15 Java Thread Concurrency Interview Questions and Answers

#1. Now there are threads T1, T2 and T3. How do you ensure that the T2 thread executes after T1, and the T3 thread executes after T2?

This thread interview question is usually asked during the first round interview or phone interview. This multi-thread question is designed to test whether the interviewer is familiar with the concept of the join method. The answer is also very simple - you can use the join method of the Thread class to achieve this effect.

2. What are the advantages of the new Lock interface in Java over synchronized blocks? If you are asked to implement a high-performance cache that supports concurrent reads and single writes, how do you ensure data integrity.

The biggest advantage of using the lock interface in multi-threaded and concurrent programming is that it provides two separate locks for reading and writing, allowing you to build high-performance data structures such as ConcurrentHashMap and conditional blocking.

This Java thread interview question is becoming more and more common, and subsequent interview questions are based on the interviewer's answer to this question.

I strongly recommend reading more about locks before any Java multi-threading interview, because locks are frequently used to build caches in client and data interactions in electronic trading systems today.

3. What is the difference between wait and sleep methods in Java?

Let’s take a look at another frequently asked thread interview question. This question often comes up in phone interviews. The main difference between the two is waiting for the lock to be released and the monitor. The sleep method does not release any locks or monitors while waiting. The wait method is mostly used for inter-thread communication, while sleep only pauses during execution. You can read my other article about wait and sleep in Java.

4. How to implement a blocking queue in Java?

This is a relatively difficult Java multi-threading interview question with many inspection points. It examines whether the interviewer has actually written Java multi-threaded code and examines the interviewer's understanding of concurrency scenarios. And you can ask a lot of follow-up questions based on the interviewer's code. If he successfully implements the blocking queue using the wait() and notify() methods, you can let him re-implement it using the Java 5 concurrency class.

5. How to write code to solve the producer-consumer problem in Java?

The answer is similar to the above question about threads. This question is very typical at work, but sometimes interviewers will ask this type of question, such as "How to solve the producer-consumer problem in Java?" Actually, there are many solutions. I have shared the solution using BlockingQueue in Java. Sometimes they'll even ask you to give you a solution to the dining philosophers problem.

6. Write a deadlock code. How do you solve deadlock in Java?

This is my favorite Java multi-threading interview question because even though deadlocks are very common in multi-threaded concurrent programming, many interviewees are still scratching their heads and unable to write deadlock-free code.

Just ask them if there are N resources and N threads to perform an operation, and then request all resources.

N here can be 2 as the simplest case, or it can be a large number to complicate the problem.

7. What is an atomic operation? What are the atomic operations in Java?

This is a simple Java thread interview question. Another immediate question would be: do you need to synchronize atomic operations? You can read this article to learn more about Java synchronization.

8. What is the volatile keyword in Java? How do you use it? How is it different from synchronized methods in Java?

Since the adjustments to the volatile keyword and the Java memory model in Java 5, threading issues related to the volatile keyword have become increasingly common. It is important to understand how volatile variables ensure visibility, ordering, and consistency in a concurrent environment.

9. What is a race condition? How do you detect and resolve race conditions?

This Java multi-threading question usually appears in senior interviews. Most interviewers will ask you about the last race condition you encountered and how you solved it. Sometimes they will also write some simple code to let you discover the race condition. You can check out my article Race Conditions in Java. In my opinion, this is one of the best Java threading interview questions, and it tests the interviewer's experience in solving race conditions, or writing code that is free of data races and race conditions.

10. How do you dump threads in Java? How to analyze it?

In UNIX, you can use kill -3 and the thread dump log will be printed on the screen and can be viewed using CTRL Break. This is just a simpler thread interview question, if you're a bit crafty they'll ask you how to analyze dump logs. Thread dump logs are very useful for analyzing deadlock situations.

11. Since the start() method will call the run() method, why do we call the start() method instead of calling the run() method directly?

This is a basic Java multi-threading interview question. Initially, I was a little confused about multi-threaded programming when I first started. Nowadays I usually meet in a phone interview or one round interview for Java Intermediate level interviews.

The answer to this question is this. When you call the start() method, it creates a new thread and executes the code in the run() method. If you call the run() method directly, no new thread will be created, and the code in the method will be executed in the thread of the current caller.

12. How do you wake up a blocked thread in Java?

This is a tricky question about threads. There are many reasons for blocking, if it is IO blocking, I don't think there is a way to interrupt the thread (please tell me if there is). On the other hand, if the thread is blocked due to a call to the wait(), sleep() or join() methods, you can interrupt the thread and wake it up by throwing an InterruptedException exception.

13. What is the difference between CyclicBarriar and CountdownLatch in Java?

Most of the recent Java thread interview questions test your mastery of the JDK 5 concurrent package. One of the differences between the two is that CyclicBarrier can be reused after the barrier is opened (all threads reach the barrier point). But CountDownLatch doesn't work. If you want to learn more, you can take the course Multithreading and Parallel Computing in Java.

14. What is an immutable class? How does it help with writing concurrent applications?

Although this interview question is not directly related to threads, the indirect impact is also great. If the interviewer then asks you to write an immutable class, or asks you why String in Java is immutable, it will make the interview questions more complicated.

15. What are the most common problems you encounter in a multi-threaded environment? How did you solve it?

Memory interference, race conditions, deadlock, livelock, and thread starvation are relatively representative problems in multi-threading and concurrent programming. These problems are endless and difficult to locate and debug.

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