private static String checkCollectionSizes(int size1, int size2) {
    if (size2 == 0) {
      return null; // for cases when an empty collection is cached and then filled lazily on request
    }
    if (size1 != size2) {
      return reportProblem(size1, size2);
    }
    return null;
  }
private static String checkCollectionSizes(int size1, int size2)