code: public String hash(String key) {
          if (digester != null) {
              byte[] bytes = digester.digest(key.getBytes());
              return bytesToHexString(bytes);
          } else {
              return Integer.toHexString(key.hashCode());
          }
      }
docs: A hashing method that changes a string (like a URL) into a hash suitable
  for using as a disk filename.