code: public DirectoryPollerBuilder addListener(Rdp4jListener listener) {
          if (listener == null) {
              throw new NullPointerException(NULL_ARGUMENT_ERROR_MESSAGE);
          }
          listeners.add(listener);
          return this;
      }
docs: Adds the given <code>listener</code> to the list of {@link Rdp4jListener}'s
  that receives notifications.
  <p>
  Once the {@link DirectoryPoller} has been built, it can be used to
  add additional listeners, or remove listeners.
  
  @param listener Implementation of any of the sub-interfaces of {@link Rdp4jListener}
  -interface.
  @return {@link DirectoryPollerBuilder}
  @throws NullPointerException if the given argument is null.