Threads - General Questions Questions and Answers

Q: Which of these correctly obtains a random number generator intended for concurrent use?

Correct answer: ThreadLocalRandom.current().nextInt(10)ThreadLocalRandom gives each thread its own generator, which removes the contention that a shared Random suffers under concurrency. The instance is reached through the static current(); the class cannot be instantiated directly, and the generating methods are not themselves static.

Q: Which of these correctly declares a stamped lock's optimistic read?

Correct answer: long stamp = sl.tryOptimisticRead();An optimistic read returns a stamp rather than a lock, and no lock is actually taken - the reader does its work and then calls validate(stamp) to find out whether a writer intervened. A zero stamp means the attempt failed, and a failed validation means the work must be redone under a real read lock.

91.

Which of these correctly obtains a random number generator intended for concurrent use?

A.ThreadLocalRandom.nextInt(10)
B.new Random().nextThreadLocalInt(10)
C.ThreadLocalRandom.current().nextInt(10)
D.new ThreadLocalRandom().nextInt(10)

Answer: C

Correct answer: ThreadLocalRandom.current().nextInt(10)

ThreadLocalRandom gives each thread its own generator, which removes the contention that a shared Random suffers under concurrency. The instance is reached through the static current(); the class cannot be instantiated directly, and the generating methods are not themselves static.

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92.

Which of these correctly runs work asynchronously and returns a handle to the eventual result?

A.CompletableFuture<String> f = new CompletableFuture<String>(() -> load());
B.CompletableFuture<String> f = CompletableFuture.runAsync(() -> load());
C.Future<String> f = CompletableFuture.async(() -> load());
D.CompletableFuture<String> f = CompletableFuture.supplyAsync(() -> load());

Answer: D

Correct answer: CompletableFuture<String> f = CompletableFuture.supplyAsync(() -> load());

supplyAsync takes a Supplier and returns a future of its result, which is what is wanted when the work produces a value. runAsync takes a Runnable and gives CompletableFuture<Void>, so it cannot carry the loaded string.

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93.

Which of these is the correct way to count the threads in a ThreadGroup into an array?

A.int n = group.enumerate(threads);
B.int n = group.toArray(threads);
C.Thread[] threads = group.getThreads();
D.int n = ThreadGroup.list(threads);

Answer: A

enumerate copies references into a caller-supplied array and returns how many it wrote. The result is an estimate, because threads may start or finish during the copy, which is why the API does not simply return a fresh array.

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94.

Which of these correctly declares a Thread subclass with a name fixed at construction?

A.class Worker extends Thread { Worker(String n) { super.setName(); } }
B.class Worker extends Thread { Worker() { super("worker"); } }
C.class Worker extends Thread { Worker() { this.name = "worker"; } }
D.class Worker extends Thread { Worker() { Thread("worker"); } }

Answer: B

Thread has a constructor taking a name, so a subclass passes it up with super. The name field is private, a superclass constructor cannot be called like a method, and setName takes an argument. setName may also be used after construction.

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95.

Which of these declarations makes a field visible to other threads without making compound updates atomic?

A.synchronized int count;
B.static int count;
C.volatile int count;
D.final int count = 0;

Answer: C

volatile guarantees that a write is seen by later reads on other threads, which is exactly what a plain or static field does not promise. It says nothing about a read-modify-write pair, and the keyword cannot be applied to a field in place of synchronized, which is not legal there at all.

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96.

Which of these is the correct signature of Object.wait with a timeout in milliseconds and nanoseconds?

A.public final void wait(long timeout, long nanos) throws InterruptedException
B.public final boolean wait(long timeout, int nanos)
C.public void wait(int timeout, int nanos) throws InterruptedException
D.public final void wait(long timeout, int nanos) throws InterruptedException

Answer: D

The milliseconds argument is a long and the nanoseconds a small int, and the method is final and void. Returning a boolean would have let a caller distinguish a timeout from a notification, which this API deliberately does not offer.

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97.

Which of these types is generic?

A.ThreadLocal
B.Thread
C.ThreadGroup
D.InterruptedException

Answer: A

ThreadLocal is declared with a type parameter, so its get and set exchange that type without a cast. Thread, ThreadGroup and InterruptedException carry no type parameter, which is why a Thread cannot be declared to produce a particular result type.

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98.

Which of these correctly declares a phaser for three registered parties?

A.Phaser p = new Phaser(3, TimeUnit.SECONDS);
B.Phaser p = new Phaser(3);
C.Phaser p = new Phaser();
D.Phaser p = Phaser.withParties(3);

Answer: B

The constructor takes the initial party count, and unlike CyclicBarrier that count can change afterwards with register and arriveAndDeregister. The no-argument form exists but starts with none registered, and there is no static factory.

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99.

Which of these correctly declares a stamped lock's optimistic read?

A.long stamp = sl.readLock().tryOptimistic();
B.Lock l = sl.tryOptimisticRead();
C.long stamp = sl.tryOptimisticRead();
D.boolean ok = sl.tryOptimisticRead();

Answer: C

Correct answer: long stamp = sl.tryOptimisticRead();

An optimistic read returns a stamp rather than a lock, and no lock is actually taken - the reader does its work and then calls validate(stamp) to find out whether a writer intervened. A zero stamp means the attempt failed, and a failed validation means the work must be redone under a real read lock.

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