Which of these correctly declares a thread pool that reuses idle threads and creates new ones as needed?
Answer: C
newCachedThreadPool takes no size argument, which is the point: the pool grows on demand and retires idle threads. A bounded pool is what newFixedThreadPool provides, and there is no public CachedThreadPool class or newElasticPool method.
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Which of these is a legal use of the volatile keyword?
Answer: D
Correct answer: private volatile boolean running;
volatile applies only to fields. It cannot qualify a method or a class, and it cannot be combined with final - a final field never changes, so there is nothing for the visibility guarantee to do. A volatile boolean flag is its commonest legitimate use.
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Which of these correctly declares a barrier for four parties?
Answer: A
The constructor takes the number of parties, with an optional Runnable to run when the barrier trips. There is no no-argument form, no static factory, and a TimeUnit belongs to the await call rather than to construction.
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Which of these is NOT a value of the TimeUnit enum?
Answer: B
TimeUnit runs from NANOSECONDS through MICROSECONDS, MILLISECONDS and SECONDS up to DAYS. There is no centisecond constant. Using a constant that does not exist is caught at compile time, which is part of why the enum is preferred to a bare millisecond count.
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Which of these correctly declares a map safe for concurrent use?
Answer: C
ConcurrentMap is the interface and ConcurrentHashMap the implementation, so the interface is used as the variable type and the class is constructed. Neither Map nor HashMap offers a method to convert an existing map in place.
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A pool needs its worker threads named and marked as daemons. Which method does it call on the ThreadFactory it was given?
Answer: D
A ThreadFactory turns a Runnable into a Thread, so it takes the task and returns the thread, which lets a pool name its threads or mark them as daemons consistently. Returning void would leave the pool with nothing to start.
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Which of these correctly submits work that returns no result?
Answer: A
ExecutorService accepts a Runnable through submit, returning a Future of Object that completes with null, and through execute, which returns nothing. There is no run or start method on the interface; those belong to Runnable and Thread.
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Which of these is the correct declaration for a read-write lock?
Answer: B
ReadWriteLock is the interface and ReentrantReadWriteLock the only implementation in the API. It is not a ReentrantLock, so the two are not assignable to each other, and its read and write locks are obtained afterwards with readLock and writeLock.
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Which of these correctly declares a task that both returns a value and may throw a checked exception?
Answer: C
Only Callable's method is declared to throw Exception, so a lambda body that calls something with a checked exception fits it and nothing else here. A Runnable returns void, a raw Callable loses the type, and Supplier declares no exception.
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Which of these is the correct package declaration for importing every concurrency utility including the locks?
Answer: D
Correct answer: import java.util.concurrent.*; import java.util.concurrent.locks.*;
A wildcard import covers one package and not its subpackages, so java.util.concurrent.* does not bring in the locks. Two imports are needed, and there is no double-asterisk form in Java for reaching a package tree.
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