private static String checkCollectionElements(@NotNull Collection<?> existing, @NotNull Collection<?> fresh) {
    if (fresh.isEmpty()) {
      return null; // for cases when an empty collection is cached and then filled lazily on request
    }
    return checkArrayEquivalence(existing.toArray(), fresh.toArray(), existing);
  }
private static String checkCollectionElements(@NotNull Collection<?> existing, @NotNull Collection<?> fresh)