java.lang.NullPointerException : Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference in GridView 异常

Question

我正在尝试从电影数据库中获取有关电影的数据。然后在我的布局中使用 gridview 我想在我的设备上显示它。我通过使用 ArrayAdapter 来做到这一点。我只想在网格中显示电影的名称。但我不断收到“java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法'java.lang.String java.lang.Object.toString()'”错误。我的代码如下:

public class MainActivityFragment extends Fragment {
    ArrayAdapter<String> mMovieAdapter;

    public MainActivityFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        mMovieAdapter = new ArrayAdapter<String>(getActivity(), R.layout.item_movies, R.id.image_view_movie, new ArrayList<String>());
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);
        GridView listView = (GridView) rootView.findViewById(R.id.gridview_movies);
        listView.setAdapter(mMovieAdapter);
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
                Intent intent = new Intent(getActivity(), DetailActivity.class);
                String forecast = mMovieAdapter.getItem(i);
                intent.putExtra(Intent.EXTRA_TEXT, forecast);
                startActivity(intent);
                /*
                 * Context context = getActivity(); String forecast =
                 * mForecastAdapter.getItem(i); int duration =
                 * Toast.LENGTH_SHORT; Toast.makeText(context, forecast,
                 * duration).show();
                 */
            }
        });
        return rootView;
    }

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setHasOptionsMenu(true);
    }

    @Override
    public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        // Inflate the menu; this adds items to the action bar if it is present.
        inflater.inflate(R.menu.moviefragment, menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == R.id.action_refresh) {
            updateMovie();
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    // TO update weather when activity starts
    @Override
    public void onStart() {
        super.onStart();
        updateMovie();
    }

    private void updateMovie() {
        FetchMovieTask movieTask = new FetchMovieTask();
        movieTask.execute();
    }

    class FetchMovieTask extends AsyncTask<Void, Void, String[]> {
        private final String LOG_TAG = FetchMovieTask.class.getSimpleName();

        @Override
        protected String[] doInBackground(Void... params) {
            /* if(params.length == 0) { return null; } */
            // These two need to be declared outside the try/catch
            // so that they can be closed in the finally block.
            HttpURLConnection urlConnection = null;
            BufferedReader reader = null;

            // Will contain the raw JSON response as a string.
            String movieJsonStr = null;

            try {
                // Construct the URL for the OpenWeatherMap query
                // Possible parameters are avaiable at OWM's forecast API page,
                // at
                // http://openweathermap.org/API#forecast
                final String BASE_URL = "http://api.themoviedb.org/3/discover/movie?";
                final String QUERY_PARAM = "sort_by";
                final String KEY_PARAM = "&api_key";
                // Using UriBuilder
                Uri builtUri = Uri.parse(BASE_URL).buildUpon().appendQueryParameter(QUERY_PARAM, "popularity.desc").appendQueryParameter(KEY_PARAM, "c20129fdf73b5df3ab44548ad7f73586").build();
                URL url = new URL(builtUri.toString());
                // Printing the url in the log
                // Log.v(LOG_TAG, "Built URI " + builtUri.toString());

                // Create the request to OpenWeatherMap, and open the connection
                urlConnection = (HttpURLConnection) url.openConnection();
                urlConnection.setRequestMethod("GET");
                urlConnection.connect();

                // Read the input stream into a String
                InputStream inputStream = urlConnection.getInputStream();
                StringBuffer buffer = new StringBuffer();
                if (inputStream == null) {
                    // Nothing to do.
                    return null;
                }
                reader = new BufferedReader(new InputStreamReader(inputStream));

                String line;
                while ((line = reader.readLine()) != null) {
                    // Since it's JSON, adding a newline isn't necessary (it
                    // won't affect parsing)
                    // But it does make debugging a *lot* easier if you print
                    // out the completed
                    // buffer for debugging.
                    buffer.append(line + "\n");
                }

                if (buffer.length() == 0) {
                    // Stream was empty. No point in parsing.
                    return null;
                }
                movieJsonStr = buffer.toString();

            } catch (IOException e) {
                Log.e(LOG_TAG, "Error ", e);
                // If the code didn't successfully get the weather data, there's
                // no point in attemping
                // to parse it.
                return null;
            } finally {
                if (urlConnection != null) {
                    urlConnection.disconnect();
                }
                if (reader != null) {
                    try {
                        reader.close();
                    } catch (final IOException e) {
                        Log.e(LOG_TAG, "Error closing stream", e);
                    }
                }
            }
            try {
                return getMovieDataFromJson(movieJsonStr);
            } catch (JSONException j) {
                Log.e(LOG_TAG, "JSON Error", j);
            }
            return null;
        }

        private String[] getMovieDataFromJson(String forecastJsonStr) throws JSONException {
            String title;
            JSONObject movieJson = new JSONObject(forecastJsonStr);
            JSONArray movieArray = movieJson.getJSONArray("results");

            String[] resultStrs = new String[100];
            for (int i = 0; i < movieArray.length(); i++) {
                // Get the JSON object representing the day
                JSONObject movie = movieArray.getJSONObject(i);

                title = movie.getString("original_title");
                resultStrs[i] = title;
                Log.v(LOG_TAG, resultStrs[i]);
            }

            /*
             * for (String s : resultStrs) { Log.v(LOG_TAG, "Forecast entry: " +
             * s); }
             */
            return resultStrs;
        }

        @Override
        protected void onPostExecute(String[] strings) {
            super.onPostExecute(strings);
            mMovieAdapter.clear();
            mMovieAdapter.addAll(strings);
        }
    }
}

我的错误日志是:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
        at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:394)
        at android.widget.ArrayAdapter.getView(ArrayAdapter.java:362)
        at android.widget.AbsListView.obtainView(AbsListView.java:2344)
        at android.widget.GridView.makeAndAddView(GridView.java:1433)
        at android.widget.GridView.makeRow(GridView.java:361)
        at android.widget.GridView.fillDown(GridView.java:302)
        at android.widget.GridView.fillGap(GridView.java:262)
        at android.widget.AbsListView.trackMotionScroll(AbsListView.java:4968)
        at android.widget.AbsListView$FlingRunnable.run(AbsListView.java:4512)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
        at android.view.Choreographer.doCallbacks(Choreographer.java:580)
        at android.view.Choreographer.doFrame(Choreographer.java:549)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5312)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901)

Answer 1

我怀疑这是 Blackbelt 在评论中也提到的错误

String[] resultStrs = new String[100];

应该是

String[] resultStrs = new String[movieArray.length()];

因为您现在对 GridView 说您有 总是 100 部电影，但是当电影少于 100 部时，您将向 GridView 提供空元素