Android开发教程:关于Cursor空指针的问题

Android开发教程:关于Cursor空指针的问题

最近自己动手写个音乐播放器,用到Cursor来取得数据库中音乐文件的信息,但是当用到Cursor的时候总是报空指针错误,后来发现是模拟器上没有音乐文件,使用Cursor的时候 ,若Cursor中没有结果,则Cursor的对象则为空,所以在使用Cursor的时候最好先判断Cursor对象是否为空且其结果是否大于等于1,

以下是最开始的代码:

private void getSongMessage(){
  String TITLE = MediaStore.Audio.Media.TITLE;
  String DATA = MediaStore.Audio.Media.DATA;
  String ALBUM = MediaStore.Audio.Media.ALBUM;
  String ARTIST = MediaStore.Audio.Media.ARTIST;
  Uri uri = MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;
  ContentResolver cr = getContentResolver();
  Cursor cs = cr.query(uri, null, null, null, null); 

while (cs.moveToNext()) {
   String title = cs.getString(cs.getColumnIndex(TITLE));
   String data = cs.getString(cs.getColumnIndex(DATA));
   String album = cs.getString(cs.getColumnIndex(ALBUM));
   String artist = cs.getString(cs.getColumnIndex(ARTIST));
   MusicData.title.add(title);
   MusicData.data.add(data);
   MusicData.album.add(album);
   MusicData.artist.add(artist);
  }
 }

以下是修改后的代码:

private void getSongMessage(){
  String TITLE = MediaStore.Audio.Media.TITLE;
  String DATA = MediaStore.Audio.Media.DATA;
  String ALBUM = MediaStore.Audio.Media.ALBUM;
  String ARTIST = MediaStore.Audio.Media.ARTIST;
  Uri uri = MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;
  ContentResolver cr = getContentResolver();
  Cursor cs = cr.query(uri, null, null, null, null); 
  if(cs != null && cs.getCount() >= 1){
   while (cs.moveToNext()) {
    String title = cs.getString(cs.getColumnIndex(TITLE));
    String data = cs.getString(cs.getColumnIndex(DATA));
    String album = cs.getString(cs.getColumnIndex(ALBUM));
    String artist = cs.getString(cs.getColumnIndex(ARTIST));
    MusicData.title.add(title);
    MusicData.data.add(data);
    MusicData.album.add(album);
    MusicData.artist.add(artist);
   }
  }else{
   Toast.makeText(this, "No Data", Toast.LENGTH_LONG).show();
  }
 }

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