code: public void addPolledDirectory(PolledDirectory directory) {
          if (directory == null) {
              throw new NullPointerException(NULL_ARGUMENT_ERROR);
          }
          scheduledRunnable.addDirectory(directory);
      }
docs: Adds the given <code>directory</code> into this instance.
  The <code>directory</code> will be polled in the next coming poll-cycle.
  <p>
  Registering an already registered directory will be ignored.
  
  @param directory implementation of {@link PolledDirectory}.
  @throws NullPointerException if the given argument is null.