我的主要活动是使用gridview，但是在更改设备方向后点击网格图像时，应用程序崩溃

Question

仅当更改设备方向后，应用程序才会崩溃。 这是主要活动：

public class MainActivity extends AppCompatActivity implements Listener {

    boolean mTwoPane;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        if (findViewById(R.id.fragment_two) != null) {
            mTwoPane = true;

        } else {
            mTwoPane = false;
        }

        if (null == savedInstanceState) {
            MoviesFragment moviesFragment = new MoviesFragment();
            moviesFragment.setListener(this);
            getSupportFragmentManager().beginTransaction().add(R.id.fragment_movies, moviesFragment).commit();
        }
    }

    @Override
    public void setSelectedMovie(MoviesData.ResultsBean movie) {

        if (mTwoPane == true) {
            DetailFragment detailsFragment = new DetailFragment();
            Bundle extras = new Bundle();
            extras.putSerializable("MoviesData", movie);
            detailsFragment.setArguments(extras);

            getSupportFragmentManager().beginTransaction().replace(R.id.fragment_two, detailsFragment).commit();

        } else {

            Intent intent = new Intent(this, DetailActivity.class);
            intent.putExtra("MoviesData", movie);
            startActivity(intent);

        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.sort_menu, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {

        int id = item.getItemId();

        if (id == R.id.action_settings) {
            startActivity(new Intent(this, SettingsActivity.class));
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

}

主要的片段代码：

public class MoviesFragment extends Fragment {

    ArrayList<MoviesData.ResultsBean> moviesList;
    private ImagesAdapter moviesAdapter;
    private ArrayList<String> mImageUrlList;
    Listener mListener;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_movies, container, false);

        mImageUrlList = new ArrayList<String>();

        moviesAdapter = new ImagesAdapter(getActivity(), mImageUrlList);
        GridView list = (GridView) rootView.findViewById(R.id.gridview_movies);
        list.setAdapter(moviesAdapter);
        list.setOnItemClickListener(new AdapterView.OnItemClickListener() {

            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int position, long l) {
                Toast.makeText(getActivity(), "title: " + moviesList.get(position).getOriginal_title(), Toast.LENGTH_SHORT).show();

                mListener.setSelectedMovie(moviesList.get(position));
            }
        });

        return rootView;

    }

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setHasOptionsMenu(true);

    }

    public void updateMoviesDisplay() {
        FetchMoviesTask fetchMoviesTask = new FetchMoviesTask();
        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(getActivity());
        String type = prefs.getString(getString(R.string.pref_display_type_key),
                getString(R.string.pref_display_categeory_defualt));
        String pop = "popular";

        //String displayType="popular?";
        Log.i("type00", type);
        System.out.println("type= " + type);
        if (type.equals("favourite")) {
            System.out.println("type= " + type);
            mImageUrlList.clear();
            MovieDb db = new MovieDb(getActivity());
            moviesList = db.getAllMovie();
            List<MoviesData.ResultsBean> mResultsBean;
            mResultsBean = db.getAllMovie();
            for (int i = 0; i < mResultsBean.size(); i++) {
                mImageUrlList.add(mResultsBean.get(i).getPoster_path());

            }

            moviesAdapter.notifyDataSetChanged();
        } else if (type.equals("popular")) {
            fetchMoviesTask.execute("popular");
        } else if (type.equals("top_rated")) {
            fetchMoviesTask.execute("top_rated");
        }
        Log.i("kkkk", String.valueOf(type.equals(pop)));

    }

    public boolean onOptionsItemSelected(MenuItem item) {


        /* FetchMoviesTask task = new FetchMoviesTask();
        if (id == R.id.popular) {
            task.execute("popular");
            return true;
        } else if (id == R.id.top_rated) {
            task.execute("top_rated");
            return true;
        } else if (id == R.id.favourite_sort) {
            mImageUrlList.clear();
            MovieDb db = new MovieDb(getActivity());
            moviesList = db.getAllMovie();
            List<MoviesData.ResultsBean> mResultsBean;
            mResultsBean = db.getAllMovie();
            for (int i = 0; i < mResultsBean.size(); i++) {
                mImageUrlList.add(mResultsBean.get(i).getPoster_path());

            }
            moviesAdapter.notifyDataSetChanged();
        }
        moviesAdapter.notifyDataSetChanged();*/
        return true;
    }

    public void setListener(Listener listener) {
        this.mListener = listener;
    }

    public static String BuildImageUrl(String imagePath) {
        Uri uri = Uri.parse("http://image.tmdb.org/t/p/").buildUpon().appendEncodedPath("w342").appendEncodedPath(imagePath).build();
        return uri.toString();
    }

    public class FetchMoviesTask extends AsyncTask<String, Void, MoviesData> {

        @Override
        protected MoviesData doInBackground(String... params) {

            try {
                return new FetchMovies().fetchMovieData(params[0], 1 + "");
            } catch (IOException e) {
                e.printStackTrace();
            }
            return null;
        }

        @Override
        protected void onPostExecute(MoviesData movie) {
            super.onPostExecute(movie);
            if (movie == null) {
                Toast toast = Toast.makeText(getActivity(), "No internet connection", Toast.LENGTH_LONG);
                toast.show();
                return;
            }

            moviesList = movie.getResults();

            mImageUrlList.clear();
            for (ResultsBean bean : movie.getResults()) {

                mImageUrlList.add(BuildImageUrl(bean.getPoster_path()));

            }

            moviesAdapter.notifyDataSetChanged();

        }
    }

    @Override
    public void onStart() {
        super.onStart();

        updateMoviesDisplay();
    }

}

我得到一个nullpointerexception：

尝试调用接口方法“ void”
com.example.mohamed.movi​​eapp.MoviesFragment $ 1.onItem上的空对象引用上的com.example.mohamed.movi​​eapp.Listener.setSelectedMovie（com.example.mohamed.movi​​eapp.Data.MoviesData $ ResultsBean）' MoviesFragment“ mListener.setSelectedMovie（moviesList.get（position））; 在setOnItemClickListener方法中

Answer 1

旋转设备时，savedInstanceState不为null。 在您的活动中，如果saveInstanceState为null，则仅设置侦听器。 因此，在设备旋转的情况下，片段中的mListener对象为NULL。 因此，对其进行任何调用都会产生NPI。

如果saveInstanceState不为null，例如在进行任何配置更改（例如设备旋转）的情况下，您需要使用SupportFragmentManager按标签或ID查找片段，在这种情况下，将Listener设置为找到的片段对象。