code: @Override
      public long lastModified() throws IOException {
          long lastModified = getFileForLastModifiedCheck().lastModified();
          if (lastModified == 0L) {
              throw new FileNotFoundException(
                  getDescription() + " cannot be resolved in the file system for resolving its last-modified timestamp");
          }
          return lastModified;
      }
docs: This implementation checks the timestamp of the underlying File, if
  available.
  
  @see #getFileForLastModifiedCheck()