Chapter TEN
Concurrency and Multithreading

Answers

1. The correct answer is C.

Explanation:

• A) Thread thread = Thread.ofVirtual(); thread.start(task);
  • This option is incorrect because Thread.ofVirtual() returns a Thread.Builder, not a Thread. The start() method on Thread.Builder takes a Runnable, but this syntax is incorrect.
• B) Thread thread = Thread.ofVirtual().unstarted(task).run();
  • This option is incorrect because calling run() directly does not start a new thread. It executes the task in the current thread.
• C) Thread thread = Thread.ofVirtual().start(task);
  • This option is correct. It uses the new thread builder API in Java 21 to create and start a virtual thread in one step.
• D) Thread thread = Thread.ofVirtual(); task.run();
  • This option is incorrect because it doesn’t actually start a new thread. It creates a thread builder but doesn’t use it, and then runs the task in the current thread.
• E) Thread thread = Thread.start(task);
  • This option is incorrect because Thread.start(task) is not a valid static method in Java 21.

2. The correct answer is B.

Explanation:

• A) synchronized (this) { counter++; }
  • This option is incorrect because this cannot be used in a static context. In the main method, this is not available. For a static field like counter, you need to synchronize on a static object or class.
• B) synchronized (Main.class) { counter++; }
  • This option is correct because synchronizing on Main.class ensures that only one thread can enter the synchronized block at a time for all instances of Main, which is appropriate for protecting static fields like counter.
• C) synchronized (task) { counter++; }
  • This option is incorrect because task is a Runnable object, and synchronizing on it does not effectively control access to the shared static field counter.
• D) synchronized (counter) { counter++; }
  • This option is incorrect because counter is a primitive type (int), and you cannot synchronize on a primitive type. Synchronization requires an object.
• E) synchronized (System.out) { counter++; }
  • This option is incorrect because synchronizing on System.out is not related to controlling access to counter. It would also interfere with other potential uses of System.out.

3. The correct answers are B and C.

Explanation:

• A) AtomicInteger is part of the java.util.concurrent.atomic package, but it does not provide atomic operations for increment and decrement.
  • This statement is incorrect. AtomicInteger provides atomic operations for increment and decrement, such as incrementAndGet() and decrementAndGet().
• B) AtomicReference can only be used with reference types, not primitive types.
  • This statement is correct. AtomicReference is designed to work with reference types and cannot be used with primitive types directly.
• C) AtomicLong supports atomic operations on long values, including getAndIncrement() and compareAndSet() methods.
  • This statement is correct. AtomicLong provides atomic operations on long values, including getAndIncrement() and compareAndSet() methods.
• D) AtomicBoolean can be used to perform atomic arithmetic operations on boolean values.
  • This statement is incorrect. AtomicBoolean is used for atomic updates to boolean values, but it does not support atomic arithmetic operations.

4. The correct answer is A.

Explanation:

• A)

  lock.lock();
  try {
    count++;
  } finally {
    lock.unlock();
  }
  

  • This option correctly acquires the lock before modifying the shared resource and ensures the lock is released in the finally block, which is the proper use of the Lock interface.
• B)

  lock.lock();
  count++;
  lock.unlock();
  

  • This option is incorrect because if an exception occurs between lock.lock() and lock.unlock(), the lock will not be released, potentially causing a deadlock.
• C)

  try {
    lock.lock(() -> {
        count++;
    });
  } finally {
    lock.unlock();
  }
  

  • This option is incorrect because that’s not a valid lock.lock() call.
• D)

  synchronized(lock) {
    count++;
  }
  

  • This option is incorrect because the synchronized block is used with the lock object itself, which is not the correct usage of the Lock interface and does not provide the intended functionality.

5. The correct answer is A.

Explanation:

• A)

  try (ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor()) {
    executor.submit(() -> System.out.println("Task executed"));
  }
  

  • This option is correct. It uses the try-with-resources block with the new Executors.newVirtualThreadPerTaskExecutor() method introduced in Java 21. The ExecutorService will be automatically closed when the try block exits, eliminating the need for explicit shutdown calls.
• B)

  try (ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor()) {
    executor.submit(() -> System.out.println("Task executed"));
  } finally {
    executor.shutdown();
  }
  

  • This option is incorrect because it unnecessarily calls shutdown() in the finally block. With try-with-resources, the ExecutorService is automatically closed, making the explicit shutdown() call redundant and potentially harmful.
• C)

  ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor();
  try {
    executor.submit(() -> System.out.println("Task executed"));
  } finally {
    executor.close();
  }
  

  • This option is incorrect because it doesn’t use the try-with-resources syntax. While it does correctly close the ExecutorService, it doesn’t take advantage of the automatic resource management provided by try-with-resources.
• D)

  try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    executor.submit(() -> System.out.println("Task executed"));
    executor.awaitTermination(1, TimeUnit.SECONDS);
  }
  

  • This option is incorrect because it unnecessarily calls awaitTermination(). In a try-with-resources block, the ExecutorService is automatically closed when the block exits, making the explicit wait for termination unnecessary and potentially causing the thread to block for 1 second.

6. The correct answer is D.

Explanation:

• A)

  try (ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor()) {
    Future<Integer> future = executor.submit(task);
    System.out.println(future.get());
  }
  

  • This option is incorrect because it doesn’t handle the potential InterruptedException and ExecutionException that future.get() can throw.
• B)

  try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    Future<Integer> future = executor.submit(task);
    Integer result = future.get(1, TimeUnit.SECONDS);
    System.out.println(result);
  }
  

  • This option is incorrect because it doesn’t handle the potential exceptions (InterruptedException, ExecutionException, and TimeoutException) that future.get(long, TimeUnit) can throw.
• C)

  try (ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor()) {
    Future<Integer> future = executor.submit(task);
    executor.shutdown();
    System.out.println(future.get());
  }
  

  • This option is incorrect because it unnecessarily calls executor.shutdown(). In a try-with-resources block, the ExecutorService is automatically closed when the block exits. Also, it doesn’t handle the potential exceptions from future.get().
• D)

  try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    Future<Integer> future = executor.submit(task);
    try {
        Integer result = future.get();
        System.out.println(result);
    } catch (InterruptedException | ExecutionException e) {
        e.printStackTrace();
    }
  }
  

  • This option is correct. It uses try-with-resources to automatically close the ExecutorService, properly submits the Callable task, retrieves the result using Future.get(), and handles the potential InterruptedException and ExecutionException that might be thrown.

7. The correct answer is A.

Explanation:

• A) ConcurrentHashMap allows concurrent read and write operations, and retrieval operations do not block even when updates are being made.
  • This statement is correct. ConcurrentHashMap is designed to handle concurrent access, allowing multiple threads to read and write simultaneously without blocking read operations during updates.
• B) CopyOnWriteArrayList is optimized for scenarios with a high number of write operations compared to read operations.
  • This statement is incorrect. CopyOnWriteArrayList is optimized for scenarios where read operations are far more frequent than write operations because it creates a new copy of the array on each write, which can be costly if writes are frequent.
• C) ConcurrentSkipListSet does not kept elements sorted.
  • This statement is incorrect. ConcurrentSkipListSet keep elements according to their natural ordering, or by a Comparator provided at set creation time.
• D) BlockingQueue implementations like LinkedBlockingQueue allow elements to be added and removed concurrently without any internal locking mechanisms.
  • This statement is incorrect. BlockingQueue implementations like LinkedBlockingQueue do use internal locking mechanisms to handle concurrent access safely.

8. The correct answer is B.

Explanation:

• A) Parallel streams always improve the performance of a program by utilizing multiple threads.
  • This statement is incorrect because parallel streams do not always improve performance. The overhead of managing multiple threads can sometimes outweigh the benefits, especially for small datasets or simple operations.
• B) Parallel streams can lead to incorrect results if the operations performed are not thread-safe.
  • This statement is correct. When using parallel streams, care must be taken to ensure that the operations performed on the elements are thread-safe. Failure to do so can lead to race conditions and incorrect results.
• C) The order of elements in a parallel stream is always preserved compared to the original stream.
  • This statement is incorrect. The order of elements in a parallel stream is not guaranteed to be the same as in the original stream unless special care is taken to preserve the order, such as using ordered stream operations.
• D) Using parallel streams guarantees that the operations on elements will execute in a fixed order.
  • This statement is incorrect because parallel streams do not guarantee the order of execution of operations on elements. The operations may execute in a non-deterministic order due to the concurrent nature of parallel processing.

9. The correct answer is B.

Explanation:

• A)

  int sum = numbers.parallelStream().reduce(1, Integer::sum);
  System.out.println(sum);
  

  • This option is incorrect because it uses 1 as the identity value. The identity value for sum should be 0, as it is the neutral element for addition. Starting the reduction with 1 will result in an incorrect sum that is incremented by 1.
• B)

  int sum = numbers.parallelStream().reduce(0, Integer::sum).collect();
  System.out.println(sum);
  

  • This option is correct. It correctly uses parallelStream() to create a parallel stream and the reduce method with the identity value 0 and the method reference Integer::sum to sum the elements.
• C)

  int sum = numbers.stream().reduce(0, Integer::sum);
  System.out.println(sum);
  

  • This option is incorrect because it uses a sequential stream (stream()) instead of a parallel stream. While it correctly sums the elements, it does not demonstrate the use of a parallel stream as specified in the question.
• D)

  int sum = numbers.parallelStream().collect(reduce(0, Integer::sum));
  System.out.println(sum);
  

  • This option is incorrect because it attempts to use the collect() method in combination with reduce(), which is not the correct syntax. The collect() method is used for mutable reduction and is typically used with collectors, not with the reduce() operation directly.