Android : How to change subtitle in mediaplayer

I can assign subtitle once but after that if I tried to change subtitle. It won't accept new one, it display previous one every-time.

Below code I'm using to change subtitle.

 MediaPlayer mediaPlayer;

    public void changeSubtitle()
    {
    //disable subtitle if it has
    if(textTrackIndex!=0){    
    mediaPlayer.deselectTrack(textTrackIndex);
    textTrackIndex=0;
    mediaPlayer.setOnTimedTextListener(null);
    }

    //try to asssign new subtitle

                mediaPlayer
                        .addTimedTextSource(
                                (Environment.getExternalStorageDirectory()
                                        .getPath()
                                        + "/"+"filename",
                                MediaPlayer.MEDIA_MIMETYPE_TEXT_SUBRIP);
    int textTrackIndex = findTrackIndexFor(TrackInfo.MEDIA_TRACK_TYPE_TIMEDTEXT,mediaPlayer.getTrackInfo());

            if (textTrackIndex >= 0) {
                mediaPlayer.selectTrack(textTrackIndex);
            } else {
                Log.w(TAG, "Cannot find text track!");
            }
            mediaPlayer.setOnTimedTextListener(this);
    }

    private int findTrackIndexFor(int mediaTrackType, TrackInfo[] trackInfo) 
    {
            int index = -1;
            for (int i = 0; i < trackInfo.length; i++) {
                if (trackInfo[i].getTrackType() == mediaTrackType) {
                    return i;
                }
            }
            return index;
        }