code: public static String approximateContentHash(long length, long modTime) {
      // approximating the content hash with the file length and modtime.
      StringBuilder sb = new StringBuilder();
      sb.append('(');
      sb.append("len:");
      sb.append(length);
      sb.append(", ");
      sb.append("modtime:");
      sb.append(modTime);
      sb.append(')');
      return sb.toString();
    }
docs: Returns an approximate content hash, using the length and modification time.
  
  @param length the content length
  @param modTime the content last modification time
  @return the content hash