我调试了整个程序，mForecastAdapter中的错误onPostExecution为空，因此任何使用mForecastAdapter.clear()的尝试都会导致错误，我甚至无法理解mForecastAdapter是全局变量它在onCreate（）函数中初始化，请帮助
代码
public class ForecastFragment extends Fragment {
private ArrayAdapter<String> mForecastAdapter=null;
private List<String> weekForecast=null;
private Context con;
public ForecastFragment(){
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView= inflater.inflate(R.layout.fragment_main, container, false);
//con=getActivity().getApplicationContext();
String[] forecastArray={
"Mon 6/23 - Sunny -31/17",
"Tue 6/24 - Foggy -21/8",
"Wed 6/25 - Cloudy -22/17",
"Thurs 6/26 - Rainy -18/11",
"Fri 6/27 - Foggy -21/10",
"Sat 6/28 - TRAPPED IN WEATHERSTATION -23/18",
"Sun 6/29 - Sunny -31/17"};
weekForecast=new ArrayList<String>(Arrays.asList(forecastArray));
mForecastAdapter=new ArrayAdapter<String>(getActivity(),R.layout.list_item_forecast,R.id.list_item_forecast_textview,forecastArray );
ListView listview=(ListView)rootView.findViewById(R.id.listview_forecast);
listview.setAdapter(mForecastAdapter);
listview.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
String forecast=mForecastAdapter.getItem(position);
callActivity(forecast);
}
});
return rootView;
}
private void callActivity(String forecast) {
Intent myIntent=new Intent(getActivity(), DetailedForecast.class).putExtra("INFORMATION",forecast);
startActivity(myIntent);
}
public class FetchWeatherTask extends AsyncTask<String,Void,String[]>{
@Override
protected String[] doInBackground(String... params) {
if(params.length==0){
return null;
}
HttpURLConnection urlConnection=null;
BufferedReader reader=null;
String forecastJsonStr =null;
String format="json";
String units="metric";
int numDays=7;
try {
final String FORECAST_BASE_URL="http://api.openweathermap.org/data/2.5/forecast/daily?";
final String QUERY_PARAM="q";
final String FORMAT_PARAM="mode";
final String UNITS_PARAM="units";
final String DAYS_PARAM="cnt";
Uri builtUri= Uri.parse(FORECAST_BASE_URL).buildUpon()
.appendQueryParameter(QUERY_PARAM,params[0])
.appendQueryParameter(FORMAT_PARAM,format)
.appendQueryParameter(UNITS_PARAM,units)
.appendQueryParameter(DAYS_PARAM,Integer.toString(numDays))
.build();
Log.i("ALEX",builtUri.toString());
URL url =new URL(builtUri.toString());
urlConnection=(HttpURLConnection)url.openConnection();
urlConnection.setRequestMethod("GET");
urlConnection.connect();
InputStream inputStream=urlConnection.getInputStream();
StringBuffer buffer=new StringBuffer();
if(inputStream==null){
return null;
}
reader=new BufferedReader(new InputStreamReader(inputStream));
String line;
while((line=reader.readLine())!=null){
buffer.append(line+"\n");
}
if(buffer.length()==0){
return null;
}
forecastJsonStr=buffer.toString();
Log.v("ALEX","JSON"+forecastJsonStr);
} catch (MalformedURLException ee) {
ee.printStackTrace();
}catch (IOException ee){
Log.e("PlaceholderFragment", "Error ", ee);
}finally {
if(urlConnection!=null){
urlConnection.disconnect();
}
if(reader!=null){
try{
reader.close();
} catch (IOException ee) {
Log.e("PlaceholderFragment", "Error closing stream", ee);
}
}
}
try{
return getWeatherDataFromJson(forecastJsonStr,numDays);
}catch (JSONException e){
Log.e("ALEX",e.getMessage(),e);
e.printStackTrace();
}
return null;
}
//CODE FROM UDACITY STARTS
private String getReadableDateString(long time){
SimpleDateFormat shortenedDateFormat = new SimpleDateFormat("EEE MMM dd");
return shortenedDateFormat.format(time);
}
//CODE FROM UDACITY ENDS
private String[] getWeatherDataFromJson(String forecastJsonStr, int numDays) throws JSONException{
final String OWM_LIST="list";
final String OWM_WEATHER="weather";
final String OWM_TEMP="temp";
final String OWM_MAX="max";
final String OWM_MIN="min";
final String OWM_DESCRIPTION="main";
JSONObject weather=new JSONObject(forecastJsonStr);
JSONArray days=weather.getJSONArray(OWM_LIST);
//CODE FROM UDACITY STARTS
Time dayTime = new Time();
dayTime.setToNow();
int julianStartDay = Time.getJulianDay(System.currentTimeMillis(), dayTime.gmtoff);
//CODE FROM UDACITY ENDS
String[] resultStrs=new String[numDays];
for(int i=0;i<days.length();i++){
String day="";
String description;
String highAndLow;
JSONObject dayForecast=days.getJSONObject(i);
//CODE FROM UDACITY STARTS
long dateTime;
dateTime = dayTime.setJulianDay(julianStartDay+i);
day = getReadableDateString(dateTime);
//CODE FROM UDACITY ENDS
JSONObject weatherObject=dayForecast.getJSONArray(OWM_WEATHER).getJSONObject(0);
description=weatherObject.getString(OWM_DESCRIPTION);
JSONObject temp=dayForecast.getJSONObject(OWM_TEMP);
double high=temp.getDouble(OWM_MAX);
double low=temp.getDouble(OWM_MIN);
//highAndLow=formatHighLows(high, low);
highAndLow=Double.toString(Math.round(high))+"/"+Double.toString(Math.round(low));
resultStrs[i]=day+" - "+description+" - "+highAndLow;
}
for (String s : resultStrs) {
Log.v("ALEX", "Forecast entry: " + s);
}
return resultStrs;
}
@Override
protected void onPostExecute(String[] strings) {
mForecastAdapter.clear();
mForecastAdapter.addAll(strings);
}
}
}
这是错误，请看第一行
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ArrayAdapter.clear()' on a null object reference
at com.betatech.alex.sunshine.ForecastFragment$FetchWeatherTask.onPostExecute(ForecastFragment.java:269)
at com.betatech.alex.sunshine.ForecastFragment$FetchWeatherTask.onPostExecute(ForecastFragment.java:89)
at android.os.AsyncTask.finish(AsyncTask.java:632)
at android.os.AsyncTask.access$600(AsyncTask.java:177)
at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
at android.os.Handler.dispatchMessage(Handler.java:102)
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)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:696)
答案 0 :(得分：0)
要进行ListView更新，您应该更新提供给适配器的阵列并致电adapter.notifyDataSetCHanged()。适配器将为您处理其余部分。
即。您应该forecastArray全局（在课程顶部声明），然后在onPostExecute()中，您可以执行以下操作：
@Override
protected void onPostExecute(String[] strings) {
forecastArray = strings;
mForecastAdapter.notifyDataSetChanged();
}
如果这有帮助，或者您需要更多信息，请告诉我。
编辑：这是我在评论中试图向您解释的代码。
public class ForecastFragment extends Fragment {
private ArrayAdapter<String> mForecastAdapter;
private List<String> weekForecast=null;
private Context con;
ListView listview;
public ForecastFragment(){
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView= inflater.inflate(R.layout.fragment_main, container, false);
//con=getActivity().getApplicationContext();
listview=(ListView)rootView.findViewById(R.id.listview_forecast);
listview.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
String forecast=mForecastAdapter.getItem(position);
callActivity(forecast);
}
});
return rootView;
}
private void callActivity(String forecast) {
Intent myIntent=new Intent(getActivity(), DetailedForecast.class).putExtra("INFORMATION",forecast);
startActivity(myIntent);
}
public class FetchWeatherTask extends AsyncTask<String,Void,String[]> {
@Override
protected String[] doInBackground(String... params) {
if(params.length==0){
return null;
}
HttpURLConnection urlConnection=null;
BufferedReader reader=null;
String forecastJsonStr =null;
String format="json";
String units="metric";
int numDays=7;
try {
final String FORECAST_BASE_URL="http://api.openweathermap.org/data/2.5/forecast/daily?";
final String QUERY_PARAM="q";
final String FORMAT_PARAM="mode";
final String UNITS_PARAM="units";
final String DAYS_PARAM="cnt";
Uri builtUri= Uri.parse(FORECAST_BASE_URL).buildUpon()
.appendQueryParameter(QUERY_PARAM,params[0])
.appendQueryParameter(FORMAT_PARAM,format)
.appendQueryParameter(UNITS_PARAM,units)
.appendQueryParameter(DAYS_PARAM,Integer.toString(numDays))
.build();
Log.i("SANJEEV", builtUri.toString());
URL url =new URL(builtUri.toString());
urlConnection=(HttpURLConnection)url.openConnection();
urlConnection.setRequestMethod("GET");
urlConnection.connect();
InputStream inputStream=urlConnection.getInputStream();
StringBuffer buffer=new StringBuffer();
if(inputStream==null){
return null;
}
reader=new BufferedReader(new InputStreamReader(inputStream));
String line;
while((line=reader.readLine())!=null){
buffer.append(line+"\n");
}
if(buffer.length()==0){
return null;
}
forecastJsonStr=buffer.toString();
Log.v("SANJEEV_TAG","JSON"+forecastJsonStr);
} catch (MalformedURLException ee) {
ee.printStackTrace();
}catch (IOException ee){
Log.e("PlaceholderFragment", "Error ", ee);
}finally {
if(urlConnection!=null){
urlConnection.disconnect();
}
if(reader!=null){
try{
reader.close();
} catch (IOException ee) {
Log.e("PlaceholderFragment", "Error closing stream", ee);
}
}
}
try{
return getWeatherDataFromJson(forecastJsonStr,numDays);
}catch (JSONException e){
Log.e("SANJEEV",e.getMessage(),e);
e.printStackTrace();
}
return null;
}
//CODE FROM UDACITY STARTS
private String getReadableDateString(long time){
SimpleDateFormat shortenedDateFormat = new SimpleDateFormat("EEE MMM dd");
return shortenedDateFormat.format(time);
}
//CODE FROM UDACITY ENDS
private String[] getWeatherDataFromJson(String forecastJsonStr, int numDays) throws JSONException{
final String OWM_LIST="list";
final String OWM_WEATHER="weather";
final String OWM_TEMP="temp";
final String OWM_MAX="max";
final String OWM_MIN="min";
final String OWM_DESCRIPTION="main";
JSONObject weather=new JSONObject(forecastJsonStr);
JSONArray days=weather.getJSONArray(OWM_LIST);
//CODE FROM UDACITY STARTS
Time dayTime = new Time();
dayTime.setToNow();
int julianStartDay = Time.getJulianDay(System.currentTimeMillis(), dayTime.gmtoff);
//CODE FROM UDACITY ENDS
String[] resultStrs=new String[numDays];
for(int i=0;i<days.length();i++){
String day="";
String description;
String highAndLow;
JSONObject dayForecast=days.getJSONObject(i);
//CODE FROM UDACITY STARTS
long dateTime;
dateTime = dayTime.setJulianDay(julianStartDay+i);
day = getReadableDateString(dateTime);
//CODE FROM UDACITY ENDS
JSONObject weatherObject=dayForecast.getJSONArray(OWM_WEATHER).getJSONObject(0);
description=weatherObject.getString(OWM_DESCRIPTION);
JSONObject temp=dayForecast.getJSONObject(OWM_TEMP);
double high=temp.getDouble(OWM_MAX);
double low=temp.getDouble(OWM_MIN);
//highAndLow=formatHighLows(high, low);
highAndLow=Double.toString(Math.round(high))+"/"+Double.toString(Math.round(low));
resultStrs[i]=day+" - "+description+" - "+highAndLow;
}
for (String s : resultStrs) {
Log.v("SANJEEV", "Forecast entry: " + s);
}
return resultStrs;
}
@Override
protected void onPostExecute(String[] strings) {
mForecastAdapter=new ArrayAdapter<String>(getActivity(),R.layout.list_item_forecast,R.id.list_item_forecast_textview,strings );
listview.setAdapter(mForecastAdapter);
}
}
}
我尽可能快地把它扔在一起，所以如果有错误我很抱歉。但是，要关注的部分是类顶部的声明，我在onCreateView()和onPostExecute()中所做的一些更改。
另外，我敦促你要求一点点要求，并且对你获得的免费帮助有点感激。如果我心情不好，我可能会（理所当然地）在你最后几次评论后告诉你在其他地方找到帮助。
答案 1 :(得分：0)
**只是把条件放在** **
if（！mForecastAdapter.isempty（））{
mForecastAdapter.clear()
}