배열에 음악 파일을 add를 통해서 넣고 리스트를 만든다.

아래 코드는 stackoverflow에 있는 해당 코드를 그대로 가져왔다.  코드를 참고해서 만들면 될 듯 하다.

import android.media.MediaPlayer;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.Menu;
import android.view.MenuItem;

import java.util.ArrayList;
import java.util.Timer;
import java.util.TimerTask;


public class MainActivity extends AppCompatActivity {

    Timer timer;
    MediaPlayer mp;
    ArrayList<Integer> playlist;
    int i=0;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        playlist = new ArrayList<>();
        playlist.add(R.raw.a1);
        playlist.add(R.raw.a2);
        mp = MediaPlayer.create(this,playlist.get(0));
        mp.start();
        timer = new Timer();
        if (playlist.size()>1) playNext();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    public void playNext() {
        timer.schedule(new TimerTask() {
            @Override
            public void run() {
                mp.reset();
                mp = MediaPlayer.create(MainActivity.this,playlist.get(++i));
                mp.start();
                if (playlist.size() > i+1) {
                    playNext();
                }
            }
        },mp.getDuration()+100);
    }

    @Override
    public void onDestroy() {
        if (mp.isPlaying())
            mp.stop();
        timer.cancel();
        super.onDestroy();
    }
}

https://stackoverflow.com/questions/30681002/play-list-of-mp3-file-with-mediaplayer-in-android