Multithreading is a crucial concept for Java developers to know. When employers interview for a Java developer position, they likely want to know the extent of your knowledge in multithreading. As part of your interview preparation, take time to review common interview questions about multithreading so you can display your experience and abilities to a hiring manager. In this article, we look at some frequently asked multithreading interview questions and provide you with example answers.
What are common Java multithreading interview questions?
These are some common interview questions a hiring manager may ask about your knowledge of Java multithreading with example answers:
1. What is a thread?
The answer to this question should indicate your programming level and your familiarity with multithreading. Give the hiring manager a definition that shows you understand what a thread is and are able to explain the importance of it.
Example: "A thread is a distinct execution path, or a subprocess. Within a process, you can have either a single thread or multiple threads sharing the same process resources."
2. What is the difference between a thread and a process?
By asking this, the interviewer wants to see that you understand fundamental multithreading concepts. Explain the concept clearly as if you're speaking to someone who doesn't have experience in this field.
Example: "A process is a single application or program, whereas a thread is a subprocess within that application or program. Each process has its own address space in memory. Threads share their address space."
3. What are the benefits of multithreading?
Interviewers may ask this question to understand how well you know the advantages of multithreading and to assess your experience level.
Example: "Because each thread runs concurrently, multithreading makes efficient use of the CPU. You can have background processes running while the application receives user input. Also, tasks can execute faster because each thread runs independently."
4. What is a thread pool?
Interviewers may ask you this question to evaluate the performance efficiency of your code. Consider including an advantage to show your understanding.
Example: “A thread pool is a collection of worker threads created at startup that you can assign tasks as needed, then put back in the pool when complete. The main advantage of using a thread pool is having a supply of already-created threads when you need them, which improves application performance.”
5. What states can a thread go through in its lifetime?
An interviewer might ask this question to understand how well you know the operation of threads and related debugging mechanisms. Consider taking a pause to think of the states before answering.
Example: "There are five states a thread can have: new, runnable, running, blocked and terminated."
6. What is a race condition?
Your answer to this question can illustrate your understanding of the dangers of multithreading.
Example: "A race condition occurs when multiple concurrent threads race to be the first to run. If the thread that wins the race was not the one that was supposed to run first, the code may exhibit unexpected behaviour. You can resolve the problem with synchronisation.”
7. What is the difference between synchronous and asynchronous programming?
Interviewers may ask you this question to understand whether you know how to code efficiently.
Example: “Synchronous programming is when you assign a single task to a single thread. Asynchronous programming is when multiple threads share a single task.”
8. What is synchronisation?
Your answer should illustrate your knowledge of ways to mitigate some dangers of multithreading.
Example: "Synchronisation forces threads to run one at a time to prevent a race condition or multiple threads trying to perform the same task."
9. Why might you use a synchronised block?
This question can help you illustrate your understanding of synchronisation methods for managing multiple threads, further showcasing your knowledge of multithreading to an employer.
Example: “A synchronised block allows you to designate a particular portion of a method as synchronised. It allows only a single thread to run until it completes, prioritising that thread above others.”
10. Between synchronised method and synchronised block, which do you prefer?
This question lets you illustrate your knowledge of the differences between synchronised method and synchronised blocks.
Example: "**Synchronised block is preferable because the object does not get locked. Synchronised methods can lead to the object getting locked and can stop the execution of multiple synchronisation blocks in the class."
11. What is context switching?
Your answer to this question should illustrate your knowledge of multithreading at the computational level.
Example: “Context switching allows you to store the current state of a thread, or process, so that you can resume it at a later time. This enables a single CPU to manage multiple threads or processes.”
12. What is a thread scheduler, and how is it related to thread priority?
Usually asked as an extension of the previous question, this question helps interviewers know how well you understand computational mechanisms.
Example: "The thread scheduler allocates CPU time to threads and determines the order in which threads execute."
13. What is time slicing?
As a further extension, your answer to this question can show the interviewer you understand how the thread scheduler works.
Example: "Time slicing is the process used by the thread scheduler to divide CPU time between the available active threads."
Related: 50 Essential Java Interview Programs
What are common interview questions about how you use Java multithreading?
These are some common interview questions an employer may ask about your use of Java multithreading with example answers:
1. Why is thread behaviour unpredictable?
Your answer to this question should illustrate the risks of multithreading when used in different CPUs.
Example: “Because the CPU determines thread scheduling, different CPUs may give priority to different threads. This means two different CPUs may not run your threads in the same order, creating unpredictability in your code execution.”
2. What is the busy spin technique?
Interviewers ask this question to understand how well versed you are in thread manipulation techniques.
Example: "Busy spin is a technique in which you pause a thread by making it run an empty loop for a certain period. Unlike other methods, like wait() or sleep(), a busy spin doesn't give up CPU control and therefore preserves CPU cache."
3. What is thread starvation?
Your awareness of thread starvation can be useful for code debugging.
Example: "Thread starvation is when there is insufficient CPU capacity to execute a thread. This can happen with low-priority threads or threads that are demoted in favour of other threads."
4. Can you start a thread twice?
This question shows you understand thread state and the life cycle of a thread.
Example: “Once you execute a thread, it is considered dead. You cannot restart a dead thread.”
5. What is a deadlock situation?
Deadlocks cause code to stall and malfunction. Employers may want to know if you know how to identify and resolve deadlocks.
Example: "In a deadlock situation, multiple threads may wait on each other to release shared resources in order to run. This can happen when, for example, a single thread has exclusive priority but needs resources from a waiting thread."
6. What is a livelock?
Your answer to this question can illustrate your knowledge of the potential problems that multithreading can incur.
Example: “A livelock is like a deadlock situation where the state of the threads changes without making progress. This can happen when all the threads are in infinite loops.”
7. Explain how threads communicate with each other.
This question allows you to illustrate your knowledge of working with multiple threads and having them communicate with each other.
Example: "There should be effective communication between threads when they use shared resources. Threads use the methods like notify(), wait() and notifyAll() in the object class to communicate the status of resources."
Additional Java multithreading interview questions
To assess the depth of your knowledge in multithreading, you may receive these questions:
- Why are the methods notify(), wait() and notifyAll() in object class?
- How are the methods wait() and sleep() different?
- What is a daemon thread?
- How do you create a daemon thread in Java?
- Why is it important to override the run() method in a thread class?
- How can you achieve thread safety?
- In Java, what does the keyword volatile mean?
- Explain how you would stop the execution of a thread in Java.
- Explain what ThreadLocal is.
- What is a timer class in Java?
What is multithreading in Java?
In Java, multithreading is a feature that allows concurrent execution of multiple parts of a program. This maximises CPU utilisation. Each part that is executed is called a thread, and they act like components within a process.
Multithreading has many benefits and uses in the following real-time scenarios:
- Running background processes like Tomcat webserver
- Performing executions when there is a blocked input or output
- Developing video games in which every interactive element is a thread
- Programming reservation and ticketing systems to allow several users to interact with the system simultaneously
Please note that none of the companies mentioned in this article are affiliated with Indeed.