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();
}
}