java.lang.NullPointerException：尝试在 GridView 中的空对象引用上调用虚拟方法“java.lang.String java.lang.Object.toString()”

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

提问by Shadab Khan

I am trying to get data about movies from the movie database. Then using a gridview in my layout I want to display it on my device. This I do by using an ArrayAdapter. I just want to diplay the names of the movies in a grid. But I keep getting "java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference" error. My code is as follows:

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: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference”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);
        }
    }
}

And my error log is:

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)

回答by Ivo Beckers

I suspect this is the error as also mentioned by Blackbelt in the comments

我怀疑这是 Blackbelt 在评论中也提到的错误

String[] resultStrs = new String[100];

This should be

这应该是

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

Because what you now do is say to the GridView you have always100 movies but when there are less than 100 movies you are giving null elements to the GridView

因为您现在所做的是对 GridView 说您总是有100 部电影，但是当电影少于 100 部电影时，您将向 GridView 提供空元素