code: public void cleanDirectory() {
          final File[] directories = _parentDirectory.listFiles((dir, name) -> name.startsWith(DIRECTORY_PREFIX));
          for (final File directory : directories) {
              delete(directory);
          }
      }
docs: Cleans the parent directory of this storage provider. This action will
  delete all previous collection storages made in this directory, and thus
  it should only be invoked either before any collections has been made or
  when all collections are ensured to be unused.