Match each concurrency utility to its purpose.
Drag a concept onto its matching description — or click a concept then click the description.
Allows one or more threads to wait until a count reaches zero
Allows a set of threads to wait for each other to reach a common barrier point
Controls access to a resource via a permit system
Allows two threads to exchange objects at a synchronization point
A reusable barrier that supports dynamic number of parties
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
CountDownLatch: A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.
This question tests understanding of common concurrency utilities in java.util.concurrent such as CountDownLatch, CyclicBarrier, Semaphore, and Exchanger. A common mistake is confusing CountDownLatch (one-time use) with CyclicBarrier (reusable) and mixing up Semaphore's permit-based control with barrier synchronization.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
CountDownLatch: A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.
Why this is correct
CountDownLatch is used for one-time synchronization where threads wait for a set of operations to finish. It cannot be reused once the count reaches zero.
CyclicBarrier: A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point, and can be reused.
Why this is correct
CyclicBarrier is reusable after the barrier is tripped, making it suitable for multi-phase computations where threads wait at each phase.
Semaphore: A counting semaphore that maintains a set of permits for controlling access to a resource.
Why this is correct
Semaphore controls access to a shared resource via permits. It can be used for limiting concurrent access or signaling.
Exchanger: A synchronization point at which two threads can exchange objects.
Why this is correct
Exchanger allows two threads to swap data at a common rendezvous point, useful for producer-consumer patterns with two threads.
CyclicBarrier: A synchronization aid that allows one or more threads to wait until a set of operations completes.
Why it's wrong here
Incorrect — this describes CountDownLatch, which is one-time use and does not automatically reset. CyclicBarrier is reusable and requires all threads to arrive.
Semaphore: A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point.
Why it's wrong here
Incorrect — this describes CyclicBarrier. Semaphore controls access via permits, not thread rendezvous.
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