java - 为什么我在这里收到微调器的空引用错误？

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

findViewById in Fragment (37 个回答)

已关闭 5 年前。

我正在 android studio 中编程，这是我的代码 fragment :

public class settingsView extends Fragment {

    public settingsView() {
    }

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

        final Spinner citySpinner = (Spinner) getActivity().findViewById(R.id.citySpinner);

        FirebaseDatabase.getInstance().getReference("/movies/karnataka").addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                ArrayList<String> cityList = new ArrayList<>();

                for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
                    cityList.add(snapshot.getKey());
                }

                Log.d("firebasedata", cityList.toString());

                ArrayAdapter<String> adapter = new ArrayAdapter<String>(getActivity().getApplicationContext(), android.R.layout.simple_spinner_item, cityList );
                adapter.setDropDownViewResource(android.R.layout.simple_spinner_item);
                citySpinner.setAdapter(adapter);
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
            }
        });


        return inflater.inflate(R.layout.settingsview, container, false);
    }
}

我正在获取

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Spinner.setAdapter(android.widget.SpinnerAdapter) on a null object reference exception 

在行:

citySpinner.setAdapter(adapter)

Answer 1

您需要首先在 oncreateview 中填充包含 fragment View 的 xml 文件。

然后使用该 View 执行 findviewbyid:

view = inflater.inflate(R.layout.fragment_xml, container, false);
final Spinner citySpinner = (Spinner) view.findViewById(R.id.citySpinner);