Java Concurrency Flashcards

Which `Phaser` method allows a thread to arrive at a phase and deregister itself from future phases?

Answer: arriveAndDeregister()

`arriveAndDeregister()` records arrival and reduces the registered party count, so the thread no longer participates in subsequent phases.

What distinguishes `ScheduledThreadPoolExecutor` from a regular `ThreadPoolExecutor`?

Answer: It can schedule commands to run after a delay or periodically

`ScheduledThreadPoolExecutor` adds the ability to schedule tasks with fixed-delay, fixed-rate, or one-shot delay execution.

When using `synchronized` on an instance method, which object is used as the monitor lock?

Answer: The instance (`this`) on which the method is called

An instance `synchronized` method acquires the intrinsic lock on `this`, the object the method is invoked on.

Which scenario can cause a `ConcurrentModificationException` in a non-concurrent collection?

Answer: Modifying a collection while iterating over it with a for-each loop

Structurally modifying a collection outside of the iterator's own `remove()` during iteration triggers a `ConcurrentModificationException`.

What is the key difference between `ReentrantLock` and `ReentrantReadWriteLock`?

Answer: ReadWriteLock allows multiple concurrent readers but only one writer at a time

`ReentrantReadWriteLock` separates read and write locks, allowing concurrent reads but exclusive writes for better throughput in read-heavy workloads.

Which `CompletableFuture` method combines two futures and applies a function to both results when both complete?

Answer: thenCombine()

`thenCombine()` waits for both `CompletableFuture`s to complete and applies a `BiFunction` to their results, returning a new `CompletableFuture`.

What is the effect of calling `LockSupport.park()` on a thread that already has a pending unpark permit?

Answer: It returns immediately without blocking

If a permit is available from a prior `unpark()`, `park()` consumes it and returns immediately without blocking.