Java Concurrency Flashcards

Which method of the `CountDownLatch` class blocks the calling thread until the count reaches zero?

Answer: await()

`await()` blocks the current thread until the latch's count reaches zero, while `countDown()` decrements the count.

What happens when a thread calls `CyclicBarrier.await()` and it is the last thread to arrive at the barrier?

Answer: It triggers the optional barrier action and releases all waiting threads

The last thread to call `await()` optionally runs the barrier action, then all waiting threads are released.

Which `ExecutorService` method submits a `Callable` and returns a `Future` representing the pending result?

Answer: submit(Callable)

`submit(Callable)` accepts a `Callable` and returns a `Future` that can be used to retrieve the result.

What is the purpose of the `volatile` keyword in Java concurrency?

Answer: Ensures visibility of variable changes across threads without atomicity guarantees

`volatile` guarantees that reads/writes go directly to main memory, ensuring visibility, but does not provide atomicity for compound operations.

Which class in `java.util.concurrent` provides a resizable-array implementation of a blocking queue?

Answer: LinkedBlockingQueue

`LinkedBlockingQueue` is backed by linked nodes and can optionally be bounded, while `ArrayBlockingQueue` uses a fixed-size array.

What does `Future.cancel(true)` do to a running task?

Answer: Attempts to interrupt the thread executing the task

Passing `true` to `cancel()` sends an interrupt signal to the thread running the task, but the task must check for interruption to stop.

Which `ReentrantLock` feature allows a thread to acquire the lock only if it is not held by another thread at the time of invocation?

Answer: tryLock()

`tryLock()` returns `true` immediately if the lock is available or `false` if another thread holds it, without blocking.