I am learning Android with Udacity nanodegree (having no support) and this is my assignment of music app. It is just an interface and no music I have to add. So far so the app is running without any error but I wold like to get expert feedback to write code in better and optimized way to achieve the same result.

My entire project is in my Bitbucket repo.

SongAdapter.java

public class SongAdapter extends ArrayAdapter<Song> {



    int     playingIndex = -1;

    boolean currentTrack = false;



    public SongAdapter(@NonNull Context context, int resource, @NonNull List<Song> objects) {

        super(context, resource, objects);

    }



    @NonNull

    @Override

    public View getView(final int position, @Nullable View convertView, @NonNull ViewGroup parent) {



        View             playlistItemView = convertView;

        final ViewHolder holder;



        if (playlistItemView == null) {

            playlistItemView = LayoutInflater.from(getContext()).inflate(R.layout.playlist_item, parent, false);



            holder = new ViewHolder();



            holder.albumCoverThumbnail = playlistItemView.findViewById(R.id.playlist_album_thumbnail);

            holder.songTitle = playlistItemView.findViewById(R.id.playlist_song_title);

            holder.songAlbumTitle = playlistItemView.findViewById(R.id.playlist_song_album_title);

            holder.songArtist = playlistItemView.findViewById(R.id.playlist_song_artist);

            holder.songPlayButton = playlistItemView.findViewById(R.id.playlist_play_button);



            playlistItemView.setTag(holder);

        } else {

            holder = (ViewHolder) playlistItemView.getTag();

        }



        final Song currentSong = getItem(position);



        // set data to the list item

        assert currentSong != null;

        holder.albumCoverThumbnail.setImageResource(currentSong.getSongAlbumCoverId());

        holder.songTitle.setText(currentSong.getSongTitle());

        holder.songAlbumTitle.setText(currentSong.getSongAlbumTitle());

        holder.songArtist.setText(currentSong.getSongSingers());



        // check the play status of the song item

        if (playingIndex == position) {

            holder.songPlayButton.setImageResource(R.drawable.ic_pause_black_24dp);

        } else {

            holder.songPlayButton.setImageResource(R.drawable.ic_play_arrow_black_24dp);

        }



        // set song button action

        holder.songPlayButton.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View v) {



                holder.songPlayButton.setImageResource(R.drawable.ic_pause_black_24dp);



                // pause current song on other click

                if (position == playingIndex) {

                    playingIndex = -1;

                } else {

                    playingIndex = position;

                    notifyDataSetChanged();

                }



                // pause play current track

                if (currentTrack) {

                    holder.songPlayButton.setImageResource(R.drawable.ic_play_arrow_black_24dp);

                    currentTrack = !currentTrack;

                } else {

                    holder.songPlayButton.setImageResource(R.drawable.ic_pause_black_24dp);

                }

                currentTrack = !currentTrack;

            }

        });



        return playlistItemView;

    }



    static class ViewHolder {

        ImageView   albumCoverThumbnail;

        TextView    songTitle;

        TextView    songAlbumTitle;

        TextView    songArtist;

        ImageButton songPlayButton;

    }

}