第 55 条：谨慎返回Optional

// Returns maximum value in collection as an Optional<E>
public static <E extends Comparable<E>> Optional<E> max(Collection<E> c) { 
  if (c.isEmpty())
    return Optional.empty();
  E result = null; 
  for (E e : c)
    if (result == null || e.compareTo(result) > 0) 
      result = Objects.requireNonNull(e);
  return Optional.of(result); 
}