Question

我无法从其他类访问getsongList()方法。我想从“Songs”类访问MainActivity类中的方法。我尝试通过以下代码，但我收到一个错误。请检查我的代码是否正确。

帮我修复代码。

歌曲类

public class Songs extends Fragment {
    private ArrayList<SongModel> songList;
    public void getsongList(){
        //retrieve song info
    ContentResolver musicResolver = getActivity().getContentResolver();
    Uri musicUri = android.provider.MediaStore.Audio.Media.EXTERNAL_CONTENT_URI;
    Cursor musicCursor = musicResolver.query(musicUri, null, null, null, null);

    if(musicCursor!=null && musicCursor.moveToFirst()){
        //get columns
        int titleColumn = musicCursor.getColumnIndex
                (android.provider.MediaStore.Audio.Media.TITLE);
        int idColumn = musicCursor.getColumnIndex
                (android.provider.MediaStore.Audio.Media._ID);
        int artistColumn = musicCursor.getColumnIndex
                (android.provider.MediaStore.Audio.Media.ARTIST);
        //add songs to list
        do {
            long thisId = musicCursor.getLong(idColumn);
            String thisTitle = musicCursor.getString(titleColumn);
            String thisArtist = musicCursor.getString(artistColumn);
            songList.add(new SongModel(thisId, thisTitle, thisArtist));
        }
        while (musicCursor.moveToNext());
    }
    }
}

MainActivity Class

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        Songs obj1 = new Songs();
        song = obj1.getsongList();
    }
}

Answer 1

只需尝试一次将您的方法返回类型void更改为您的arraylit

private  songList;
public ArrayList<SongModel> getsongList(){
return song;}

MainActivity Class

public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
    Songs obj1 = new Songs();
  ArrayList<SongModel>  song = obj1.getsongList();
}

}

Answer 2

方法getsongList（）返回void。访问类似

song = obj1.getsongList（）;您应该将方法签名从void修改为ArrayList并返回该方法中的歌曲列表

public void getsongList() 

to 

public ArrayList<SongModel> getsongList() {
return songList;
}

Answer 3

您应该创建一个返回列表的方法，记住始终programming to interface：

private List<SongModel> songList;

public List<SongModel> getsongList() {
  return songList;
}

但是以下一行有一个缺陷：

public class MainActivity extends Activity {
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    Songs obj1 = new Songs();

    // You can't call it here!!!!
    song = obj1.getsongList();
  }
}

因为歌曲是片段（您还应该将其名称更改为SongsFragment），所以需要时间来初始化和创建自己。因此，您需要添加Callback，以便在片段完成创建时告诉MainActivity。

请阅读Communicating with Other Fragments

从另一个类访问方法时出错

3 个答案: