Java OCP Concurrency and Multithreading

1. What does an AtomicInteger provide that a regular int does not?

Atomic operations without explicit synchronization

Better memory efficiency

Automatic null handling

Type safety for multi-threaded access

2. Which exception is thrown if a thread is interrupted while in wait()?

InterruptedException

ThreadInterruptedException

IllegalMonitorStateException

ConcurrentModificationException

3. A synchronized method is equivalent to synchronizing on which object?

This (the current object instance)

The Class object

A static lock variable

The thread itself

4. When a thread is in WAITING state, what must happen for it to progress?

Another thread must notify() or notifyAll()

The timeout expires automatically

The JVM scheduler resumes it

Thread.interrupt() is called

5. Which Semaphore constructor argument controls the number of permits?

The single int parameter

The second boolean parameter

Configured after instantiation only

Semaphore uses a fixed 10 permits

6. Which method of ExecutorService waits for all submitted tasks to complete?

Submit()

Execute()

AwaitTermination()

InvokeAll()

7. Which interface must a class implement to be eligible for execution by a Thread?

Runnable

Callable

Executor

ThreadFactory

8. Which collection is safe for concurrent iteration without external synchronization?

ArrayList

LinkedList

CopyOnWriteArrayList

HashSet

9. What is the purpose of a ReadWriteLock?

Allow multiple readers concurrently, exclusive writer access

Prevent all concurrent access

Replace synchronized blocks entirely

Improve performance by reducing thread creation

10. What is the result of calling notifyAll() when no threads are waiting?

An exception is thrown

The method does nothing

All threads in the JVM are interrupted

The calling thread enters BLOCKED state

11. A thread calls wait() on an object. Which thread can wake it with notify()?

Any thread that holds the same object's lock

Only the thread that called wait()

The main thread only

Any thread in the JVM

12. Which class provides thread-safe operations without explicit synchronization?

HashMap

ArrayList

ConcurrentHashMap

Vector

13. What does the volatile keyword guarantee in Java?

Atomicity of all operations

Visibility of changes across threads

Mutual exclusion like synchronized

Prevention of thread context switching

14. What is the primary difference between Runnable and Callable?

Callable can return a value and throw checked exceptions

Runnable is faster than Callable

Callable requires a Thread object to execute

Runnable cannot be used in ExecutorService

15. A CountDownLatch initialized with count 3. How many times must countDown() be called?

Once (the latch counts internally)

Three times

Depends on thread count

Unlimited until await() returns

16. Which method transitions a thread from RUNNABLE to BLOCKED state?

Thread.sleep()

Synchronized block entry when lock unavailable

Thread.yield()

Wait() without timeout

17. Which Lock interface method does NOT acquire the lock if unavailable?

Lock()

TryLock()

LockInterruptibly()

All methods acquire the lock

18. What is the key difference between CountDownLatch and CyclicBarrier?

Latch is reusable; Barrier is one-time use

Barrier is reusable; Latch cannot be reset

Latch requires equal thread participation

Barrier only works with Executor framework

19. What does a Phaser control that CountDownLatch and CyclicBarrier do not?

Multiple phases of synchronization

Thread pool size

Lock fairness

Timeout behavior

20. What does the ForkJoinPool optimize for compared to ExecutorService?

Divide-and-conquer recursive tasks

Single long-running background tasks

I/O-bound blocking operations

Scheduled delayed execution

Oracle Java OCP Concurrency and Multithreading Quiz