java.lang.NullPointerException:尝试调用虚拟方法';java.lang.String java.lang.Object.toString()';关于GridView中的空对象引用
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。这是通过使用ArrayAdapter实现的。我只想把电影的名字放在一个格子里。 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:java.lang.NullPointerException:尝试调用虚拟方法';java.lang.String java.lang.Object.toString()';关于GridView中的空对象引用,java,android,Java,Android,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。这是通过使用ArrayAdapter实现的。我只想把电影的名字放在一个格子里。 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 Frag
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);
}
}
}
我怀疑这就是布莱克贝尔在评论中提到的错误
String[] resultStrs = new String[100];
这应该是
String[] resultStrs = new String[movieArray.length()];
因为你现在要做的是对GridView说,你总是有100部电影,但是当电影少于100部时,你会给GridView提供空元素。问题1)你的JSON是什么样子的?2) 你确定你有一系列的电影吗?3) 您的JSON结构正确吗?如果出现JSON异常,我不会只记录错误并返回null。我宁愿抛出一个相关的异常。您解析的字符串数组不是null吗?doInBackground返回的结果似乎是null。你能证实这一点吗?为什么
100
而不是新字符串[movieArray.length()]
?@Blackbelt:我添加了你说的行,我的应用程序现在运行良好。我猜是因为循环运行了整整100次迭代,所以在最后几次迭代中发现了空值。谢谢你的帮助。是的,我接受了布莱克贝尔的建议,现在一切都好了。谢谢
String[] resultStrs = new String[movieArray.length()];