implement the basic functionality

app/src/main/java/musicplayer/simplemobiletools/com/MainActivity.java

@@ -61,12 +61,15 @@ public class MainActivity extends AppCompatActivity {
 
     private void songPicked(int pos) {
         musicService.setSong(pos, true);
-        updateSongInfo(songs.get(pos));
+        updateSongInfo(musicService.getCurrSong());
+        playPauseBtn.setImageDrawable(getResources().getDrawable(R.mipmap.pause));
     }
 
     private void updateSongInfo(Song song) {
-        titleTV.setText(song.getTitle());
-        artistTV.setText(song.getArtist());
+        if (song != null) {
+            titleTV.setText(song.getTitle());
+            artistTV.setText(song.getArtist());
+        }
     }
 
     private void getSortedSongs() {
@@ -129,24 +132,85 @@ public class MainActivity extends AppCompatActivity {
     public void previousClicked() {
         if (isPlaylistEmpty())
             return;
+
+        playPreviousSong();
     }
 
     @OnClick(R.id.playPauseBtn)
     public void playPauseClicked() {
         if (isPlaylistEmpty())
             return;
+
+        resumePauseSong();
     }
 
     @OnClick(R.id.nextBtn)
     public void nextClicked() {
         if (isPlaylistEmpty())
             return;
+
+        playNextSong();
     }
 
     @OnClick(R.id.stopBtn)
     public void stopClicked() {
         if (isPlaylistEmpty())
             return;
+
+        stopMusic();
+    }
+
+    public void stopMusic() {
+        if (musicService != null) {
+            musicService.stopMusic();
+            playPauseBtn.setImageDrawable(getResources().getDrawable(R.mipmap.play));
+        }
+    }
+
+    public void resumePauseSong() {
+        if (songs.isEmpty() || musicService == null)
+            return;
+
+        if (musicService.isPlaying()) {
+            pauseSong();
+        } else {
+            resumeSong();
+        }
+
+        // in case we just launched the app and pressed play, also update the song and artist name
+        if (artistTV.getText().toString().trim().isEmpty())
+            updateSongInfo(musicService.getCurrSong());
+    }
+
+    private void resumeSong() {
+        playPauseBtn.setImageDrawable(getResources().getDrawable(R.mipmap.pause));
+        musicService.resumePlayer();
+    }
+
+    private void pauseSong() {
+        if (musicService == null)
+            return;
+
+        playPauseBtn.setImageDrawable(getResources().getDrawable(R.mipmap.play));
+        musicService.pausePlayer();
+    }
+
+    private void playPreviousSong() {
+        if (songs.isEmpty() || musicService == null)
+            return;
+
+        musicService.playPrevious();
+        playPauseBtn.setImageDrawable(getResources().getDrawable(R.mipmap.pause));
+        updateSongInfo(musicService.getCurrSong());
+    }
+
+    private void playNextSong() {
+        if (songs.isEmpty() || musicService == null)
+            return;
+
+        musicService.playNext();
+        playPauseBtn.setImageDrawable(getResources().getDrawable(R.mipmap.pause));
+        updateSongInfo(musicService.getCurrSong());
     }
 
     private boolean isPlaylistEmpty() {

app/src/main/java/musicplayer/simplemobiletools/com/MusicService.java

@@ -44,10 +44,6 @@ public class MusicService extends Service
         player.setOnErrorListener(this);
     }
 
-    public void playNext() {
-        setSong(getNewSongId(), true);
-    }
-
     private int getNewSongId() {
         final int cnt = songs.size();
         if (cnt == 0) {
@@ -65,6 +61,50 @@ public class MusicService extends Service
         }
     }
 
+    public boolean isPlaying() {
+        return player != null && player.isPlaying();
+    }
+
+    public void playPrevious() {
+        if (player == null)
+            initMediaPlayer();
+
+        // remove the latest song from the list
+        if (playedSongIDs.size() > 1) {
+            playedSongIDs.remove(playedSongIDs.size() - 1);
+            setSong(playedSongIDs.get(playedSongIDs.size() - 1), false);
+        }
+    }
+
+    public void pausePlayer() {
+        if (player == null)
+            initMediaPlayer();
+
+        player.pause();
+    }
+
+    public void resumePlayer() {
+        if (player == null)
+            initMediaPlayer();
+
+        if (currSong == null)
+            playNext();
+        else
+            player.start();
+    }
+
+    public void playNext() {
+        setSong(getNewSongId(), true);
+    }
+
+    public void stopMusic() {
+        if (player != null) {
+            // .stop() seems to misbehave weirdly
+            player.pause();
+            player.seekTo(0);
+        }
+    }
+
     public void setSong(int songId, boolean addNewSong) {
         if (player == null)
             initMediaPlayer();
@@ -85,6 +125,10 @@ public class MusicService extends Service
         player.prepareAsync();
     }
 
+    public Song getCurrSong() {
+        return currSong;
+    }
+
     @Nullable
     @Override
     public IBinder onBind(Intent intent) {

app/src/main/java/musicplayer/simplemobiletools/com/Song.java

@@ -22,4 +22,13 @@ public class Song {
     public String getTitle() {
         return title;
     }
+
+    @Override
+    public String toString() {
+        return "Song{"
+                + "id=" + getId()
+                + ", title=" + getTitle()
+                + ", artist=" + getArtist()
+                + "}";
+    }
 }

app/src/main/res/layout/activity_main.xml

@@ -54,7 +54,6 @@
         android:layout_marginRight="@dimen/activity_margin"
         android:layout_marginTop="@dimen/activity_margin"
         android:gravity="center_horizontal"
-        android:text="Song Title"
         android:textSize="18sp"/>
 
     <TextView
@@ -64,7 +63,6 @@
         android:layout_below="@+id/songTitle"
         android:layout_marginRight="@dimen/activity_margin"
         android:gravity="center_horizontal"
-        android:text="Song Artist"
         android:textSize="16sp"/>
 
     <ListView