Interview Questions
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1.What is Thread in java?

Threads consumes CPU in best possible manner, hence enables multi processing. Multi threading reduces idle time of CPU which improves performance of application. Thread are light weight process.
A thread class belongs to java.lang package.
We can create multiple threads in java, even if we don’t create any Thread, one Thread at least do exist i.e. main thread. Multiple threads run parallely in java.
Threads have their own stack.
Advantage of Thread : Suppose one thread needs 10 minutes to get certain task, 10 threads used at a time could complete that task in 1 minute, because threads can run parallely.

2.What is difference between Process and Thread in java?

One process can have multiple Threads,
Thread are subdivision of Process. One or more Threads runs in the context of process. Threads can execute any part of process. And same part of process can be executed by multiple Threads.
Processes have their own copy of the data segment of the parent process while Threads have direct access to the data segment of its process. Processes have their own address while Threads share the address space of the process that created it.
Process creation needs whole lot of stuff to be done, we might need to copy whole parent process, but Thread can be easily created.
Processes can easily communicate with child processes but interprocess communication is difficult. While, Threads can easily communicate with other threads of the same process using wait() and notify() methods.
In process all threads share system resource like heap Memory etc. while Thread has its own stack.
Any change made to process does not affect child processes, but any change made to thread can affect the behavior of the other threads of the process.
Example to see where threads on are created on different processes and same process.

3.How to implement Threads in java?

This is very basic threading question. Threads can be created in two ways i.e. by implementing java.lang.Runnable interface or extending java.lang.Thread class and then extending run method.
Thread has its own variables and methods, it lives and dies on the heap. But a thread of execution is an individual process that has its own call stack.
Thread are lightweight process in java.
Thread creation by implementingjava.lang.Runnableinterface.
We will create object of class which implements Runnable interface :
MyRunnable runnable=new MyRunnable();
Thread thread=new Thread(runnable);
2) And then create Thread object by calling constructor and passing reference of Runnable interface i.e. runnable object :
Thread thread=new Thread(runnable);

4.We should implement Runnable interface or extend Thread class. What are differences between implementing Runnable and extending Thread?

Well the answer is you must extend Thread only when you are looking to modify run() and other methods as well. If you are simply looking to modify only the run() method implementing Runnable is the best option (Runnable interface has only one abstract method i.e. run() ).
Differences between implementing Runnable interface and extending Thread class -
1.Multiple inheritance in not allowed in java : When we implement Runnable interface we can extend another class as well, but if we extend Thread class we cannot extend any other class because java does not allow multiple inheritance. So, same work is done by implementing Runnable and extending Thread but in case of implementing Runnable we are still left with option of extending some other class. So, it’s better to implement Runnable.
2.Thread safety: When we implement Runnable interface, same object is shared amongst multiple threads, but when we extend Thread class each and every thread gets associated with new object.
3.Inheritance (Implementing Runnable is lightweight operation) : When we extend Thread unnecessary all Thread class features are inherited, but when we implement Runnable interface no extra feature are inherited, as Runnable only consists only of one abstract method i.e. run() method. So, implementing Runnable is lightweight operation.
4.>Coding to interface : 5.Don’t extend unless you wanna modify fundamental behaviour of class, Runnable interface has only one abstract method i.e. run() : We must extend Thread only when you are looking to modify run() and other methods as well. If you are simply looking to modify only the run() method implementing Runnable is the best option (Runnable interface has only one abstract method i.e. run() ). We must not extend Thread class unless we're looking to modify fundamental behaviour of Thread class.
6.Flexibility in code when we implement Runnable : When we extend Thread first a fall all thread features are inherited and our class becomes direct subclass of Thread , so whatever action we are doing is in Thread class. But, when we implement Runnable we create a new thread and pass runnable object as parameter,we could pass runnable object to executorService & much more. So, we have more options when we implement Runnable and our code becomes more flexible.
7.ExecutorService : If we implement Runnable, we can start multiple thread created on runnable object with ExecutorService (because we can start Runnable object with new threads), but not in the case when we extend Thread (because thread can be started only once).

5.How can you say Thread behaviour is unpredictable?

Answer. The solution to question is quite simple, Thread behaviour is unpredictable because execution of Threads depends on Thread scheduler, thread scheduler may have different implementation on different platforms like windows, unix etc. Same threading program may produce different output in subsequent executions even on same platform.
To achieve we are going to create 2 threads on same Runnable Object, create for loop in run() method and start both threads. There is no surety that which threads will complete first, both threads will enter anonymously in for loop.

6.When threads are not lightweight process in java?

Threads are lightweight process only if threads of same process are executing concurrently. But if threads of different processes are executing concurrently then threads are heavy weight process.

7.How can you ensure all threads that started from main must end in order in which they started and also main should end in last?

Interviewers tend to know interviewees knowledge about Thread methods. So this is time to prove your point by answering correctly. We can use join() method to ensure all threads that started from main must end in order in which they started and also main should end in last.In other words waits for this thread to die. Calling join() method internally calls join(0);
DETAILED DESCRIPTION : Join() method - ensure all threads that started from main must end in order in which they started and also main should end in last. Types of join() method with programs- 10 salient features of join.

8.What is difference between starting thread with run() and start() method?

This is quite interesting question, it might confuse you a bit and at time may make you think is there really any difference between starting thread with run() and start() method.
When you call start() method, main thread internally calls run() method to start newly created Thread, so run() method is ultimately called by newly created thread.
When you call run() method main thread rather than starting run() method with newly thread it start run() method by itself.

9.What is significance of using Volatile keyword?

Java allows threads to access shared variables. As a rule, to ensure that shared variables are consistently updated, a thread should ensure that it has exclusive use of such variables by obtaining a lock that enforces mutual exclusion for those shared variables.
If a field is declared volatile, in that case the Java memory model ensures that all threads see a consistent value for the variable
Few small questions>
Q. Can we have volatile methods in java?
No, volatile is only a keyword, can be used only with variables.
Q. Can we have synchronized variable in java?
No, synchronized can be used only with methods, i.e. in method declaration.

10.Differences between synchronized and volatile keyword in Java?

Its very important question from interview perspective. Volatilecan be used as a keyword against the variable, we cannot use volatile against method declaration.
volatile void method1(){} //it’s illegal, compilation error.
While synchronization can be used in method declaration or we can create synchronization blocks (In both cases thread acquires lock on object’s monitor). Variables cannot be synchronized.
Synchronized method:
synchronized void method2(){} //legal
Synchronized block:
void method2(){
synchronized (this) {
//code inside synchronized block.
}
}
Synchronized variable (illegal):
synchronized int i;//it’s illegal, compilatiomn error.
Volatile does not acquire any lock on variable or object, but Synchronization acquires lock on method or block in which it is used.
Volatile variables are not cached, but variables used inside synchronized method or block are cached.
When volatile is used will never create deadlock in program, as volatile never obtains any kind of lock . But in case if synchronization is not done properly, we might end up creating dedlock in program.
Synchronization may cost us performance issues, as one thread might be waiting for another thread to release lock on object. But volatile is never expensive in terms of performance.
DETAILED DESCRIPTION : Differences between synchronized and volatile keyword in detail with programs.

11.Can you again start Thread?

we cannot start Thread again, doing so will throw runtimeException java.lang.IllegalThreadStateException. The reason is once run() method is executed by Thread, it goes into dead state.
Let’s take an example-
Thinking of starting thread again and calling start() method on it (which internally is going to call run() method) for us is some what like asking dead man to wake up and run. As, after completing his life person goes to dead state.

12.What is race condition in multithreading and how can we solve it?

This is very important question, this forms the core of multi threading, you should be able to explain about race condition in detail. When more than one thread try to access same resource without synchronization causes race condition.
So we can solve race condition by using either synchronized block or synchronized method. When no two threads can access same resource at a time phenomenon is also called as mutual exclusion.
Few sub questions>
Q 1.What if two threads try to read same resource without synchronization?
When two threads try to read on same resource without synchronization, it’s never going to create any problem.
Q 2.What if two threads try to write to same resource without synchronization?
When two threads try to write to same resource without synchronization, it’s going to create synchronization problems.

13.How threads communicate between each other?

This is very must know question for all the interviewees, you will most probably face this question in almost every time you go for interview.
Threads can communicate with each other by using wait(), notify() and notifyAll() methods.

14.Why wait(), notify() and notifyAll() are in Object class and not in Thread class?

Every Object has a monitor, acquiring that monitors allow thread to hold lock on object. But Thread class does not have any monitors.
wait(), notify() and notifyAll()are called on objects only >When wait() method is called on object by thread it waits for another thread on that object to release object monitor by calling notify() or notifyAll() method on that object.
When notify() method is called on object by thread it notifies all the threads
which are waiting for that object monitor that object monitor is available now.
So, this shows that wait(), notify() and notifyAll() are called on objects only.
Now, Straight forward question that comes to mind is how thread acquires object lock by
acquiring object monitor? Let’s try to understand this basic concept in detail?
8 Wait(), notify() and notifyAll() method being in Object class allows all the threads created on that object to communicate with other.
9 As multiple threads exists on same object. Only one thread can hold object monitor at a time. As a result thread can notify other threads of same object that lock is available now. But, thread having these methods does not make any sense because multiple threads exists on object its not other way around (i.e. multiple objects exists on thread).
10 Now let’s discuss one hypothetical scenario, what will happen if
Thread class contains wait(), notify() and notifyAll() methods?
Having wait(), notify() and notifyAll() methods means Thread class also must have their monitor.
Every thread having their monitor will create few problems -
>Thread communication problem.
>Synchronization on object won’t be possible- Because object has monitor, one object can have multiple threads and thread hold lock on object by holding object monitor. But if each thread will have monitor, we won’t have any way of achieving synchronization.
>Inconsistency in state of object (because synchronization won't be possible).

15.Is it important to acquire object lock before calling wait(), notify() and notifyAll()?

Yes, it’s mandatory to acquire object lock before calling these methods on object. As discussed above wait(), notify() and notifyAll() methods are always called from Synchronized block only, and as soon as thread enters synchronized block it acquires object lock (by holding object monitor). If we call these methods without acquiring object lock i.e. from outside synchronize block then java.lang. IllegalMonitorStateException is thrown at runtime. Wait() method needs to enclosed in try-catch block, because it throws compile time exception i.e. InterruptedException.

16.How can you solve consumer producer problem by using wait() and notify() method? (Important)

Here come the time to answer very very important question from interview perspective. Interviewers tends to check how sound you are in threads inter communication. Because for solving this problem we got to use synchronization blocks, wait() and notify() method very cautiously. If you misplace synchronization block or any of the method, that may cause your program to go horribly wrong. So, before going into this question first i’ll recommend you to understand how to use synchronized blocks, wait() and notify() methods.
Key points we need to ensure before programming :
>Producer will produce total of 10 products and cannot produce more than 2 products at a time until products are being consumed by consumer.
Example> when sharedQueue’s size is 2, wait for consumer to consume (consumer will consume by calling remove(0) method on sharedQueue and reduce sharedQueue’s size). As soon as size is less than 2, producer will start producing.
>Consumer can consume only when there are some products to consume. Example> when sharedQueue’s size is 0, wait for producer to produce (producer will produce by calling add() method on sharedQueue and increase sharedQueue’s size). As soon as size is greater than 0, consumer will start consuming.
Explanation of Logic >
We will create sharedQueue that will be shared amongst Producer and Consumer. We will now start consumer and producer thread.
Note: it does not matter order in which threads are started (because rest of code has taken care of synchronization and key points mentioned above) First we will start consumerThread >
consumerThread.start();
consumerThread will enter run method and call consume() method. There it will check for sharedQueue’s size.
-if size is equal to 0 that means producer hasn’t produced any product, wait for producer to produce by using below piece of code-
synchronized (sharedQueue) {
while (sharedQueue.size() == 0) {
sharedQueue.wait();
}
}
-if size is greater than 0, consumer will start consuming by using below piece of code.
synchronized (sharedQueue) {
Thread.sleep((long)(Math.random() * 2000));
System.out.println("consumed : "+ sharedQueue.remove(0));
sharedQueue.notify();
}
Than we will start producerThread >
producerThread.start();
producerThread will enter run method and call produce() method. There it will check for sharedQueue’s size.
-if size is equal to 2 (i.e. maximum number of products which sharedQueue can hold at a time), wait for consumer to consume by using below piece of code-
synchronized (sharedQueue) {
while (sharedQueue.size() == maxSize) { //maxsize is 2
sharedQueue.wait();
}
}
-if size is less than 2, producer will start producing by using below piece of code.
synchronized (sharedQueue) {
System.out.println("Produced : " + i);
sharedQueue.add(i);
Thread.sleep((long)(Math.random() * 1000));
sharedQueue.notify();
}
DETAILED DESCRIPTION with program : Solve Consumer Producer problem by using wait() and notify() methods in multithreading.

17.How to solve Consumer Producer problem without using wait() and notify() methods, where consumer can consume only when production is over.?

In this problem, producer will allow consumer to consume only when 10 products have been produced (i.e. when production is over). We will approach by keeping one boolean variable productionInProcess and initially setting it to true, and later when production will be over we will set it to false.

18.How can you solve consumer producer pattern by using BlockingQueue?

Now it’s time to gear up to face question which is most probably going to be followed up by previous question i.e. after how to solve consumer producer problem using wait() and notify() method. Generally you might wonder why interviewer's are so much interested in asking about solving consumer producer problem using BlockingQueue, answer is they want to know how strong knowledge you have about java concurrent Api’s, this Api use consumer producer pattern in very optimized manner, BlockingQueue is designed is such a manner that it offer us the best performance.
BlockingQueue is a interface and we will use its implementation class LinkedBlockingQueue.
Key methods for solving consumer producer pattern are >
put(i); //used by producer to put/produce in sharedQueue.
take();//used by consumer to take/consume from sharedQueue.

19.What is deadlock in multithreading? Write a program to form DeadLock in multi threading and also how to solve DeadLock situation. What measures you should take to avoid deadlock?

This is very important question from interview perspective. But, what makes this question important is it checks interviewees capability of creating and detecting deadlock. If you can write a code to form deadlock, than I am sure you must be well capable in solving that deadlock as well. If not, later on this post we will learn how to solve deadlock as well.
First question comes to mind is, what is deadlock in multi threading program? Deadlock is a situation where two threads are waiting for each other to release lock holded by them on resources.
But how deadlock could be formed :
Thread-1 acquires lock on String.class and then calls sleep() method which gives Thread-2 the chance to execute immediately after Thread-1 has acquired lock on String.class and Thread-2 acquires lock on Object.class then calls sleep() method and now it waits for Thread-1 to release lock on String.class. Conclusion:
Now, Thread-1 is waiting for Thread-2 to release lock on Object.class and Thread-2 is waiting for Thread-1 to release lock on String.class and deadlock is formed.
//Code called by Thread-1
public void run() {
synchronized (String.class) {
Thread.sleep(100);
synchronized (Object.class) {
}
}
}
//Code called by Thread-2
publicvoid run() {
synchronized (Object.class) {
Thread.sleep(100);
synchronized (String.class) {
}
}
}
Here comes the important part, how above formed deadlock could be solved :
Thread-1 acquires lock on String.class and then calls sleep() method which gives Thread-2 the chance to execute immediately after Thread-1 has acquired lock on String.class and Thread-2 tries to acquire lock on String.class but lock is holded by Thread-1. Meanwhile, Thread-1 completes successfully. As Thread-1 has completed successfully it releases lock on String.class, Thread-2 can now acquire lock on String.class and complete successfully without any deadlock formation.
Conclusion: No deadlock is formed.
//Code called by Thread-1
publicvoid run() {
synchronized (String.class) {
Thread.sleep(100);
synchronized (Object.class) {
}
}
}
//Code called by Thread-2
publicvoid run() {
synchronized (String.class) {
Thread.sleep(100);
synchronized (Object.class) {
}
}
}
Few important measures to avoid Deadlock >
Lock specific member variables of class rather than locking whole class: We must try to lock specific member variables of class rather than locking whole class. Use join() method: If possible try touse join() method, although it may refrain us from taking full advantage of multithreading environment because threads will start and end sequentially, but it can be handy in avoiding deadlocks.
If possible try avoid using nested synchronization blocks.

20.Have you ever generated thread dumps or analyzed Thread Dumps?

Answering this questions will show your in depth knowledge of Threads. Every experienced must know how to generate Thread Dumps. VisualVM is most popular way to generate Thread Dump and is most widely used by developers. It’s important to understand usage of VisualVM for in depth knowledge of VisualVM. I’ll recommend every developer must understand this topic to become master in multi threading.
It helps us in analyzing threads performance, thread states, CPU consumed by threads, garbage collection and much more. For detailed information see Generating and analyzing Thread Dumps using VisualVM - step by step detail to setup VisualVM with screenshots
jstack is very easy way to generate Thread dump and is widely used by developers. I’ll recommend every developer must understand this topic to become master in multi threading. For creating Thread dumps we need not to download any jar or any extra software. For detailed information see Generating and analyzing Thread Dumps using JSATCK - step by step detail to setup JSTACK with screenshots.

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