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Interview Questions and Answers for 'Multithreading' - 23 question(s) found - Order By Newest

 Q1. Which keyword is used to provide explicit access of a code block to single thread ?

a. Transient
b. Final
c. Explicit
d. Synchronized
Core Java
Anonymous
Ans. Synchronized

  Sample Code for Synchronized

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Frequently asked to fresh graduates and less experienced developers.
 Q2. Explain multithreading in Java ?Core Java
Anonymous
Ans. 1. Multithreading provides better interaction with the user by distribution of task

2. Threads in Java appear to run concurrently, so it provides simulation for simultaneous activities.The processor runs each thread for a short time and switches among the threads to simulate sim-ultaneous execution (context-switching) and it make appears that each thread has its own processor.By using this feature, users can make it appear as if multiple tasks are occurring simultaneously when, in fact, each is running for only a brief time before the context is switched to the next thread.

3. We can do other things while waiting for slow I/O operations.In the java.iopackage, the class InputStreamhas a method, read(), that blocks until a byte is read from the stream or until an IOExceptionis thrown. The thread that executes this method cannot do anything elsewhile awaiting the arrival of another byte on the stream.

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 Q3. Can constructors be synchronized in Java ?Core Java
Anonymous
Ans. No. Java doesn't allow multi thread access to object constructors so synchronization is not even needed.

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 Q4. What is race condition ?Operating System
Admin
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Ans. A source of possible errors in parallel programming, where one thread can cause an error in another thread by changing some aspect of the state of the program that the second thread is depending on (such as the value of variable).

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 Q5. What is the use of runnable interface if we can always create a new thread using Thread class ?Core Java
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Ans. It's a choice to be made whether to use first approach ( Thread class ) or second approach ( runnable interface ) by the programmer. The second facility has been given for cases where your class is already extending some parent class and hence cannot extend another class ( for Thread ) as Java doesn't allow multiple inheritance.

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 Q6. Why do we need Thread class even in case we execute thread using runnable interface ?Core Java
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Ans. Thread class holds the definition of start method ( This is the method that starts execution of new thread and then calls run method within the scope of new thread ). Interfaces don't hold any definition and so does runnable. So it makes it necessary the usage of Thread class , whatever implementation you choose.

When your class extends the thread class, it carries the definition of start method from parent Thread class onto itself and hence new yourClass.start() helps starting a new thread and then executing run method in that new thread scope.

When you implement runnable interface , you are just making it sure to the JVM that you have implemented the required method ( run() ) which the Thread start method will look for upon executing start method.

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Usually asked only to fresh graduates.
 Q7. Why threads block or enters to waiting state on I/O?Operating System
Anonymous
Ans. Threads enters to waiting state or block on I/O because other threads can execute while the I/O operations are performed.

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 Q8. Write a Program to add 1-100, 101-200, 201-300 in different threads and then sum the numbers ?Core Java2016-11-30 20:51:58

Ans. public class TotalUsingThreads extends Thread{
   
   private static List<TotalUsingThreads> collector = new ArrayList<TotalUsingThreads>();
   
   private int startFrom = 0;
   
   private Integer threadTotal = null;
   
   Test(int startFrom){
      this.startFrom = startFrom;
   }
   
   
   public static void main(String[] args) throws InterruptedException{
      int totalSum = 0;
      
      // Create all the threads and set the count starting point for all of them
      for(int count=0;count<10;count++){
         TotalUsingThreads newThread = new TotalUsingThreads(count*100);
         newThread.start();
         collector.add(newThread);
      }
      
      boolean allCollected = false;
      
      
      // Make sure that all thread totals are collected and all threads have completed their tasks
      while(!allCollected){
         for(int count=0;count<10;count++){
            if(collector.get(count).threadTotal == null){
               Thread.sleep(100);
               break;
            }
         }
         allCollected = true;
      }
      
      // sum totals of all threads
      for(int count=0;count<10;count++){
         totalSum += collector.get(count).threadTotal;
      }
      
      System.out.println(totalSum);
      
      
   }
   
   public void run(){
      threadTotal = 0;
      for(int count=startFrom;count<startFrom+100;count++){
       threadTotal += count;   
      }
   }
}

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 Q9. Name few classes that extend Thread class ?Core Java
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Ans. http://www.buggybread.com/2015/02/java-threads-classes-that-inherit.html

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 Q10. Name few Threads related classes and interfaces ?
Admin
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Ans. http://www.buggybread.com/2015/01/java-threads-classes-and-interfaces.html

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 Q11. What is the initial state of a thread when it is created and started ?

a. Wait
b. Running
c. Ready
d. Sleep
Anonymous
Ans. Ready

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 Q12. What state does a thread enter when it terminates its processing?

a. Wait
b. Ready
c. Dead
d. Running
Anonymous
Ans. Dead

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 Q13. Print natural numbers sequentially with two threads.Core Java

Ans. http://stackoverflow.com/questions/18799591/print-natural-sequence-with-help-of-2-threads1-is-printing-even-and-2nd-is-pri

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 Q14. What is the use of Synchronized block ?Core Java

Ans. The goal of a synchronised block is to achieve mutual exclusion i.e at one time, the segment of the code should be executed by single thread only and hence the lock needs to be retrieved before executing the segment and then released.

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 Q15. Difference between Yielding and Sleeping ?

Ans. When a task invokes its yield method, it returns to the ready state. When a task invokes its sleep method, it returns to the waiting state.

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 Q16. In case of a synchronized method, Does the method only gets locked or the whole object ?Core Java2016-10-18 14:22:40

Ans. Just the method gets locked.

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 Q17. What are the disadvantages of multithreading ?Core Java2016-10-11 08:47:41

Ans. 1. Switching Overheads - Even though multi threading aims at improving performance by reducing the wait time and hence improving overall throughput, there is a cost of switching resources between threads and sometime this cost can surpass the benefits if there isnt much wait for IO or external communication.

2. Debugging is hard with multi threaded code.

3. Deadlock - Execution of multi threaded code many a times lead to deadlock due to shared resources.

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Very frequently asked.
 Q18. What is the use of synchronized keyword ?Core Java2016-11-04 08:15:53

Ans. Synchronize is used to achieve mutual exclusion i.e at one time, the segment of the code, method that has been declared synchronized should be executed by single thread only and hence the lock needs to be retrieved before executing the segment and then released.

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 Q19. Can we start the same thread twice ?Core Java2016-11-29 14:24:56

Ans. No we cannot start the same thread twice. Each thread has a lifecycle.

But Yes, we can run the same code in parallel using different threads.


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 Q20. Can you give real time examples of thread or where thread are actually used ?Core Java2017-05-29 08:35:42

 This question was recently asked at 'Persistent Systems'.This question is still unanswered. Can you please provide an answer.


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 Q21. Can we synchronize the run() method in Java?Core Java2017-07-23 20:08:58

 This question is still unanswered. Can you please provide an answer.


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 Q22. If you need to consume messages from the queue faster, which approach will you recommend - batching or concurrency ?Design2017-07-24 13:34:47

Ans. Each has it's own advantages

Batching requires less resources but may result in loosing whole batch in case of failure whereas concurrency even though is little more expensive in terms of resources but would result in minimal loss in case of failure.

In case messages are to be consumed in a particular order, batching them in that order and then consuming them makes better sense.

if incoming messages are not continuous , it makes more sense to do concurrency as we need not wait for all messages to form a batch and flush. Though time sensitivity can be added but that would add unnecessary complexity.

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 Q23. If a Map needs to be accessed simultaneously by multiple threads, which collection class should be used ? Core Java2017-08-17 11:01:07

Ans. ConcurrentHashMap

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