DatePickerDialog NPE

Question

为什么我会得到一个NPE (返回对话框为null)？

public class ViewCalendar extends DialogFragment implements OnDateSetListener {
    private Calendar c = Calendar.getInstance();
    private DatePickerDialog dialog = new DatePickerDialog(getActivity(), this, c.get(Calendar.YEAR), c.get(Calendar.MONTH), c.get(Calendar.DAY_OF_MONTH));

    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {  
        return dialog;
    }
}

以下是有用的。为什么？

public class ViewCalendar extends DialogFragment implements OnDateSetListener {
    private Calendar c = Calendar.getInstance();

    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        DatePickerDialog dialog = new DatePickerDialog(getActivity(), this, c.get(Calendar.YEAR), c.get(Calendar.MONTH), c.get(Calendar.DAY_OF_MONTH)); 
        return dialog;
    }
}

Answer 1

在Java中(我对其他语言不太了解)，在构造对象(创建实例)之后，将立即初始化全局(实例)字段。因此，当您将Dialog声明为字段时，如

public class Sampple extends Fragment {
     private DatePickerDialog dialog = new DatePickerDialog(getActivity(), this, c.get(Calendar.YEAR), c.get(Calendar.MONTH), c.get(Calendar.DAY_OF_MONTH));
 }

在这里，Dialog将在构造函数调用后初始化，而onAttach()仍然不被调用。因此，这里getActivity()返回null