Core Java - Interview Questions and Answers for 'Multithreading' | Search Interview Question

Core Java - Interview Questions and Answers for 'Multithreading' - 30 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

Ans. Synchronized

Sample Code for Synchronized

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Q2. Can constructors be synchronized in Java ?

Core Java

Ans. No. Java doesn't allow multi thread access to object constructors so synchronization is not even needed.

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Frequently asked to fresh graduates and less experienced developers.

Q3. Explain multithreading in Java ?

Core Java

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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Q4. Why do we need Thread class even in case we execute thread using runnable interface ?

Core Java

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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Q5. What is race condition ?

Operating System

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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Q6. What is the use of runnable interface if we can always create a new thread using Thread class ?

Core Java

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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Q7. If you need to consume messages from the queue faster, which approach will you recommend - batching or concurrency ?

Design

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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Usually asked only to fresh graduates.

Q8. Why threads block or enters to waiting state on I/O?

Operating System

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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Q9. Write a Program to add 1-100, 101-200, 201-300 in different threads and then sum the numbers ?

Core Java

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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Q10. Name few classes that extend Thread class ?

Core Java

Ans. http://www.buggybread.com/2015/02/java-threads-classes-that-inherit.html

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Q11. Name few Threads related classes and interfaces ?

Ans. http://www.buggybread.com/2015/01/java-threads-classes-and-interfaces.html

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Q12. What is the initial state of a thread when it is created and started ?

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

Ans. Ready

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Q13. What state does a thread enter when it terminates its processing?

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

Ans. Dead

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Q14. 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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Q15. 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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Q16. 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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Q17. In case of a synchronized method, Does the method only gets locked or the whole object ?

Core Java

Ans. Just the method gets locked.

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Q18. What are the disadvantages of multithreading ?

Core Java

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.

Q19. What is the use of synchronized keyword ?

Core Java

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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Q20. Can we start the same thread twice ?

Core Java

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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Q21. Can you give real time examples of thread or where thread are actually used ?

Core Java

Ans. Games are very good examples of threading.You can use multiple objects in games like cars, motor bikes, animals, people etc. All these objects are nothing but just threads that run your game application.

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Q22. Can we synchronize the run() method in Java?

Core Java

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

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Q23. If a Map needs to be accessed simultaneously by multiple threads, which collection class should be used ?

Core Java

Ans. ConcurrentHashMap

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Q24. Write Java code that would cause deadlock ?

Core Java

Ans. https://howtodoinjava.com/java/multi-threading/writing-a-deadlock-and-resolving-in-java/

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Q25. How executor service is better than using primitive Threading mechanism using Thread class or runnable Interface ?

Core Java

^{This question was recently asked at 'Symantec'.This question is still unanswered. Can you please provide an answer.}

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Q26. Suppose we make an api call using multithreading, How would you design to make sure that we have received response for all threads before exiting the main thread ?

Design

Ans. We can use HashMap for tracking response status for all threads. We can wait every n second by using Thread.sleep and exit the main thread only once we have received response for all threads.

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Q27. How does multi threading improve performance ?

Core Java

Ans. Every process in it's timeline require different resources. Utilization of resources can be optimized when they are shared among different processes or threads. When one thread is sleeping waiting for a peripheral to complete (e.g. a disk write, or a key press from the keyboard), other threads can continue using processor time and hence would lead to better usage of resources.

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Q28. What are the different ways to avoid multi Threading related problems in Java ?

Core Java

Ans. Synchronization,
Concurrent classes,
Volatile keyword,
Implementing concurrent Lock interface,
Immutable classes

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Q29. What is context switching wrt Threads in Java?

Core Java

Ans. Context Switching is the process of storing and restoring of CPU state so that Thread execution can be resumed from the same point at a later point of time.

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Q30. Tell something about Sleep method.

Core Java

^{This question was recently asked at 'IBM'.This question is still unanswered. Can you please provide an answer.}

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