Part 2
Interview questions on multithreading in java for experienced covered in this post:
What is deadlock? How to avoid a
situation of dead lock?
- Deadlocks are a set of blocked processes, each holding a resource and waiting to acquire a resource held by another process.
-
We can avoid the deadlock: Do not grant a resource request if
this allocation has the potential to lead to deadlock.
What happen if you don't override run()
method on Thread class?
-
If you don’t override the run() method it will execute the
run method of the Thread class that has empty implementation. The compiler
won’t throw any exception.
-
It is recommended to override the run() method and provide
your implementation.
What is the difference between sleep()
and wait() in java?
|
|
Sleep() |
Wait() |
|
1. |
The sleep () method can be used to pause the process for some time
or the time we want to. |
Wait() is a method that can be used to put the thread in a waiting
state. |
|
2. |
Sleep() method never release the lock while sleeping the
thread. |
Wait() method can release the lock while a thread is waiting. |
|
3. |
Syntax:
synchronized(LOCK) {
Thread.sleep(1000);
} |
Syntax:
synchronized(LOCK) {
LOCK.wait();
} |
 
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There are 3 Thread Thread1, Thread2 and
Thread3 how would you ensure the thread execution for Thread1, Thread2
and then Thread3?
-
If a thread wants to wait until completing some other thread
then we should go for the join(); method.
-
In above example, Thread1.join(Thread2) and
Thread2.join(Thread3) in the above case Thread3 will execute first and then
thread2 and then thread1.
What is the difference between Runnable
and Callable?
|
|
Runnable |
Callable |
|
1. |
The runnable interface has the run() method which needs to override
an implemented class. |
The callable interface has a call() method which needs to override
an implemented class. |
|
2. |
Run() method cannot return a value. |
Call() method can return a value. |
|
3. |
Runnable cannot throw checked Exception. |
Callable can throw checked exception. |
|
4. |
Runnable cannot be used in Executor Framework. |
Callable can use in Executor Framework. |
|
5. |
public interface Runnable {
void run();
} |
public interface
Callable<V> {
V
call() throws Exception;
} |
What is static synchronization?
-
Static synchronized method will acquire a lock of the class
instead of an object.
-
Static belongs to the class, not the object so Thread
acquires a lock from class.
-
Suppose there are multiple static synchronized methods (m1,
m2, m3, m4) in a class, and suppose one thread is accessing m1, then no
other thread at the same time can access any other static synchronized
methods.
What is the difference between
Synchronous and Asynchronous programming?
-
Implies that tasks will be executed one by one. The next task
is started only after the current task is finished. Task3 is not started
until task2 is finished.
-
Implies that task returns control immediately with a promise
to execute a code and notify about the result later(e.g. callback, feature).
Task 3 is executed even if Task 2 is not finished.
-
Please refer to the Real-Time Example.
-
Synchronous: You
are in a queue to get a movie ticket. You cannot get one until everybody in
front of you gets one, and the same applies to the people queued behind
you.
-
Asynchronous:
You are in a restaurant with many other people. You order your food. Other
people can also order their food, they don't have to wait for your food to
be cooked and served to you before they can order. In the kitchen,
restaurant workers are continuously cooking, serving, and taking orders.
People will get their food served as soon as it is cooked.
What is the difference between submit()
and execute() in thread pool?
|
|
Execute() |
Submit() |
|
1. |
Return type of execute() method is void. |
Return type of submit() is Future. |
|
2. |
execute() method is declared in Executor Interface. |
submit() method is declared in ExecutorService interface. |
|
3. |
This method accepts only Runnable Task. |
This method accepts Runnable as well as Callable tasks. |
-
Please refer below examples of each interface.
|
package
simplifiedjava.crackedInterview;
import
java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class ThreadExecutorServiceUsingRunnableDemo { public static void main(String[] args) {
ExecutorService executorService
= xecutors.newSingleThreadExecutor(); executorService.execute(new Runnable() {
@Override
public
void
run() {
System.out.println("Executor Service Executed with Runnable...");
for(int
i=0; i<10; i++) {
System.out.println("Child Thread...");
}
}
});
executorService.shutdown();
}
} Using Callable Interface. package simplifiedjava.crackedInterview;
import
java.util.concurrent.Callable;
import
java.util.concurrent.ExecutionException;
import
java.util.concurrent.ExecutorService;
import
java.util.concurrent.Executors; import java.util.concurrent.Future; public class ThreadExecutorServiceUsingCallableDemo { public static void main(String[] args) {
ExecutorService executorService
= Executors.newFixedThreadPool(1); Future obj = executorService.submit(new Callable() {
@Override
public
Object call() throws
Exception {
//System.out.println("Executor Service Executed with
Callable...");
return
"Executor Service Executed with Callable...";
}
});
try
{
System.out.println(obj.get());
} catch
(InterruptedException | ExecutionException e) {
e.printStackTrace();
}
}
} |
What is the difference between
CyclicBarrier and CountDownLatch?
-
CyclicBarrier and Countdown latch techniques can be used in a
multi-threaded environment to manage resources for multiple threads.
-
The difference is explained below.
|
|
CountDownLatch |
CyclicBarrier |
|
1. |
You cannot reuse the instance of CountDownLatch once the count
reaches zero and the latch is open. CountDownLatch works like a
count-down timer. |
CyclicBarrier can be reused by resetting the barrier once the
barrier is broken. |
|
2. |
CountDownLatch doesn’t have any kind of Constructor. |
CyclicBarrier has a constructor where Runnable instance can be
provided. |
|
3. |
CountDownLatch maintains the count of Tasks. |
CyclicBarrier maintains the count of Threads. |
|
4. |
The only current thread will throw InterruptedException. It will
not impact other threads. |
If one thread is interrupted while waiting then other threads will
throw BrokenBarrierException. |
What is race condition?
-
In a multithreaded environment when more than one thread
tries to access shared resources at the same time.
-
Please refer to the below real-life example to understand
race conditions.
- You are planning to go to a movie at 5 pm. You inquire about the availability of the tickets at 4 pm. The representative says that they are available. You relax and reach the ticket window 5 minutes before the show. I'm sure you can guess what happens: it's a full house. The problem here was in the duration between the check and the action. You inquired at 4 and acted at 5. In the meantime, someone else grabbed the tickets.
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