Question

我正在关注Android开发的Big Nerd Ranch Guide第2版。我正在构建DatePickerFragment来选择日期。该对话框通过按钮的onClick启动。一切都已初始化。

DatePickerFragment.java上课：

public class DatePickerFragment extends DialogFragment {

    View v = LayoutInflater.from(getActivity()).inflate(R.layout.dialog_date, null);

    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        return new AlertDialog.Builder(getActivity())
                .setView(v)
                .setTitle(R.string.date_picker_title)
                .setPositiveButton(R.string.ok, null)
                .setNegativeButton(R.string.cancel, null)
                .create();
    }
}

我是从CrimeFragment.java班级

调用此词

 @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState){

        ...

        mDateButton = (Button) v.findViewById(R.id.crime_date);
        mDateButton.setText(mCrime.getDate().toString());
        mDateButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                FragmentManager manager = getFragmentManager();
                DatePickerFragment dialog = new DatePickerFragment();
                dialog.show(manager, DIALOG_DATE);
            }
        });

        ...
    }

正如您所看到的，所有内容都已初始化（从我能看到的内容），但我收到此错误。

这是stacktrace：

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
                                                                               at android.view.LayoutInflater.from(LayoutInflater.java:232)
                                                                               at myapp.onur.criminalintent.DatePickerFragment.<init>(DatePickerFragment.java:16)
                                                                               at myapp.onur.criminalintent.CrimeFragment$2.onClick(CrimeFragment.java:77)

Answer 1

将视图通胀移至onCreateDialog，如下所示：

public class DatePickerFragment extends DialogFragment {
    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        View v = LayoutInflater.from(getActivity()).inflate(R.layout.dialog_date, null);
        return new AlertDialog.Builder(getActivity())
                .setView(v)
                .setTitle(R.string.date_picker_title)
                .setPositiveButton(R.string.ok, null)
                .setNegativeButton(R.string.cancel, null)
                .create();
    }
}

问题是getActivity()只会在调用onAttach()后返回非空值。有关详细信息，请参阅the Activity/Fragment lifecycle。

Answer 2

添加：

 public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        v = inflater.from(getActivity()).inflate(R.layout.dialog_date, container, false);
        return v;
    }

所以你的代码将是：

View v;

@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    v = inflater.from(getActivity()).inflate(R.layout.dialog_date, container, false);
    return v;
}

@NonNull
@Override
public Dialog onCreateDialog(Bundle savedInstanceState) {
    return new AlertDialog.Builder(getActivity())
            .setView(v)
            .setTitle(R.string.date_picker_title)
            .setPositiveButton(R.string.ok, null)
            .setNegativeButton(R.string.cancel, null)
            .create();
}

为什么DatePickerFragment在初始化所有内容时都返回Null Object Reference

2 个答案: