请帮助我解决一些问题。我不是Java专业人士，也无法理解为什么该代码会导致崩溃。我可以看到一些整数变为空，但是再次，我不明白为什么。 感谢您的关注。对不起，我的英语。
以下是崩溃日志：
java.lang.NullPointerException：尝试在空对象引用上调用接口方法'java.lang.Object java.util.List.get（int）' 在my.package.mpservice.MediaPlayerService $ 1.run（MediaPlayerService.java:143）
这是活动代码：
package my.package.mpservice;
import android.app.Service;
import android.content.Intent;
import android.media.AudioManager;
import android.media.MediaPlayer;
import android.media.MediaPlayer.OnPreparedListener;
import android.media.MediaPlayer.OnCompletionListener;
import android.media.MediaPlayer.OnErrorListener;
import android.os.Binder;
import android.os.IBinder;
import android.os.PowerManager;
import android.support.annotation.Nullable;
import com.google.common.base.Preconditions;
import org.greenrobot.eventbus.EventBus;
import org.greenrobot.eventbus.Subscribe;
import org.greenrobot.eventbus.ThreadMode;
import java.io.IOException;
import java.util.List;
import java.util.Random;
import my.package.constants.Constants;
import my.package.constants.Constants.State;
import my.package.listeners.listview.OnItemClickListener;
import my.package.models.ITrack;
public class MediaPlayerService extends Service implements OnPreparedListener, OnCompletionListener, OnErrorListener {
private List<ITrack> mTracks;
private int mPosition = -1;
private Thread mThread;
private final IBinder mBinder = new OnBinder();
private MediaPlayer mPlayer;
public State mState = State.PREPARING;
private Boolean isRepeat = false;
private Boolean isShuffle = false;
public class OnBinder extends Binder {
MediaPlayerService getService() {
return MediaPlayerService.this;
}
}
@SuppressWarnings("unused")
@Subscribe(threadMode = ThreadMode.BACKGROUND)
public void onEventBusGetTracks(@Nullable final List<ITrack> tracks) {
if(tracks != null){
mTracks = tracks;
}
}
private void setUpTrackToPlay(Intent intent){
// if(mPosition != Integer.parseInt(intent.getStringExtra(Constants.TRACK_POSITION))){
mPosition = Integer.parseInt(intent.getStringExtra(Constants.TRACK_POSITION));
playTrack(mPosition);
// }
if(mPosition != -1){
// if mPosition is different to -1 that means there's a track which is playing,
// so shows the FooterPlayerView when the user clicks on a item or the device orientation changes.
EventBus.getDefault().post(mTracks.get(mPosition));
}
}
@Override
public void onCreate() {
EventBus.getDefault().register(this);
}
@Override
public void onDestroy() {
EventBus.getDefault().unregister(this);
releaseMediaPlayer();
}
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
if(intent != null){
setUpTrackToPlay(intent);
}
return START_REDELIVER_INTENT; // Do not recreate the service if it gets killed.
}
@Override
public IBinder onBind(Intent intent) { return mBinder; }
@Override
public void onPrepared(MediaPlayer mp) {
mp.start();
EventBus.getDefault().post(mTracks.get(mPosition));
mState = State.PlAYING;
}
@Override
public void onCompletion(MediaPlayer mp) {
if (isRepeat) {
playTrack(mPosition);
}else if(isShuffle) {
Random r = new Random();
mPosition = r.nextInt(mTracks.size());
playTrack(mPosition);
}else{
nextSong();
}
}
@Override
public boolean onError(MediaPlayer mp, int what, int extra) {
return false;
}
private void playTrack(final int position){
terminatePreviousThread();
mThread = new Thread(){ // To make sure it's not doing too much work on the Main Thread.
@Override
public void run() {
super.run();
try {
createMediaPlayerIfNeeded();
mPlayer.setAudioStreamType(AudioManager.STREAM_MUSIC);
mPlayer.setDataSource(mTracks.get(position).getUrl());
mPlayer.prepareAsync(); // Prepares, but does not block the UI thread.
EventBus.getDefault().post(Constants.FOOTER_PLAYER_VISIBLE);
} catch (IllegalStateException | IOException e) {
e.printStackTrace();
}
}
};
mThread.start();
}
public ITrack getCurrentTrack(){
if(mTracks == null){
return (ITrack) mTracks;
}
return mTracks.get(mPosition);
}
public boolean canISaveData(){
return (mPlayer != null);
}
public void nextSong(){
try{
if(mTracks != null && !mTracks.isEmpty()){
if(mPosition < mTracks.size()-1){
mPosition++;
playTrack(mPosition);
}else{
mPosition = 0;
playTrack(mPosition);
}
}
}catch (IndexOutOfBoundsException e){
mPosition = 0;
playTrack(mPosition);
}
}
public void backSong(){
if(mTracks != null && !mTracks.isEmpty()){
if(mPosition > 0){
mPosition--;
playTrack(mPosition);
}else{
mPosition = mTracks.size() -1;
playTrack(mPosition);
}
}
}
public void onRepeatSong(){
if(isRepeat){
isRepeat = false;
}else{
isRepeat = true;
isShuffle = false;
}
}
public void onShuffleSong(){
if(isShuffle){
isShuffle = false;
}else{
isShuffle = true;
isRepeat = false;
}
}
public int getCurrentPosition(){
if(mTracks == null){
return -1;
}
return mPosition;
}
private void terminatePreviousThread(){
if(mThread != null && mThread.isAlive()){
mThread.interrupt();
}
}
private void createMediaPlayerIfNeeded(){
if(mPlayer == null){
mPlayer = new MediaPlayer();
mPlayer.setWakeMode(getApplicationContext(), PowerManager.PARTIAL_WAKE_LOCK);
mPlayer.setOnPreparedListener(this);
mPlayer.setOnCompletionListener(this);
mPlayer.setOnErrorListener(this);
}else {
mPlayer.reset();
}
}
public void pause(){
if((mPlayer != null)){
mPlayer.pause();
mState = State.PAUSED;
}
}
public void start(){
if((mPlayer != null)){
mPlayer.start();
mState = State.PlAYING;
}
}
private void releaseMediaPlayer(){
if(mPlayer != null){
mPlayer.reset();
mPlayer.release();
mPlayer = null;
}
}
public boolean isRepeatSong(){
return isRepeat;
}
public boolean isShuffleSong(){
return isShuffle;
}
public MediaPlayer mediaPlayer(){
return Preconditions.checkNotNull(mPlayer, "MediaPlayer cannot be null");
}
}