Java Threading Interview Questions

'Java thread interview questions' is covered in this topic. This topic is the facto for clearing any investment banking jobs. There are few java interviews where they ask most of questions from threading and concurrency. These topic become synonyms on how much knowledge you have in java. No interviewer takes a chance that developer is not be aware of threading. I will say it is better to do same coding to practice this topic.
  
Java Concurrency Questions.

Threading Interview Questions:-
  1. Why we need multi-threaded applications?
    Now a day most of servers are coming with multiple processors. To take benefit of these multi core processing capabilities, we need to run application with multiple processes concurrently. In Java, Thread is one of the way to create these light weight process, which can execute at same time and perform tasks. 
  2. What is synchronization ?
    Synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.                                                                               
  3. Different way of creating thread?
    The thread could be implemented by using Runnable interface or by extending the Thread class. The Runnable is more advantageous, when you are going for multiple inheritance.
    a) public class MyThread extends Thread
    b) public class MyThread implements Runnable
  4. What is the difference between Thread.start() & Thread.run() method?
    Thread.start() method (native method) of Thread class actually does the job of running the Thread.run() method in a thread. If we directly call Thread.run() method it will executed in same thread, so does not solve the purpose of creating a new thread.   In this sequence the thread changes state.
       
  5. Why do we need run() & start() methods?
    We need run() & start() method both because JVM needs to create a separate thread which can not be differentiated from a normal method call. So this job is done by start method native implementation which has to be explicitly called.
    Another advantage of having these two methods is we can have any object run as a thread if it implements Runnable interface. This is to avoid Java’s multiple inheritance problems which will make it difficult to inherit another class with Thread.
  6. What is ThreadLocal class?
    - A thread-local variable effectively provides a separate copy of its value for each thread that uses it.
    - ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread
    - In case when multiple threads access a ThreadLocal instance, separate copy of Threadlocal variable is maintained for each thread.
    - Common use is seen in DAO pattern where the DAO class can be singleton but the Database connection can be maintained separately for each thread. (Per Thread Singleton)
    Good article on ThreadLocal: http://www-128.ibm.com/developerworks/java/library/j-threads3.html
    Good example : http://javaboutique.internet.com/tutorials/localdata/
    Multithreading in Swing: http://java.sun.com/developer/JDCTechTips/2003/tt1208.html
    Refer API Docs: http://java.sun.com/j2se/1.4.2/docs/api/java/lang/ThreadLocal.html
       
  7. When InvalidMonitorStateException is thrown? Why?
    This exception is thrown when you try to call wait()/notify()/notifyAll() any of these methods for an Object from a point in your program where u are NOT having a lock on that object.(i.e. u r not executing any synchronized block/method of that object and still trying to call wait()/notify()/notifyAll())
          wait(), notify() and notifyAll() all throw IllegalMonitorStateException. since This exception is a subclass of RuntimeException so we r not bound to catch it (although u may if u want to). and being a RuntimeException this exception is not mentioned in the signature of wait(), notify(), notifyAll() methods.
  8. What is the difference between sleep(), suspend() and wait() ?
    Thread.sleep() sends the current thread into the "Not Runnable" state for some amount of time. The thread keeps the monitors it has aquired -- i.e. if the thread is currently in a synchronized block or method no other thread can enter this block or method. If another thread calls t.interrupt() it will wake up the sleeping thread. Note that sleep is a static method, which means that it always affects the current thread (the one that is executing the
    sleep method). A common mistake is to call t.sleep() where t is a different thread; even then, it is the current thread that will sleep, not the t thread.
          t.suspend() is deprecated. Using it is possible to halt a thread other than the current thread. A suspended thread keeps all its monitors and since this state is not interruptable it is deadlock prone.
          object.wait() sends the current thread into the "Not Runnable" state, like sleep(), but with a twist. Wait is called on a object, not a thread; we call this object the "lock object." Before lock.wait() is called, the current thread must synchronize on the lock object; wait() then releases this lock, and adds the thread to the "wait list" associated with the lock. Later, another thread can synchronize on the same lock object and call lock.notify(). This wakes up the original,
    waiting thread. Basically, wait()/notify() is like sleep()/interrupt(), only the active thread does not need a direct pointer to the sleeping thread, but only to the shared lock object.
  9. What happens when I make a static method as synchronized?
    Synchronized static methods have a lock on the class "Class", so when a thread enters a synchronized static method, the class itself gets locked by the thread monitor and no other thread can enter any static synchronized methods on that class. This is unlike instance methods, as multiple threads can access "same synchronized instance methods" at same time for different instances.
  10. Can a thread call a non-synchronized instance method of an Object when a synchronized method is being executed ?
    Yes, a Non synchronized method can always be called without any problem. In fact Java does not do any check for a non-synchronized method. The Lock object check is performed only for synchronized methods/blocks. In case the method is not declared synchronized Jave will call even if you are playing with shared data. So you have to be careful while doing such thing. The decision of declaring a method as synchronized has to be based on critical section access. If your method does not access a critical section (shared resource or data structure) it need not be declared synchronized.
    Below is the example which demonstrates this, The Common class has two methods
    synchronizedMethod1() and method1()
    MyThread class is calling both the methods in separate threads,
      

package com.learning.thread;

class MyThread extends Thread {
 private int id = 0;
 private Common common;   
 public MyThread(String name, int no, Common object) {  
  super(name);
  common = object;
  id = no;
 
 }
 public void run() {
  System.out.println("Running Thread" + this.getName());
  try {
   if (id == 0) {
    common.synchronizedMethod1();
   } else {
    common.method1();
   }
  } catch (Exception e) {
   e.printStackTrace();
  }
 }
}
 public class Common {
  public synchronized void synchronizedMethod1() {
   System.out.println("synchronizedMethod1 called");
   try {
    Thread.sleep(1000);
   } catch (InterruptedException e) {
    e.printStackTrace();
   }
   System.out.println("synchronizedMethod1 done");
  }

  public void method1() {
   System.out.println("Method 1 called");
   try {
    Thread.sleep(1000);
   } catch (InterruptedException e) {
    e.printStackTrace();
   }
   System.out.println("Method 1 done");
  }

  public static void main(String... strings){
   Common c = new Common();
   MyThread t1 = new MyThread("MyThread-1", 0, c);
   MyThread t2 = new MyThread("MyThread-2", 1, c);
   t1.start();
   t2.start(); 
  }

 }



OutPut-->
1.  Running ThreadMyThread-1
2.  synchronizedMethod1 called
3.  Running ThreadMyThread-2
4.  Method 1 called
5.  synchronizedMethod1 done
6.  Method 1 done
This shows that method1() - is called even though the synchronizedMethod1() was in execution.
11. Can two threads call two different synchronized instance methods of an Object?
No. If a object has synchronized instance methods then the Object itself is used a lock object for controlling the synchronization. Therefore all other instance methods need to wait until previous method call is completed.
See the below sample code which demonstrate it very clearly. The Class Common has 2 methods called synchronizedMethod1() and synchronizedMethod2() MyThread class is calling both the methods.
 public class Common {
  public synchronized void synchronizedMethod1() {
   System.out.println("synchronizedMethod1 called");
   try {
    Thread.sleep(1000);
   } catch (InterruptedException e) {
    e.printStackTrace();
   }
   System.out.println("synchronizedMethod1 done");
  }

  public synchronized void synchronizedMethod2() {
   System.out.println("synchronizedMethod1 called");
   try {
    Thread.sleep(1000);
   } catch (InterruptedException e) {
    e.printStackTrace();
   }  
 }
12. What is a deadlock?
Deadlock is a situation where two or more threads are blocked forever, waiting for each other. This may occur when two threads, each having a lock on one resource, attempt to acquire a lock on the other's resource. Each thread would wait indefinitely for the other to release the lock, unless one of the user processes is terminated. In terms of Java API, thread deadlock can occur in following conditions:
o    When two threads call Thread.join() on each other.
o    When two threads use nested synchronized blocks to lock two objects and the blocks lock the same objects in different order.

13. What is Starvation? and What is a Livelock?
Starvation and livelock are much less common a problem than deadlock, but are still problems that every designer of concurrent software is likely to encounter.
LiveLock
Livelock occurs when all threads are blocked, or are otherwise unable to proceed due to unavailability of required resources, and the non-existence of any unblocked thread to make those resources available. In terms of Java API, thread livelock can occur in following conditions:

  •    When all the threads in a program execute Object.wait(0) on an object with zero parameter. The program is live-locked and cannot proceed until one or more threads call Object.notify() or Object.notifyAll() on the relevant objects. Because all the threads are blocked, neither call can be made.
  •     When all the threads in a program are stuck in infinite loops.
Starvation
Starvation describes a situation where a thread is unable to gain regular access to shared resources and is unable to make progress. This happens when shared resources are made unavailable for long periods by "greedy" threads. For example, suppose an object provides a synchronized method that often takes a long time to return. If one thread invokes this method frequently, other threads that also need frequent synchronized access to the same object will often be blocked.
Starvation occurs when one thread cannot access the CPU because one or more other threads are monopolizing the CPU.
In Java, thread starvation can be caused by setting thread priorities inappropriately. A lower-priority thread can be starved by higher-priority threads if the higher-priority threads do not yield control of the CPU from time to time.

14. How to make sure Thread 1 , 2, 3 can run in sequence?
We can use join() to run the thread in sequence.
thread1.join();
thread2.join();
thread3.join();
package com.learning.thread;
public class Join implements Runnable {
 class ├žounter {
  private int c = 0;
  public void increment() {
   c++;
  }
  public void decrement() {
   c--;
  }
  public int value() {
   return c;
  }
 }
 @Override
 public void run() {
  for (int x = 1; x <= 5; x++) {
   System.out.println("this is thread "
     + Thread.currentThread().getName());
  }
 }
 public static void main(String[] args) throws Exception {
  Join j1 = new Join();
  Thread t1 = new Thread(j1, "1");
  Thread t2 = new Thread(j1, "2");
  Thread t3 = new Thread(j1, "3");
  t1.start();
  t1.join(0);
  t2.start();
  t2.join();
  t3.start();
 }
}
output :
this is thread 1
this is thread 1
this is thread 2
this is thread 2
this is thread 3
this is thread 3

 If you do not use join then you will get the result in random order.

15. What is ThreadGroup and its benefit?
The ThreadGroup class manages groups of threads for Java applications. A ThreadGroup can contain any number of threads, presumably related in some way such as who created them, what function they perform, or when they should be started and stopped. ThreadGroup provides benefit of assigning similar threads to same group.
       ThreadGroups can contain not only threads but also other ThreadGroups. The top-most thread group in a Java application is the thread group named "main". You can create threads and thread groups in the "main" group. You can also create threads and thread groups in subgroups of "main" and so on. The result is a root-like hierarchy of threads and thread groups.  Below example shows how to get all threads running under current threadGroup.

class EnumerateTest {
    void listCurrentThreads() {
 ThreadGroup currentGroup = Thread.currentThread().getThreadGroup();
 int numThreads;
 Thread listOfThreads[];

 numThreads = currentGroup.activeCount();
 listOfThreads = new Thread[numThreads];
 currentGroup.enumerate(listOfThreads);
 for (int i = 0; i < numThreads; i++) {
     System.out.println("Thread #" + i + " = " + listOfThreads[i].getName());
 }
    }
}
15. Create Producer and Consumer of data using synchronization ?
class DataHolder {
    private int seq;
    private boolean available = false;

    public synchronized int get() {
        while (available == false) {
     try {
           wait();
     } catch (InterruptedException e) {
     }
    }
        available = false;
        notify();
        return seq;
    }

    public synchronized void put(int value) {
 while (available == true) {
     try {
            wait();
     } catch (InterruptedException e) {
     }
    }
        seq = value;
        available = true;
        notify();
    }
}

16. Why Java locks \ object monitors are re-entrant?
The Java runtime system allows a thread to re-acquire a monitor that it already holds because Java monitors are re-entrant. Re-entrant monitors are important because it eliminates the possibility of a thread deadlocking on a monitor that it already holds.

Read Java Concurrency for latest updates to create multithreaded applications

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