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

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Q11. 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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Q12. What is an object's lock and which object's have locks?

Ans. An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.

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Q13. Different ways of implementing Threads in Java ?

Core Java

Ans. Threads in Java can be implement either by Extending Thread class or implementing runnable interface.

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Q14. How can we ensure thread safety in static method ?

Core Java

Ans. By using synchronized static method or synchronized block

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Q15. What are the difference between Threads and Processes ?

Operating System

Ans. 1. when an OS wants to start running program it creates new process means a process is a program that is currently executing and every process has at least one thread running within it.
2). A thread is a path of code execution in the program, which has its own local variables, program counter(pointer to current execution being executed) and lifetime.
3. When the JavaVirtual Machine (JavaVM, or just VM) is started by the operating system, a new process is created. Within that process, many threads can be created.
4. Consider an example : when you open Microsoft word in your OS and you check your task manger then you can see this running program as a process. now when you write something in opened word document, then it performs more than one work at same time like it checks for the correct spelling, it formats the word you enter , so within that process ( word) , due to different path execution(thread) all different works are done at same time.
5. Within a process , every thread has independent path of execution but there may be situation where two threads can interfere with each other then concurrency and deadlock come is picture.
6. like two process can communicate ( ex:u open an word document and file explorer and on word document you drag and drop another another file from file explorer), same way two threads can also communicate with each other and communication with two threads is relatively low.
7. Every thread in java is created and controlled by unique object of java.lang.Thread class.
8. prior to jdk 1.5, there were lack in support of asynchronous programming in java, so in that case it was considered that thread makes the runtime environment asynchronous and allow different task to perform concurrently.

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Q16. 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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Q17. What is the difference between yield() and sleep()?

Operating System

Ans. When a object invokes yield() it returns to ready state. But when an object invokes sleep() method enters to not ready state.

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Q18. What is the initial state of a thread when it is created and started?

Ans. Ready state.

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Q19. Can a lock be acquired on a class ?

Ans. Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.

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Q20. What state does a thread enter when it terminates its processing?

Ans. When a thread terminates its processing, it enters the dead state.

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Q21. What is suspend() method used for ?

Ans. suspend() method is used to suspend the execution of a thread for a period of time. We can then restart the thread by using resume() method.

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Q22. Difference between suspend() and stop() ?

Ans. Suspend method is used to suspend thread which can be restarted by using resume() method. stop() is used to stop the thread, it cannot be restarted again.

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Q23. Difference between yield() and sleeping()?

Ans. When a task invokes yield(), it changes from running state to runnable state. When a task invokes sleep(), it changes from running state to waiting/sleeping state.

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Q24. What is a daemon thread? Give an Example ?

Ans. These are threads that normally run at a low priority and provide a basic service to a program or programs when activity on a machine is reduced. garbage collector thread is daemon thread.

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Q25. What is ThreadFactory ?

Core Java

Ans. ThreadFactory is an interface that is meant for creating threads instead of explicitly creating threads by calling new Thread(). Its an object that creates new threads on demand. Using thread factories removes hardwiring of calls to new Thread, enabling applications to use special thread subclasses, priorities, etc.

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Q26. When you will synchronize a piece of your code?

Ans. When you expect your code will be accessed by different threads and these threads may change a particular data causing data corruption.

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Q27. 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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Q28. 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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Q29. Name few Threads related classes and interfaces ?

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

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Q30. 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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Q31. 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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Q32. Invoking start twice on same thread leads to ..

a. ClassCastException
b. NullPointerException
c. InterruptedException
d. IllegalStateException

Ans. IllegalStateException

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Q33. 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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Q34. 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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Q35. 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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Q36. 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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Q37. 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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Q38. Can there be a Thread without a Process ?

Core Java

Ans. No.

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Q39. 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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Q40. 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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