Wednesday, July 20, 2011

Top 15 Java Multithreading, Concurrency Interview Questions Answers asked in Investment banks

Thread interview questions Java
Multi-threading and concurrency questions are essential part of any Java interview. If you are going for any Java interview on any Investment bank for equities front office position expect lots of muti-threading interview questions on your way. Multi-threading and concurrency is a favorite topics on Investment banking specially on electronic trading development and they grill candidate on many confusing java thread interview questions. They just want to ensure that the guy has solid knowledge of multi-threading and concurrent programming in Java  because most of them are in business of performance. High volume and low latency Electronic trading System which is used for Direct to Market (DMA) trading is usually concurrent in nature. These are my favorite thread interview questions on Java  asked on different on different time. I am not providing answer of these thread interview questions but I will give you hint whenever possible, some time hint is enough to answer. I will update the post further with detailed answers just like I did for 10 Singleton interview questions in Java recently.  With introduction of concurrency package in Java 5 questions on concurrent utility and concurrent collections are on rise as well. ThreadLocal, BlockingQueue, Counting Semaphore and ConcurrentHashMap are popular among those.

 

15 Java Thread Interview Questions and answers

1) You have thread T1, T2 and T3, how will you ensure that thread T2 run after T1 and thread T3 run after T2?
This thread interview questions is mostly asked in first round or phone screening round of interview and purpose of this multi-threading question is to check whether candidate is familiar with concept of "join" method or not. Answer of this multi-threading questions is simple it can be achieved by using join method of Thread class.


2) What is the advantage of new Lock interface over synchronized block in Java? You need to implement a high performance cache which allows multiple reader but single writer to keep the integrity how will you implement it?
The major advantage of lock interfaces on multi-threaded and concurrent programming is they provide two separate lock for reading and writing which enables you to write high performance data structure like ConcurrentHashMap and conditional blocking. This java threads interview question is getting increasingly popular and more and more follow-up questions come based upon answer of interviewee. I would strongly suggest reading Locks before appearing for any java multi-threading interview because now days Its  heavily used to build cache for electronic trading system on client and exchange connectivity space.


3) What are differences between wait and sleep method in java?
Another frequently asked thread interview question in Java mostly appear in phone interview. Only major difference is wait release the lock or monitor while sleep doesn't release any lock or monitor while waiting. Wait is used for inter-thread communication while sleep is used to introduce pause on execution. See my post wait vs sleep in Java for more differences


4) Write code to implement blocking queue in Java?
This is relatively tough java multi-threading interview question which servers many purpose, it checks whether candidate can actually write Java code using thread or not, it sees how good candidate is on understanding concurrent scenarios and you can ask lot of follow-up question based upon his code. If he uses wait() and notify() method to implement blocking queue, Once interviewee successfully writes it  you can ask him to write it again using new java 5 concurrent classes etc.


5) Write code to solve the Produce consumer problem in Java?
Similar to above questions on thread but more classic in nature, some time interviewer ask follow up questions How do you solve producer consumer problem in Java, well it can be solved in multiple way, I have shared one way to solve producer consumer problem using BlockingQueue in Java , so be prepare for surprises. Some time they even ask to implement solution of dining philosopher problem as well.

6) Write a program which will result in deadlock? How will you fix deadlock in Java?
Java thread interview questions and answers This is my favorite java thread interview question because even though deadlock is quite common while writing multi-threaded concurrent program many candidates not able to write deadlock free code and they simply struggle. Just ask them you have n resources and n thread and to complete an operation you require all resources. Here n can be replace with 2 for simplest case and higher number to make question more intimidating. see  How to avoid deadlock in java  for more information on deadlock in Java.


7) What is atomic operation? What are atomic operations in Java?
Simple java thread interview questions, another follow-up is do you need to synchronized an atomic operation? :) You can read more about java synchronization here.


8) What is volatile keyword in Java? How to use it? How is it different from synchronized method in Java?
Thread questions based on volatile keyword in Java has become more popular after changes made on it on Java 5 and Java memory model. It’s good to prepare well about how volatile variables ensures visibility, ordering and consistency in concurrent environment.


9) What is race condition? How will you find and solve race condition?
Another multi-threading question in Java which appear mostly on senior level interviews. Most interviewer grill on recent race condition you have faced and how did you solve it and some time they will write sample code and ask you detect race condition. See my post on Race condition in Java for more information. In my opinion this is one of the best java thread interview question and can really test the candidate's experience on solving race condition or writing code which is free of data race or any other race condition. Best book to get mastery of this topic is "Concurrency practices in Java'".


10) How will you take thread dump in Java? How will you analyze Thread dump?
In UNIX you can use kill -3 and then thread dump will print on log on windows you can use "CTRL+Break". Rather simple and focus thread interview question but can get tricky if he ask how you analyze it. Thread dump can be useful to analyze deadlock situations as well.


11) Why we call start() method which in turns calls run() method, why not we directly call run() method ?
Another classic java multi-threading interview question This was my original doubt when I started programming in thread. Now days mostly asked in phone interview or first round of interview at mid and junior level java interviews. Answer to this question is that, when you call start() method it creates new Thread and execute code declared in run() while directly calling run() method doesn’t create any new thread and execute code on same calling thread. Read my post Difference between start and run method in Thread for more details.


12) How will you awake a blocked thread in java?
This is tricky question on threading, blocking can result on many ways, if thread is blocked on IO then I don't think there is a way to interrupt the thread, let me know if there is any, on the other hand if thread is blocked due to result of calling wait(), sleep() or join() method you can interrupt the thread and it will awake by throwing InterruptedException. See my post How to deal with blocking methods in Java for more information on handling blocked thread.



13) What is difference between CyclicBarriar and CountdownLatch in Java ?
New java thread interview questions mostly to check familiarity with JDK 5 concurrent packages. One difference is that you can reuse CyclicBarrier once barrier is broken but you can not reuse ContdownLatch.


14) What is immutable object? How does it help on writing concurrent application?
Another classic interview questions on multi-threading, not directly related to thread but indirectly helps a lot. This java interview question can become more tricky if ask you to write an immutable class or ask you Why String is immutable in Java as follow-up.


15) What are some common problems you have faced in multi-threading environment? How did you resolve it?
Memory-interference, race conditions, deadlock, live lock and starvation are example of some problems comes in multi-threading and concurrent programming. There is no end of problem if you get it wrong and they will be hard to detect and debug. This is mostly experienced based interview question on java thread instead of fact based.

These were my favorite Java thread interview questions and mostly asked on Investment banks. This list is by no means complete so please contribute some of interesting java thread questions you have faced during interview. Purpose of this article is to collect and share great interview questions on multi-threading concept which not only helps on interview but opens door for learning new threading concept.

Update:
One of Javarevisited reader, Hemant has contributed some more thread interview questions in Java, though he hasn’t provide answer and left that job for me, I will certainly do when time allows, just like I have recently updated 10 Singleton interview question in Java with answers. If you guys know answers of this java concurrency questions than please post as comment:

Here is his comment “Good questions on multi-threading though you may need to prepare more in order to clear any multi-threading interview, you need to be familiar with concept of immutability, thread-safety, race condition and many more. 10 or 15 question is good for quick recap but you at-least need to prepare more than 50 questions on threading and concurrency to perform better on Java interview. You can find some interesting thread question below which is no doubt highly popular –

1)  Difference between green thread and native thread in Java?
2)  Difference between thread and process?
3)  What is context switching in multi-threading?
4)  Difference between deadlock and livelock, deadlock and starvation?
5)  What thread-scheduling algorithm is used in Java?
6)  What is thread-scheduler in Java?
7)  How do you handle un-handled exception in thread?
8)  What is thread-group, why its advised not to use thread-group in Java?
9)  Why Executor framework is better than creating and managing thread by application ?
10) Difference between Executor and Executors in Java?
10) How to find which thread is taking maximum cpu in windows and Linux server?

Apart from practicing these question answers, more important is to understand the concept behind these multi-threading questions simply mugging the answers of these thread interview questions is not going to help because there would be a lot of follow-ups based upon your answer and if you haven't master the particular thread topic it would be difficult to answer them.

Related post:

26 comments :

Anonymous said...

Indeed a great collection of java multi-threading interview questions and I agree with you. wall street firms really need people who are very strong in multi-threading and concurrent programming. you could probably include more interview questions from new Thread classes added on JDK5

Anonymous said...

answers of your thread interview questions are not proper. you should work to make your interview questions and answers more relevant, accurate and better. Though I agree that multi-threading is not an easy thing to achieve and multi-threading interview is real tough.

Anonymous said...

Thanks for another great post! You're very smart, and you're writing really important articles. That being said, you have to get an editor! Your spelling, grammar, and punctuation distract from otherwise amazing content. Writing is a great way to get better at writing, but you can get better even faster with feedback from writers with more skill than yourself. In any case, you rock and have amazing potential.

Javin@ OutOfMemoryError in Java said...

@Anonymous, thanks for your feedback , English not being my first language, it has some characteristic or effect of Hindi in grammar and work flow.I love to make my English flawless and increase reading experience of my readers but more importantly I like to share my experience and until I connect to my readers or my fellow developers I am happy and it motivates me to improve. Once again thanks for your feedback appreciate.

Javin

Javin @ OutOfMemoryError in Perm Gen Space said...

Hi Vineet, its indeed a interview question guide, you can easily find detailed answer for these multi-threading question by doing google, Though I have also provided pointers and main point which some one will look. would be great if you could contribute some good answer and I will include in post.

Anonymous said...

Usage of Singleton Pattern in Java API

java.lang.Runtime#getRuntime() java.awt.Desktop#getDesktop()

Garima said...

I am looking for some more thread questions and answers in Java, your thread questions are good but I need more tough thread interview questions something which is new and not everybody know about those interview questions.Please help

Anonymous said...

Question: what are the benfits we will get when we extend a java.lang.Thread class instead of implimenting Runnable interface?

Arulkumaran.K said...

Good list of questions. I have listed frequently asked multi-threading interview questions and answers at http://java-success.blogspot.com.au/2011/09/java-multi-threading-interview.html that will be very handy as well.

Javin @ spring interview question answer said...

Thanks Arul. you like these thread interview questions. glad to here that.

Javin @ Servelt Interview Questions said...

Thanks Kamal. I have shared many of those in my blog, just use search box on top left. by the way here are few :
10 questions on Singleton Pattern in Java
Why String is immutable in Java

Peter Lawrey said...

1. I would call run() instead of start() ;)

2. I would make it clear there is a difference between ReentrantLock and ReentrantReadWriteLock.

4. While I started programming Java before version 5.0, many people started programming after 2004, so I wouldn't consider the concurrency library that new as there when many people started. BTW It was available as a separate library from 1998

6. My favourite answer to this is to create a Thread in a static block which access the class and does a join. This creates a deadlock with just one new thread and no explicit locking. ;)

8. Another question I get is the difference in volatile between Java and C++

10. You might add, how do you take a thread dump programmatically.

Excellent posts, keep them up.

Javin @ thread safe class in java said...

Hi Peter, Good to see you back here :) difference between volatile in Java and C++ is a good question if you mention C++ in your resume and also taking a thread dump programmatically and with command prompt in case of deadlock is a good skill to have. What is current trend, have you spotted any new trend related to concurrency in Java interviews ?

Anonymous said...

Can you please share some multi-threading questions asked in Google Interviews. I am very interested on what these top companies like Google, Amazon, Facebook or Investment banks like Nomura or Goldman asked in Java, JVM and Threading. Thanks

Jon said...

Is it possible that interviewers do not always know the answers, and are taking advantage of job interviews to get free software consultancy?

Javin @ transient vs volatile said...

@Jon, You bring the very interesting point about Java interviews,Many do and which I guess is not bad because if some one can solve the problem you are facing than it's good signal that guy can be useful.

Anonymous said...

Please try to answer question dont leave reader in dark room

Anonymous said...

Javin, really good questions. Just another query which core java book you recommned to understand concepts.

mayank tripathi said...

@Javin : The main advantage of new Locks over Synchronized block is to handle Deadlock better way. In blocks once deadlock happens you have no way to fix it unless restart the jvm but Lock gives you tools to handle deadlock situations. Other benefits are bonus :)

Rashi said...

Hi Guys, I would like to share one of the Interview Question related to ThreadPool implementation. I was asked in JP Morgan and Chase that , What should you take care, if you have to make your own Thread Pool? I thought about Executors framework of Java 5 but couldn't answer much. I am interested in points, what are the important points one should take care while creating there own thread pool in Java?

Thx
Rashi

Anonymous said...

I was asked to implement thread pool in java as well and they were interested in knowing key things you would consider to design pool

Andrew said...

Why knowledge of multithreading in Java is so important to get a Job? What makes it so desirable and same time tough to get master?

Aravind Datta said...

Javin, I am a Java developer with 10+ years of experience and accidentally bumped into your blog around 15 days ago. Since then, I am stuck with it :-). I tried to read thru so many articles that you wrote and they are extremely useful and extremely well written with a lot of clarity on the fundamental concepts! Its a great job! Keep it up!

I have a couple of questions on threads:
1. Is "a thread of execution" same as "thread object" ?
2. Lets say, there is a java application (in real production scenario) and it can accept concurrent requests.. implies, multiple users can get data from this application while it is running in production.. and I can see the logs that multiple "threads" are running in the log.. like say, Thread-1, Thread-2, Thread-23, Thread-50.. etc.. based on the number of "concurrent" requests it is processing.
My question is, is this concurrency same as "multi threading within an application"? Or is it different? Can a java application handle concurrent requests (from multiple users/sessions) simultaneously by default? How does it work? Please let us know.

Best wishes..

Sekhar babu Karri said...

package com.learning.material;

public class MyThread {

/**
* @param args
*/
public static void main(String[] args) throws InterruptedException {

System.out.println("i am the Main thread...starting......");

final Thread t1 = new Thread(){
public void run(){
System.out.println("i am thread 1 starting......");
try{
Thread.sleep(5000);
System.out.println("I am Thread 1 finished ");
}catch(InterruptedException ie){
System.out.println("Exception:"+ie.getMessage());
}
}
};

final Thread t2 = new Thread(){
public void run(){
try{
t1.join();
System.out.println("I am Thread 2 finished ");
}catch(InterruptedException ie){
System.out.println("Exception:"+ie.getMessage());
}
}
};

Thread t3 = new Thread(){
public void run(){
try{
t2.join();
System.out.println("I am Thread 3 finished ");
}catch(InterruptedException ie){
System.out.println("Exception:"+ie.getMessage());
}
}
};
t1.start();
t2.start();
t3.start();
t3.join();

System.out.println("i am Main thread finished");

}

}

Gopal said...

I think you could do better by adding few more multithreading and concurrency questions from java.util.concurrent package e.g.

1) Difference between Callable and Runnable interface in Java?
2) What is FutureTask in Java? What is benefit of using it?
3) Difference between Thread.isInterrupted() and Thread.interrupt() method?
4) How ConcurrentHashMap achieves scalability?
5) What is Semaphore in Java?
6) Difference between ReentrantLock and synchronized keyword in Java?
7) What is ReadWriteLock in Java?
8) What is busy spinning?
9) Difference between notify() and notifyAll() in Java? When do you use notifyAll() over notify()?
10) Difference between wait, sleep and yield in Java?
11) Difference between Runnable vs Thread in Java?
12) Why wait and notify methods are called from synchronized block or method in Java?
13) What is doubled checked locking of Singleton?
14) Difference between LiveLock and Starvation in Java?
15) What is daemon thread in Java? What is use of daemon threads?
16) Why wait, notify and notifyAll are in Object class instead of Thread?
17) Why wait method is called from loop in Java
18) Difference between wait method and Condition object in Java?
19) Write code for getInstance() method of thread-safe Singleton in Java?
20) What is Java Memory Model?
21) Can we make static method synchronized in Java?
22) What is lock stripping?
23) How many thread is created by Timer class?
24) What is ThreadLocal in Java?
25) What is ThreadLocal memory leak in Java? How to fix that?
26) What is race condition in Java? Have you ever faced race conditions?
27) What is thread-safety? What are few ways to achieve thread-safety in multithreading code?
28) Difference between synchronized method and block in Java? Which one is better?
29) Difference between volatile and transient variable in Java?
30) How to check if a Thread holds a Lock in Java?

cbp said...

Thanks for the list of questions and all the great comments. I wanted to point out an error in #11 though.

When you call the start() method in the Thread class, it does not directly call the run() method. It calls a native code method start0(). This native code will only schedule the thread to be executed with the ThreadScheduler. The JVM will then grab the item from the ThreadScheduler and execute the run() function. It is very similar to a Strategy Pattern from GoF where the JVM knows how to execute a strategy and a Thread class defines what the strategy is that is to be executed.

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