MainActivity.java

package com.example.android.musicalstructureapp;

import android.media.MediaPlayer;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.ListAdapter;
import android.widget.ListView;

import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.List;


public class MainActivity extends AppCompatActivity {

    ListView listView;

    List<String> list;

    ListAdapter adapter;

    MediaPlayer mediaPlayer;

    ImageView previousButton;

    ImageView playButton;

    ImageView pauseButton;

    ImageView nextButton;

    int resID;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        previousButton = (ImageView) findViewById(R.id.imageViewPrevious);

        playButton = (ImageView) findViewById(R.id.imageViewPlay);

        pauseButton = (ImageView) findViewById(R.id.imageViewPause);

        nextButton = (ImageView) findViewById(R.id.imageViewNext);

        listView = (ListView) findViewById(R.id.listView);

        list = new ArrayList<>();

        Field[] fields = R.raw.class.getFields();
        for(int i = 0; i < fields.length; i++ ){
            list.add(fields[i].getName());
        }


        adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, list);
        listView.setAdapter(adapter);

        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
                if(mediaPlayer != null){
                    mediaPlayer.release();
                }

                resID = getResources().getIdentifier(list.get(i), "raw", getPackageName());
                mediaPlayer = MediaPlayer.create(MainActivity.this, resID);
                mediaPlayer.start();
            }
        });

        pauseButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mediaPlayer.pause();
            }
        });

        playButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mediaPlayer.start();
            }
        });

        nextButton.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick(View v) {
                resID += 1;
                mediaPlayer.start();
            }
        });

        previousButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                
                mediaPlayer.start();
            }
        });
        
        list.remove(0);
        list.remove(3);
    }
}