我在MainActivityFragment中初始化了数组适配器，我用虚假数据初始化了适配器，然后尝试通过从服务器获取数据来更新适配器。在mforecastadapter方法中使用onPostExecute()获取空指针异常，该方法是内联类FetchWeatherTask的成员方法。
package com.example.puneet.sunshine;
import android.os.AsyncTask;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.util.Log;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import android.text.format.Time;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Arrays;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
/**
* A placeholder fragment containing a simple view.
*/
public class MainActivityFragment extends Fragment {
public ArrayAdapter<String> mforecastadapter;
public MainActivityFragment() {
}
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View rootview = inflater.inflate(R.layout.fragment_main,container,true);
String[] forecast = {
"Today - Sunny - 80/66",
"Tomorrow - Cloudy - 75/55",
"Weds - Foggy - 86/65",
"Thurs - Asteroids - 76/43",
"Fri - Sunny - 87/56",
"Sat - Foggy - 76/55",
"Sun - Cloudy - 66/44"
};
ArrayList<String> weekforecast = new ArrayList<String>(Arrays.asList(forecast));
mforecastadapter = new ArrayAdapter<String>(getActivity(), R.layout.list_forecast_layout, R.id.list_item_forecast_textview, weekforecast);
ListView l = (ListView) rootview.findViewById(R.id.listView_forecast);
l.setAdapter(mforecastadapter);
return rootview;
}
public class FetchWeatherTask extends AsyncTask<String, Void, String[]> {
private final String LOG_TAG = FetchWeatherTask.class.getSimpleName();
/* The date/time conversion code is going to be moved outside the asynctask later,
* so for convenience we're breaking it out into its own method now.
*/
private String getReadableDateString(long time){
// Because the API returns a unix timestamp (measured in seconds),
// it must be converted to milliseconds in order to be converted to valid date.
SimpleDateFormat shortenedDateFormat = new SimpleDateFormat("EEE MMM dd");
return shortenedDateFormat.format(time);
}
/**
* Prepare the weather high/lows for presentation.
*/
private String formatHighLows(double high, double low) {
// For presentation, assume the user doesn't care about tenths of a degree.
long roundedHigh = Math.round(high);
long roundedLow = Math.round(low);
String highLowStr = roundedHigh + "/" + roundedLow;
return highLowStr;
}
/**
* Take the String representing the complete forecast in JSON Format and
* pull out the data we need to construct the Strings needed for the wireframes.
*
* Fortunately parsing is easy: constructor takes the JSON string and converts it
* into an Object hierarchy for us.
*/
private String[] getWeatherDataFromJson(String forecastJsonStr, int numDays)
throws JSONException {
// These are the names of the JSON objects that need to be extracted.
final String OWM_LIST = "list";
final String OWM_WEATHER = "weather";
final String OWM_TEMPERATURE = "temp";
final String OWM_MAX = "max";
final String OWM_MIN = "min";
final String OWM_DESCRIPTION = "main";
JSONObject forecastJson = new JSONObject(forecastJsonStr);
JSONArray weatherArray = forecastJson.getJSONArray(OWM_LIST);
// OWM returns daily forecasts based upon the local time of the city that is being
// asked for, which means that we need to know the GMT offset to translate this data
// properly.
// Since this data is also sent in-order and the first day is always the
// current day, we're going to take advantage of that to get a nice
// normalized UTC date for all of our weather.
Time dayTime = new Time();
dayTime.setToNow();
// we start at the day returned by local time. Otherwise this is a mess.
int julianStartDay = Time.getJulianDay(System.currentTimeMillis(), dayTime.gmtoff);
// now we work exclusively in UTC
dayTime = new Time();
int le = weatherArray.length();
String str = new String(String.valueOf(le));
Log.v(LOG_TAG,str);
String[] resultStrs = new String[numDays];
for(int i = 0; i < weatherArray.length(); i++) {
// For now, using the format "Day, description, hi/low"
String day;
String description;
String highAndLow;
// Get the JSON object representing the day
JSONObject dayForecast = weatherArray.getJSONObject(i);
// The date/time is returned as a long. We need to convert that
// into something human-readable, since most people won't read "1400356800" as
// "this saturday".
long dateTime;
// Cheating to convert this to UTC time, which is what we want anyhow
dateTime = dayTime.setJulianDay(julianStartDay+i);
day = getReadableDateString(dateTime);
// description is in a child array called "weather", which is 1 element long.
JSONObject weatherObject = dayForecast.getJSONArray(OWM_WEATHER).getJSONObject(0);
description = weatherObject.getString(OWM_DESCRIPTION);
// Temperatures are in a child object called "temp". Try not to name variables
// "temp" when working with temperature. It confuses everybody.
JSONObject temperatureObject = dayForecast.getJSONObject(OWM_TEMPERATURE);
double high = temperatureObject.getDouble(OWM_MAX);
double low = temperatureObject.getDouble(OWM_MIN);
highAndLow = formatHighLows(high, low);
resultStrs[i] = day + " - " + description + " - " + highAndLow;
}
for (String s : resultStrs) {
Log.v(LOG_TAG, "Forecast entry: " + s);
}
return resultStrs;
}
@Override
public String[] doInBackground(String... params) {
// 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;
if (params.length == 0) {
return null;
}
// Will contain the raw JSON response as a string.
String forecastJsonStr = 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
String place = params[0];
String mode = "json";
String unit = "metric";
String count = "7";
String baseUrl = "http://api.openweathermap.org/data/2.5/forecast/daily?q="+place+"&mode=json&units=metric&cnt=7";
String apiKey = "&APPID=" + BuildConfig.OPEN_WEATHER_MAP_API_KEY;
URL url = new URL(baseUrl.concat(apiKey));
String o1 = new String(String.valueOf(url));
Log.v(LOG_TAG,"url="+o1);
// 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.
Log.v(LOG_TAG,"empty input stream");
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.
Log.v(LOG_TAG,"empty buffer");
return null;
}
forecastJsonStr = buffer.toString();
Log.v(LOG_TAG, "Forecast Json string:" + forecastJsonStr);
} 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 getWeatherDataFromJson(forecastJsonStr, 7);
} catch (JSONException e) {
Log.v(LOG_TAG,"null object");
Log.e(LOG_TAG, e.getMessage(), e);
e.printStackTrace();
}
return null;
}
@Override
protected void onPostExecute(String[] result) {
super.onPostExecute(result);
if(mforecastadapter==null){
Log.v(LOG_TAG,"adapter null");
}
if(result!=null){
mforecastadapter.clear();
for(String s: result){
mforecastadapter.add(s);
}
}
mforecastadapter.notifyDataSetChanged();
}
}
}
答案 0 :(得分：0)
在setAdapter代码下运行您的任务，如下所示：
ListView l = (ListView) rootview.findViewById(R.id.listView_forecast);
l.setAdapter(mforecastadapter);
new FetchWeatherTask().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR,inputString);
或者您可以初始化mforecastAdapter
if(mforecastadapter==null){
Log.v(LOG_TAG,"adapter null");
mforecastadapter = new ArrayAdapter<String>(getActivity(), R.layout.list_forecast_layout, R.id.list_item_forecast_textview, weekforecast);
}
并确保在ListView上设置此适配器。
我推荐第一个解决方案。