Threads - General Questions Questions and Answers

Answer: C

Correct answer: CountDownLatch latch = new CountDownLatch(3);

The constructor takes the initial count, and the latch opens when countDown() has been called that many times. There is no factory method and no timeout in the constructor - a timeout belongs to the await call, where waiting actually happens.

Answer: D

Condition deliberately renames the operations to await and signal, so that code holding a Lock cannot accidentally call the Object versions, which need a monitor rather than a Lock. acquire and release belong to Semaphore.

Answer: A

The lock is taken before the try so that the finally only runs when it was actually acquired; acquiring inside the try would attempt an unlock even if lock itself threw. ReentrantLock uses lock and unlock, and the synchronized statement does not drive it at all.

Answer: B

put and take are the forms that wait for space or for an element. add and remove throw when they cannot proceed, and offer and poll return a value or null instead of waiting. All three pairs exist, which is why the names matter.

Answer: C

Correct answer: ReentrantLock

ReentrantLock lives in java.util.concurrent.locks, along with ReadWriteLock, Condition and StampedLock. The others are directly in java.util.concurrent, which is why importing that package alone leaves the locks unavailable.

Answer: D

Future exposes get to collect the outcome, cancel to request that the task stop, and the pair isDone and isCancelled to query it without blocking. The names come from the interface rather than from Thread, so run and stop do not appear.

Answer: A

Correct answer: sem.acquire();

Semaphore uses acquire() and release(), and acquire() blocks until a permit is available or the thread is interrupted. tryAcquire() is the non-blocking form. The lock, take and enter vocabularies belong to other types entirely.

Answer: B

newSingleThreadExecutor takes no arguments and gives a one-thread service. newFixedThreadPool requires a count, newCachedThreadPool takes none and grows on demand, and ThreadPoolExecutor's constructors need several arguments rather than one.

Answer: C

Runnable is an interface with one method that declares no checked exception, which is exactly why run cannot throw one and why Callable was added for tasks that need to. It extends nothing, and Thread implements it rather than the other way round.

Answer: D

Thread implements Runnable, so a Thread object satisfies the Runnable parameter and the outer thread will call the inner one's run on its own thread of execution. It compiles regardless of whether Worker overrides anything, since Thread supplies an inherited run.