Thread Programming Interview Questions in Java 2

Q) How to set & get Priority in Threads using java?

public class ThreadExample implements Runnable{

public  void run(){
System.out.println("Thread Starting");

{
for(int i=0;i<9;i++)
System.out.print(i);
}
}

public static void main(String args[]){
Thread t1=new Thread(new ThreadExample());
t1.start();

Thread t2= new Thread(new ThreadExample());
t2.start();

System.out.println("Thread 1 Priority"+t1.getPriority());

t2.setPriority(9);

System.out.println("Thread 2 Priority "+t2.getPriority());

}

}

Output
=============
Thread Starting
Thread Starting
012345678Thread 1 Priority5
Thread 2 Priority 5
012345678

Q) Difference between green thread and native thread in Java?
A) Native threads can switch between threads pre-emptively, switching control from a running thread to a non-running thread at any time. Green threads only switch when control is explicitly given up by a thread (Thread.yield(), Object.wait(), etc.) or a thread performs a blocking operation (read(), etc.).
On multi-CPU machines, native threads can run more than one thread simultaneously by assigning different threads to different CPUs. Green threads run on only one CPU.

Q)  Difference between thread and process?
A) Threads share the address space of the process that created it; processes have their own address space.
Threads have direct access to the data segment of its process; processes have their own copy of the data segment of the parent process.
Threads can directly communicate with other threads of its process; processes must use interprocess communication to communicate with sibling processes.

Q) Thread Life Cycle?

Q) What is context switching in multi-threading?
A) When doing context switching on a single-core processor, the code responsible is executed on the only CPU which takes care of switching the threads.
The kernel is multi-threaded. It can execute on any core. When a core comes to need to swap threads, it invokes the part of the kernel responsible for selecting the next thread it should execute.

5)  What thread-scheduling algorithm is used in Java?
6)  What is thread-scheduler in Java?
7)  How do you handle un-handled exception in thread?
8)  What is thread-group, why its advised not to use thread-group in Java?
9)  Why Executor framework is better than creating and managing thread by application ?
10) Difference between Executor and Executors in Java?
10) How to find which thread is taking maximum cpu in windows and Linux server?


Q) In which class would you find the methods sleep() and yield()?  Thread class

Q) Can you notify a particular thread? :   No

Q) How to make a normal thread as daemon thread?
A) We should call setDaemon(true) method on the thread object to make a thread as daemon thread.

Q) Give one good example of a daemon thread? : Garbage Collector is a low priority daemon thread.

Q) What are the two ways that a code can be synchronised?
A) Method can be declared as synchronised and A block of code be sychronised.


Q)What happens when I make a static method as synchronized?
A) 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.


Q) Can a thread execute another objects run() method?
A) A thread can execute it's own run() method or another objects run() method.

Q) What are different ways in which a thread can enter the waiting state?
A) A thread can enter the waiting state by the following ways:
1. Invoking its sleep() method,
2. By blocking on I/O
3. By unsuccessfully attempting to acquire an object's lock
4. By invoking an object's wait() method.
5. It can also enter the waiting state by invoking its (deprecated) suspend() method.

Q) What is the difference between yielding and sleeping?
When a task invokes its yield() method, it returns to the ready state, either from waiting, running or after its creation. When a task invokes its sleep() method, it returns to the waiting state from a running state.

Q)What is mutual exclusion? How can you take care of mutual exclusion using Java threads?
A)Mutual exclusion is a phenomenon where no two processes can access critical regions of memory at the same time. Using Java multithreading we can arrive at mutual exclusion. For mutual exclusion, you can simply use the synchronized keyword and explicitly or implicitly provide an Object, any Object, to synchronize on. The synchronized keyword can be applied to a class, to a method, or to a block of code. There are several methods in Java used for communicating mutually exclusive threads such as wait( ), notify( ), or notifyAll( ). For example, the notifyAll( ) method wakes up all threads that are in the wait list of an object.

Q) What is the difference between preemptive scheduling and time slicing?
A) Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then re-enters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.

Q) run(): After a thread is started, via its start() method of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.

Q) wait(), notify(), and notifyAll() methods : Methods are used to provide an efficient way for thread inter-communication.

Q) Thread & States ?
A) A thread is an independent path of execution in a system. The high-level thread states are ready, running, waiting and dead.

Q) What are synchronized methods and synchronized statements?

Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Q) What happens when a thread cannot acquire a lock on an object?
A) Yes. You can lock an object by putting it in a "synchronized" block. The locked object is inaccessible to any thread other than the one that explicitly claimed it. If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.

Q) What is the difference between process and thread?
A) A thread is a separate path of execution in a program. A Process is a program in execution.

Q) Why would you use a synchronized block vs. synchronized method?
A) Synchronized blocks place locks for shorter periods than synchronized methods.

Q) What is an object's lock and which objects have locks?
A) 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.

Q)Let`s say you have thread T1, T2 and T3, how will you ensure that thread T2 run after T1 and thread T3 run after T2?
(A) It can be achieved using join method of Thread class.

Q)How to know the thread priority?
A) isAlive() —it returns true when Thread is running
A) join() these two methods are used to know thread priority.

Q) 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. Deadlock ocurred on Two threads call Thread.join() on each other and Two threads use nested synchronized blocks to lock two objects and the blocks lock the same objects in different order.