Question

我有一个问题，我一次又一次地用Google搜索，但此时一切似乎都是不合逻辑的。我有一个片段，当主要活动中的某些事情发生时开始（我不认为这很重要）。在片段中我有这段代码，请注意onCreate部分，因为错误发生在testtext.setText(" "+parola);：

public class FinishedFragment extends Fragment {
    // TODO: Rename parameter arguments, choose names that match
    // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM2 = "param2";

    // TODO: Rename and change types of parameters
    private String mParam1;
    private String mParam2;

    private OnFragmentInteractionListener mListener;

    /**
     * Use this factory method to create a new instance of
     * this fragment using the provided parameters.
     *
     * @param param1 Parameter 1.
     * @param param2 Parameter 2.
     * @return A new instance of fragment FinishedFragment.
     */
    // TODO: Rename and change types and number of parameters
    public static FinishedFragment newInstance(String param1, String param2) {
        FinishedFragment fragment = new FinishedFragment();
        Bundle args = new Bundle();
        args.putString(ARG_PARAM1, param1);
        args.putString(ARG_PARAM2, param2);
        fragment.setArguments(args);

        return fragment;
    }

    private View rootView;

    public FinishedFragment() {
        // Required empty public constructor
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ActionBar actionBar = getActivity().getActionBar();
        actionBar.hide();
        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(getActivity());
        TextView testtext = (TextView) getActivity().findViewById(R.id.testtext);
        String parola = prefs.getString("prefUserPassword","defaultpass");
        testtext.setText(" "+parola);
        if (getArguments() != null) {
            mParam1 = getArguments().getString(ARG_PARAM1);
            mParam2 = getArguments().getString(ARG_PARAM2);
        }
    }



    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        rootView = inflater.inflate(R.layout.fragment_finished, container, false);


        return rootView;
    }

    // TODO: Rename method, update argument and hook method into UI event
    public void onButtonPressed(Uri uri) {
        if (mListener != null) {
            mListener.onFragmentInteraction(uri);
        }

    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        try {
            mListener = (OnFragmentInteractionListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString()
                    + " must implement OnFragmentInteractionListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }




    /**
     * This interface must be implemented by activities that contain this
     * fragment to allow an interaction in this fragment to be communicated
     * to the activity and potentially other fragments contained in that
     * activity.
     * <p>
     * See the Android Training lesson <a href=
     * "http://developer.android.com/training/basics/fragments/communicating.html"
     * >Communicating with Other Fragments</a> for more information.
     */
    public interface OnFragmentInteractionListener {
        // TODO: Update argument type and name
        public void onFragmentInteraction(Uri uri);
    }

}

以下是SettingsActivity.java：

public class SettingsActivity extends PreferenceActivity
{

    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        // add the xml resource
        addPreferencesFromResource(R.xml.preferences);
        ActionBar actionBar = getActionBar();
        actionBar.setDisplayHomeAsUpEnabled(true);

    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        switch (item.getItemId()) {
            case android.R.id.home:
                this.finish();
                return true;
        }
        return super.onOptionsItemSelected(item);
    }

Answer 1

您正试图在onCreateMethod内部设置UI元素，当视图尚未创建时，这就是您的应用崩溃的原因。

尝试在onViewCreated或onActivityCreated方法中设置它们，以便在创建视图时设置视图元素。

@Override
public void onViewCreated (View view, Bundle savedInstanceState) {

    ActionBar actionBar = getActivity().getActionBar();
    actionBar.hide();
    SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(getActivity());
    TextView testtext = (TextView) getActivity().findViewById(R.id.testtext);
    String parola = prefs.getString("prefUserPassword","defaultpass");
    testtext.setText(" "+parola);
    if (getArguments() != null) {
        mParam1 = getArguments().getString(ARG_PARAM1);
        mParam2 = getArguments().getString(ARG_PARAM2);
    }

}

Answer 2

在片段的生命周期中，方法调用的顺序是 -

onCreate() => onCreateView()

您无法在onCreate()设置文字，因为该视图尚未创建。

因此，对于

，testtext仍然为null

testtext.setText(" "+parola);  //NullPointerException

但是，您可以将代码转换为onCreateView()方法，如下所示 -

/*The view is created at this method call so you can access that view and 
  set the text for your TextView.*/ 
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    rootView = inflater.inflate(R.layout.fragment_finished, container, false);


    TextView testtext = (TextView) rootView.findViewById(R.id.testtext);
    String parola = prefs.getString("prefUserPassword","defaultpass");
    testtext.setText(" "+parola);

    return rootView;
}

Fragment中getDefaultSharedPreferences的NullPointerException

2 个答案: