code: public static String md5(String key) {
          String cacheKey;
          try {
              final MessageDigest mDigest = MessageDigest.getInstance("MD5");
              mDigest.update(key.getBytes());
              cacheKey = bytesToHexString(mDigest.digest());
          } catch (NoSuchAlgorithmException e) {
              cacheKey = String.valueOf(key.hashCode());
          }
          return cacheKey;
      }
docs: A hashing method that changes a string (like a URL) into a hash string