code: public void addTrackPositionListener(int player, TrackPositionListener listener) {
          listenerPlayerNumbers.put(listener, player);
          TrackPositionUpdate currentPosition = positions.get(player);
          if (currentPosition !=  null) {
              listener.movementChanged(currentPosition);
              trackPositionListeners.put(listener, currentPosition);
          } else {
              trackPositionListeners.put(listener, NO_INFORMATION);
          }
      }
docs: Add a listener that wants to closely follow track playback for a particular player. The listener will be called
  as soon as there is an initial {@link TrackPositionUpdate} for the specified player, and whenever there is an
  unexpected change in playback position, speed, or state on that player.
  
  @param player the player number that the listener is interested in
  @param listener the interface that will be called when there are changes in track playback on the player