当我尝试传递播放歌曲的意图时，应用程序正在强行关闭。我是初学者，所以无法找到错误。我从教程中获得帮助，但现在我被困在这里。我正试图从移动存储中获取音乐并在新活动中播放它。 我哪里弄错了？ 此活动正在接收意图值。
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_player);
Intent j = getIntent();
Bundle b = j.getExtras();
arrayList = (ArrayList) b.getParcelableArrayList("songlist");
int position = b.getInt("pos", 0);
Uri u;
u = Uri.parse(arrayList.get(i).toString());
mediaPlayer = MediaPlayer.create(getApplicationContext(),u);
mediaPlayer.start();
}
这是主要活动
public class MainActivity extends AppCompatActivity {
private static final int MY_PERMISSION_REQUEST = 1;
ArrayList<String> arrayList;
ListView listView;
ArrayAdapter<String> adapter;
MediaPlayer mediaPlayer;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (ContextCompat.checkSelfPermission(MainActivity.this,
Manifest.permission.READ_EXTERNAL_STORAGE) !=PackageManager.PERMISSION_GRANTED){
if (ActivityCompat.shouldShowRequestPermissionRationale(MainActivity.this,
Manifest.permission.READ_EXTERNAL_STORAGE)){
ActivityCompat.requestPermissions(MainActivity.this,
new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, MY_PERMISSION_REQUEST);
}else {
ActivityCompat.requestPermissions(MainActivity.this,
new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, MY_PERMISSION_REQUEST);
}
}else {
doStuff();
}
}
public void doStuff(){
listView = (ListView) findViewById(R.id.listView);
arrayList = new ArrayList<>();
getMusic();
adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, arrayList);
listView.setAdapter(adapter);
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
startActivity(new Intent(getApplicationContext(),Player.class).putExtra("pos",i).putExtra("songlist",arrayList));
}
});
}
public void getMusic() {
ContentResolver contentResolver = getContentResolver();
Uri songUri = MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;
Cursor songCursor = contentResolver.query(songUri, null, null, null, null);
if (songCursor != null && songCursor.moveToFirst()){
int songTitle = songCursor.getColumnIndex(MediaStore.Audio.Media.TITLE);
int songArtist = songCursor.getColumnIndex(MediaStore.Audio.Media.ARTIST);
/*int songLocation = songCursor.getColumnIndex(MediaStore.Audio.Media.DATA);*/
do {
String currentTitle = songCursor.getString(songTitle);
String currentArtist = songCursor.getString(songArtist);
/*String currentLocation = songCursor.getString(songLocation);*/
arrayList.add(currentTitle + "\n"
+ "Artist: " + currentArtist);
}while (songCursor.moveToNext());
}
}
@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
switch (requestCode){
case MY_PERMISSION_REQUEST: {
if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED){
if (ContextCompat.checkSelfPermission(MainActivity.this,
Manifest.permission.READ_EXTERNAL_STORAGE) == PackageManager.PERMISSION_GRANTED){
Toast.makeText(this, "Permission Granted", Toast.LENGTH_SHORT).show();
doStuff();
}
}else {
Toast.makeText(this, "Permission Not Granted", Toast.LENGTH_SHORT).show();
}
return;
}
}
}
这是Logcat详细信息
01-12 23:01:50.658 25516-25516/com.example.name.audioplayer D/AndroidRuntime: Shutting down VM
01-12 23:01:50.658 25516-25516/com.example.name.audioplayer E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.name.audioplayer, PID: 25516
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.name.audioplayer/com.example.name.audioplayer.Player}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.start()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2984)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045)
at android.app.ActivityThread.-wrap14(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6776)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1518)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1408)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.start()' on a null object reference
at com.example.name.audioplayer.Player.onCreate(Player.java:29)
at android.app.Activity.performCreate(Activity.java:6956)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1126)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2927)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045)
at android.app.ActivityThread.-wrap14(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6776)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1518)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1408)
答案 0 :(得分：0)
使用检查mediaPlayer.start()的if语句环绕null，并且仅当mediaPlayer不为空时才执行该块。在您的情况下，mediaPlayer对象可能是null，因为Uri对象u没有有效的Uri。插入日志以检查其值，但首先插入if块以避免崩溃。