// That means that it is possible to observe that the future isDone and that your listeners
    // don't execute 'immediately'.  By checking isDone here we avoid that.
    // A corollary to all that is that we don't need to check isDone inside the loop because if we
    // get into the loop we know that we weren't done when we entered and therefore we aren't under
    // an obligation to execute 'immediately'.
    if (!isDone()) {

          // there's a way around creating the separate sortedKeys array, and if we're allocating
          // one array of size n, we might as well allocate two -- to say nothing of the allocation
          // done in Arrays.sort.
          for (int i = 0; i < size; i++) {
            // We're careful to put only K instances in.

    long timeoutNanos = unit.toNanos(timeout);

    // For efficiency, especially in executors with limited
    // parallelism, check to see if previously submitted tasks are
    // done before submitting more of them. This interleaving
    // plus the exception mechanics account for messiness of main
    // loop.

    try {
      // Record exceptions so that if we fail to obtain any

   *
   * @throws IllegalArgumentException if the stream consists of two or more elements.
   * @throws NullPointerException if any element in the stream is {@code null}.
   * @return {@code Optional.of(onlyElement)} if the stream has exactly one element (must not be
   *     {@code null}) and returns {@code Optional.empty()} if it has none.
   */
  @SuppressWarnings("unchecked")

   * Returns a multimap containing the mappings in {@code unfiltered} whose keys satisfy a
   * predicate. The returned multimap is a live view of {@code unfiltered}; changes to one affect
   * the other.
   *
   * <p>The resulting multimap's views have iterators that don't support {@code remove()}, but all
   * other methods are supported by the multimap and its views. When adding a key that doesn't

 * java.util.concurrent}, {@code java.util.regex}, {@code com.google.common.base}, {@code
 * com.google.common.collect} and {@code com.google.common.primitives}. In addition, if the type
 * exposes at least one public static final constant of the same type, one of the constants will be
 * used; or if the class exposes a public parameter-less constructor then it will be "new"d and
 * returned.
 *

 * java.util.concurrent}, {@code java.util.regex}, {@code com.google.common.base}, {@code
 * com.google.common.collect} and {@code com.google.common.primitives}. In addition, if the type
 * exposes at least one public static final constant of the same type, one of the constants will be
 * used; or if the class exposes a public parameter-less constructor then it will be "new"d and
 * returned.
 *