if (forwardComparator == null) {
        return (Comparator) Ordering.natural().reverse();
      } else {
        return reverse(forwardComparator);
      }
    }

    // If we inline this, we get a javac error.
    private static <T extends @Nullable Object> Ordering<T> reverse(Comparator<T> forward) {
      return Ordering.from(forward).reverse();
    }

    @Override

     *
     * @param <E> the element type of {@link PriorityBlockingQueue}
     * @param initialCapacity the initial capacity of the priority queue
     * @param comparator the comparator for ordering the elements in the priority queue
     * @return a new instance of {@link PriorityBlockingQueue}
     * @see PriorityBlockingQueue#PriorityBlockingQueue(int, Comparator)
     */

   * across different segments.
   *
   * The page replacement algorithm's data structures are kept casually consistent with the map. The
   * ordering of writes to a segment is sequentially consistent. An update to the map and recording
   * of reads may not be immediately reflected on the algorithm's data structures. These structures

   * computation is {@linkplain java.util.concurrent.Future#isDone() complete} or, if the
   * computation is already complete, immediately.
   *
   * <p>The callback is run on {@code executor}. There is no guaranteed ordering of execution of
   * callbacks, but any callback added through this method is guaranteed to be called once the
   * computation is complete.
   *