tasks.At the language level, Java provides excellent support for multi-threading and this feature is also classified as one of the strongest selling points of Java.

For example, if a CPU-bound task running on a single thread takes 20 seconds to complete, splitting it to run on 10 threads would result in the same job to finish in 20/10 = 2 seconds (considering all threads are utilized equally).

2. Mention some ways in which you can create a thread in Java?

There are two methods to create a thread in Java

Implementing Runnable interface and creating a thread object from it then
Extending the thread class

3. State a method to force start a thread.

This is a tricky yet common java thread interview question, as there are no methods that can force start a thread.

Starting a thread is controlled by the thread controller and there are no APIs exposed by java that control the Java schedule.

4. Can you start any thread twice in Java?

No. A thread cannot be started twice when started once.

5. Differentiate between Process and Thread in Java.

Freshers, take note, cause this is one of the most common Java thread interview questions for freshers.

The thread is considered as a subset of the process. The process can contain up to multiple threads and run on different memory spaces. However, all threads have the same memory space.

6. Why is thread behavior considered unpredictable?

The thread behaviour is considered unpredictable because the execution of the threads depends on a thread scheduler and it is important to acknowledge here that the scheduler has a different implementation on different platforms like Unix, Windows etc.

7. What are the advantages of multi-threading?

Multi-threading is defined as the process of executing more than one thread simultaneously and some of the potential advantages are
Sharing of the same address takes place between the threads
The communication cost between the threads is low
The threads remain light

8. State a way by which you can control multiple threads

You might feel that these questions are basic, but trust us when we say that these are the most frequently asked Java thread interview questions, however, interviewees fumble upon these basic questions

To answer the above question, to control multiple threads, they should be created in the ThreadGroup object.

9. State an instance when the threads do not remain lightweight in Java.

When the threads of the same process begin to execute simultaneously, then it can be stated as an instance of threads not remaining lightweight.

Following some practices while writing concurrent codes helps in the maintenance, debugging and boosting performance.

10. State some of the best practices of multi-threading that one should follow

Listed down below are some of the best practices that an average programmer should follow
Giving meaningful names to the thread. The name should speak of the task that the thread is performing.
Reducing the scope of synchronization and avoiding locking as locking is expensive and context switching is costlier.
Using synchronizers over notify and wait. It becomes very complex to implement control flow using wait and notify. However, using synchronizers like Exchanger, CountDownLatch, Cyclic barrier or Semaphore simplifies coding to a great extent.
Preferring concurrent collection over the synchronized collection.

11. State the function of the synchronized keyword.

By using the synchronized keyword, only one thread can have access to synchronized methods.

In instances where there are multiple threads trying to access the same method, they have to wait for the execution thread then.

Synchronization keyword is applied to either static or non-static methods and also helps in applying a lock on the object to prevent the condition of the race.

12. Differentiate between concurrent and synchronized collection in Java

Both concurrent and synchronized collection provides thread-safe collection for concurrent and multiple thread access, however, the key difference is the fact concurrent is more scalable than synchronized.

Previously, if multiple threads were accessed concurrently, then the scalability of the system becomes hampered but upon the introduction of the concurrent collection, it not only improves scalability but provides thread safety too.

13. State a way in which you can synchronize the constructor of the Java class

Another tricky Java thread interview question is on your way!

Constructors of a Java class cannot be synchronized as it is not possible to see the objects before the process of thread creation is complete.

Also, there is no point in the Java objects being synchronized since it will lock the object being constructed.

14. State the reason for calling wait() and notify() method from synchronized block

Java API has made it mandatory to call wait() and notify() methods from synchronized blocks and if it is not done then the code will throw IllegalMonitorStateException.

It is also done to avoid the race condition.

15. How can data be shared between two threads in Java?

The methods that are generally used to share data between two threads in Java are
sharing objects between the two threads
using concurrent data structures like BlockingQueue

16. What is the function of the busy-spin in multi-threading?

To answer this common questions of java thread multithreading interviews, busy spin is the technique that is employed by concurrent programmers to make a thread wait in certain conditions.

This method does not require abandoning CPU unlike the traditional methods, sleep() and wait(). This method just runs the empty loop.

17. What is the benefit of context switching in multi-threading?

Context switching is defined as the process of storing and restoring the CPU state.

This function helps in restoring the thread execution from the same point one leaves behind.

This is considered one of the most essential features for multitasking operating systems.

18. What are the different states of a thread?

To answer one of the most common Java thread interview questions, you have to summarize the different states of a thread in short
New state: a thread that has been just instantiated
Ready state: when the start method is applied on the new thread
Runnable state: state in which a thread is ready to run
Running: the thread when it has been executed
Blocked state: the state when the thread is waiting for a monitor lock. This state can also occur when the thread performs an I/O operation and moves to the next state.
Waiting: the state in which the thread is waiting for another thread to a specific action
Timed_waiting: the state in which a thread is waiting for another thread to perform
Terminated: the state in which the thread has been exited.

19. What causes a thread to enter the waiting stage?

Some of the ways that can cause a thread to enter the waiting stage are
By using the sleep() method
By clicking on the object’s wait() method
Getting blocking on I/O
Due to unsuccessful attempts to acquire the object’s lock.

20. What happens when one does not override a run method?

To answer this one of the frequently asked java thread interview questions, explain how when we call the start() method, the run() method with newly created thread gets called internally.

Therefore, if one does not override the run() method, then newly created threads won’t be called and no new cycle will start.

21. When should one use Runnable or Thread in Java?

Threads can be implemented by extending the Thread class or by implementing the Runnable interface.

To answer the question, the interviewee needs to explain how it is easier to implement interface than to support multiple class inheritance, therefore, to extend another class, one should definitely implement Runnable.

22. What do you mean by LiveLock?

LiveLock is defined as a state in which all the threads are blocked and cannot execute due to the non-existence of unblocked threads or unavailability of the required resources.

Some of the conditions that can lead to livelock are
when the threads in a program get stuck in an infinite loop
when the threads executed in a program are with zero parameters. In such conditions, one or more objects need to call Object.notifyAll() or Object.notify() to process the threads

23. What is thread starvation?

Another one of the most common Java interview questions, thread starvation is defined as a situation in which a thread does not have sufficient CPU for its execution.

This situation generally arises from the following scenarios
low priority threads getting less CPU as compared to high priority ones which cause the low priority threads to starve away from the CPU required to perform calculations.
thread waiting indefinitely for a lock on objects monitor but notify() keeps repeatedly issuing more threads.

24. What is the importance of thread priority?

Thread priority helps in prioritizing a thread. The threads with higher priority get precedence in execution.

However, the execution is also dependent on thread scheduler implementation which is OS-dependent.

The priority of a thread can also be changed.

25. How will you pause the execution of a thread for a certain period of time?

To pause the execution of a thread for a certain period of time, one can use the sleep() method.

However, it should be acknowledged that the processing of the thread cannot be stopped for a specific time.

As soon as the thread awakes from sleep, its state changes into a runnable state and then according to thread scheduling, it will be executed.

26. What is the Java memory model?

One of the most common java thread interview questions, this question is quite a favourite among the interviewers.

Java memory model is defined as a set of rules and regulations that act as guidelines for java programs to run deterministically across the different operating systems, CPU and multiple memory architecture.

The Java memory model is particularly important in the case of multi-threading. The model helps in providing a guarantee which suggests that changes made in one thread will be visible to others.

For example, the happens-before relationship. This relationship defines several rules which help in anticipating and reasoning the behaviours of the concurrent java programs.

27. How can you differentiate between the wait() and sleep() method in Java multi-threading?

Both wait() and sleep() methods are introduced to perform a pause function in Java applications.

However, they have different functions.
Wait() – used for inter-thread communication. If the waiting condition is true, it relinquishes a lock but when due to action of another thread, the waiting condition becomes false, then it waits for notification. The method is defined in the Object class.

Sleep() – used for relinquishing CPU or for stopping the execution of a thread for a specified time period. Using the sleep method does not release the lock held by the current thread. The method is defined in the Thread class.

28. Differentiate between notifyAll() and notify() in Java

Notify() method: does not provide the option to choose a particular thread. This method is only useful when a single thread is working.

notifyAll() method: helps in sending notifications to all threads. The method allows multiple threads to compete for locks and also ensure that at least one thread can proceed further.

29. What is the key difference between yield and sleep methods?

Yield() method- the task which is involved returns to the ready state
Sleep() method- the task involved returns to the waiting state.

30. Differentiate between CountDownLatch and CyclicBarrier in Java

The key difference that differentiates CyclicBarrier from CountDownLatch is the fact that one cannot use CountDownLatch once it reaches zero but CyclicBarrier can be used even when the barrier is broken.

31. Differentiate between Interrupted() and isInterrupted() method in Java

The key difference that helps in differentiating the methods is the fact that the Interrupted() method clears the interrupted status whereas the other one does not.

32. What is the advantage of the fork-join framework in Java?

Introduced in JDK7, the fork-join framework is a powerful tool used by developers to take advantage of multiple processors of the modern-day servers.

The framework is designed for work that can be broken into smaller pieces. The goal of the framework is to basically enhance the performance of the application by using all the available processing power.

Since the fork-join framework uses the work-stealing algorithm, worker threads that have run out of things can steal work from other threads that are still busy.

33. Define a transient variable

A transient variable can be defined as a variable that cannot be serialized during serialization. During serialization, the transient variable gets initialized to its default value.

34. Define a volatile keyword

A volatile keyword is defined as a qualifier that gets applied to a variable when it is declared. To a compiler, the volatile keyword helps in telling that the value of the variable can change at any time, without any action taken by code.

35. Mention some functions of volatile in Java?

Threads can hold values of variables in the local memory. Therefore, if the variable is marked volatile, it will read from the main memory every time it is used.
The value gets stored in the main memory.

36. Define Java Shutdown Hook

Java shutdown hook is used to clean all the resources when the Java Virtual Machine (JVM) shuts down.

The functions that are usually done are sending alerts, closing log files and some other functions.

However, it should be kept in mind that the shutdown hook needs to execute code before the JVM shuts down.

37. Define a ThreadPool

Definitions are one of the most commonly asked Java thread interview questions, therefore moving forward with your definition list, here is one more.

ThreadPool is defined as a pool of threads that reuses a fixed number of threads to execute a specific task.

38. What will happen if one submits a task even when the thread pool is filled?

Another one of the favourite java thread interview questions, some may wonder that the task may be blocked until everything is cleared. But ThreadPoolExecuter’s submit() method throws RejectedExecutionException if the task cannot be scheduled for execution.

39. Define the race condition in Java

When Java programs get exposed to the concurrent home environment it causes subtle programming bugs which are known as the race condition.

As the name suggests, this condition is caused due to race between multiple threads.

This is one of the hardest bugs to determine since of the random nature of racing between the threads but if this situation occurs then it leads to changes in behaviour codes.

40. What is the purpose of the yield method of thread class?

The yield method is used to request the current thread to relinquish the CPU. It is the static method that guarantees that the current threads relinquish CPU but does not tell which other threads impact the CPU or not.

41. What do you mean by an immutable object? State how it helps in writing a concurrent application?

An immutable object is defined as an object whose state cannot be changed after the construction. These objects help in creating concurrent, reliable and simple applications. To make the object immutable one has to make the Class and its member final before the objects are created.

42. What do you mean by a monitor?

The monitor is defined as a body of code that can be executed by only one thread at a time. If another thread tries to access at that same time, then that thread will get suspended until the current thread releases the monitor.

43. What is the function of the blocking method in Java?

In Java, the blocking method blocks any function until a task is done. In this scenario, blocking refers to any function that the control will not return until a task is over.

For example, the accept() method of ServerSocket blocks a function until the time a client is reconnected.

44. How can you control the stack size of a thread?

Java Xss parameter is used to control the stack size of the thread.

45. What do you mean by thread-safety?

Thread safety is defined as a property of a code or object which guarantees that when executed by multiple threads in any manner, it will behave as expected.

Like for example, a thread-safe counter object would not be missing any count if shared among multiple threads.

Apparently, divide collector classes can be collected into non-thread-safe and thread-safe categories.

46. What do you mean by FutureTask in Java?

FutureTask in Java represents a cancellable asynchronous computation occurring in concurrent Java applications.

It involves a future implementation of methods to start and cancel computation, query to see if computation is complete and also retrieve the computation.

A FutureTask object can also help in wrapping a Runnable and Callable object.

47. State a method to check whether a thread holds a lock or not?

If the current thread holds a monitor lock on a specified object, then by using a method called holdsLock() on java.lang.Thread, it will return true.

48. What do you mean by semaphore in Java?

Semaphore is a new kind of synchronizer used in Java. It is a counting synchronizer. It helps in maintaining a set of permits. It counts the number of permits available and works accordingly.

Semaphores are also used to protect expensive resources that come in a fixed number.

49. What do you mean by the ThreadLocal variable in Java?

ThreadLocal variable is described as a special kind of variable that is available to Java programmers. This variable is available per thread. This helps in achieving thread safety of expensive-to-create-objects.

50. How to stop a thread in Java?

Java does not provide sure shot ways of stopping a thread.

In JDK 1.0, there were some control methods like a resume(), suspend() and stop() used but they were later removed as frequent occurrences of deadlock threat creeped out.

Since then, there has been no safe and consistent method to stop a thread.

Threads stop as soon as the call() or run() method finishes but to stop a thread manually, one has to rely on volatile boolean variables or interrupt the threads to abruptly cancel the task.

Now there you go, our compilation of the frequently asked java thread interview questions to help you ace your interview.

These common java thread interview questions mentioned above help in determining the skills and knowledge of an interview.

However, during the interview process, a hiring manager looks through various features that help them understand the personality of their candidate and sometimes this plays a huge role in your final hiring verdict.

We know preparation is the key for acing any interview and to help you further, we have prepared a short checklist that will act as a guide to help you prepare for the interview.

Quick Tips to Ace Multithreading Java Interview Questions

1. Try to research about the organization you would be having the interview with. 

While you have applied to a company, you must have already gathered information about the company but before the interview, learning more about the company. It will make the hiring managers understand that you are invested in getting into the company. 

A quick look on the company website or a check on their LinkedIn profile can do wonders. 

2. Prepare for all the possible questions that you are likely to get being asked. 

Hopefully, with our article on java thread interview questions, you can get an idea on what are the typical questions that are generally asked. 

3. Apart from the knowledge-based questions that are asked during an interview, questions like what do interviewees see themselves in 5-6 years are asked too. 

Most of them try to offer a vague answer which often leaves hiring managers unsatisfied. 

Therefore, to answer the question correctly, try to gather knowledge on career progression and the opportunities that you wish to concur in the following years. 

This will help the hiring professionals to understand that you are ambitious and invested for the long term. 

4. We are sure that you have heard of the adage that first impressions are the last impressions. 

Make sure to arrive at the interview centre prim and proper. Wear appropriate clothes. Follow a good hygiene routine and make sure to figure out the route to take to reach the interview hall. 

Punctuality must be maintained. Do not show up late. Instead, try to reach the place early. 

5. Practice for the interview that you are going to give. 

You can speak in front of the mirror to boost your confidence or you can prepare a mock interview with your family or friends. 

It has been observed that answering mock questions boosts confidence and helps in regulating your preparation procedure. 

6. Do not forget to have a good sleep the night before. 

We can understand that you are likely to feel jittery and nervous for your interview but try to calm yourself as much as possible. Having a good sleep will help you to feel calm and refreshed to tackle any situation that you are likely to face. 

7. And lastly, never lie about yourself, your experience or expertise during an interview. 

Lies will get busted and that will create a very bad impression on your recruiters. 

You are more likely to lose the job even before you get it. 

Now there you go, a short checklist that will help in preparing you for the interview. 

Hopefully, our article on java thread interview questions could be beneficial for you. 

Like as mentioned, prepare yourself thoroughly, keep calm and you will definitely be able to grab the job. 

More Resources :

Advanced Java Interview QuestionsJava Interview Questions & Answers for 5 – 6 Years Experience
Java Technical Architect Interview Questions and AnswersTips to Make a Successful Career in Java
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