我从提供的gist Udacity复制了getWeatherDataFromJson（forecastJsonStr，numDays）。但是，它给出了空指针异常。我无法弄明白。你可以检查代码并告诉我我做错了吗？
ForecastFragment.java
public class ForecastFragment extends Fragment {
public ForecastFragment() {
// Required empty public constructor
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setHasOptionsMenu(true);
}
@Override
public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
inflater.inflate(R.menu.forecast_fragment, menu);
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml
int id = item.getItemId();
if (id == R.id.action_refresh) {
new FetchWeatherTask().execute("94043");
return true;
}
return super.onOptionsItemSelected(item);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_forecast, container, false);
/*//My way
ArrayList<String> fakeData = new ArrayList<String>();
fakeData.add("Today-Sunny 50/60");
fakeData.add("Tomorrow-Cloudy 20/30");
fakeData.add("Wednesday-Snowy 40/50");
fakeData.add("Thursday-Rainy 20/40"); */
//Google's way
String[] forecastArray={
"Today-Sunny 50/60",
"Tomorrow-Cloudy 20/30",
"Wednesday-Snowy 40/50",
"Thursday-Rainy 20/40",
"Friday-Funny 20/50",
"Sat-Sunny 70/80",
"Sun-Sunny 90/100"
};
ArrayList<String> weekForecast=new ArrayList<String>(Arrays.asList(forecastArray));
ArrayAdapter adapter = new ArrayAdapter<String>
(getActivity(), R.layout.list_item_forecast, R.id.list_item_forecast_textview, weekForecast);
ListView listview = (ListView) rootView.findViewById(R.id.listview_forecast);
listview.setAdapter(adapter);
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.
Date date = new Date(time * 1000);
SimpleDateFormat format = new SimpleDateFormat("E, MMM d");
return format.format(date).toString();
}
/**
* 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_DATETIME = "dt";
final String OWM_DESCRIPTION = "main";
JSONObject forecastJson = new JSONObject(forecastJsonStr);
JSONArray weatherArray = forecastJson.getJSONArray(OWM_LIST);
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 = dayForecast.getLong(OWM_DATETIME);
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
protected 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;
// Will contain the raw JSON response as a string.
String forecastJsonStr = null;
String format="json";
String units="metric";
int numDays=7;
try {
// Construct the URL for the OpenWeatherMap query
// Possible parameters are avaiable at OWM's forecast API page, at
// http://openweathermap.org/API#forecast
//My way
/*Uri.Builder builder = new Uri.Builder();
builder.scheme("http")
.authority("api.openweathermap.org")
.appendPath("data")
.appendPath("2.5")
.appendPath("forecast")
.appendPath("daily")
.appendQueryParameter("q", params[0])
.appendQueryParameter("mode", "json")
.appendQueryParameter("units", "metric")
.appendQueryParameter("cnt", "7");
*/
//Google's way
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="metric";
final String DAYS_PARAM="cnt";
Uri.Builder builder = new Uri.Builder();
builder.scheme("http")
.authority(FORECAST_BASE_URL)
.appendQueryParameter(QUERY_PARAM,params[0])
.appendQueryParameter(FORMAT_PARAM, format)
.appendQueryParameter(UNITS_PARAM, units)
.appendQueryParameter(DAYS_PARAM, Integer.toString(numDays))
.build();
String myUrl = builder.toString();
URL url = new URL(myUrl);
// 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;
}
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.
forecastJsonStr = 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,numDays);
} catch (JSONException e) {
Log.e(LOG_TAG, e.getMessage(), e);
e.printStackTrace();
}
return null;
}
}
}
logcat的
07-20 23:01:19.030: E/AndroidRuntime(7325): FATAL EXCEPTION: AsyncTask #1
07-20 23:01:19.030: E/AndroidRuntime(7325): java.lang.RuntimeException: An error occured while executing doInBackground()
07-20 23:01:19.030: E/AndroidRuntime(7325): at android.os.AsyncTask$3.done(AsyncTask.java:299)
07-20 23:01:19.030: E/AndroidRuntime(7325): at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:273)
07-20 23:01:19.030: E/AndroidRuntime(7325): at java.util.concurrent.FutureTask.setException(FutureTask.java:124)
07-20 23:01:19.030: E/AndroidRuntime(7325): at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:307)
07-20 23:01:19.030: E/AndroidRuntime(7325): at java.util.concurrent.FutureTask.run(FutureTask.java:137)
07-20 23:01:19.030: E/AndroidRuntime(7325): at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
07-20 23:01:19.030: E/AndroidRuntime(7325): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
07-20 23:01:19.030: E/AndroidRuntime(7325): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
07-20 23:01:19.030: E/AndroidRuntime(7325): at java.lang.Thread.run(Thread.java:856)
07-20 23:01:19.030: E/AndroidRuntime(7325): Caused by: java.lang.NullPointerException
07-20 23:01:19.030: E/AndroidRuntime(7325): at org.json.JSONTokener.nextCleanInternal(JSONTokener.java:116)
07-20 23:01:19.030: E/AndroidRuntime(7325): at org.json.JSONTokener.nextValue(JSONTokener.java:94)
07-20 23:01:19.030: E/AndroidRuntime(7325): at org.json.JSONObject.<init>(JSONObject.java:154)
07-20 23:01:19.030: E/AndroidRuntime(7325): at org.json.JSONObject.<init>(JSONObject.java:171)
07-20 23:01:19.030: E/AndroidRuntime(7325): at app.sunshine.android.example.com.sunshine.ForecastFragment$FetchWeatherTask.getWeatherDataFromJson(ForecastFragment.java:149)
07-20 23:01:19.030: E/AndroidRuntime(7325): at app.sunshine.android.example.com.sunshine.ForecastFragment$FetchWeatherTask.doInBackground(ForecastFragment.java:296)
07-20 23:01:19.030: E/AndroidRuntime(7325): at app.sunshine.android.example.com.sunshine.ForecastFragment$FetchWeatherTask.doInBackground(ForecastFragment.java:103)
07-20 23:01:19.030: E/AndroidRuntime(7325): at android.os.AsyncTask$2.call(AsyncTask.java:287)
07-20 23:01:19.030: E/AndroidRuntime(7325): at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
07-20 23:01:19.030: E/AndroidRuntime(7325): ... 5 more
07-20 23:01:19.080: E/android.os.Debug(2128): !@Dumpstate > dumpstate -k -t -z -d -o /data/log/dumpstate_app_error
答案 0 :(得分：0)
问题是Uri构建器，我尝试通过查看Uri.Builder文档来做到这一点，但后来改变了我的方式，使用教程的方式并没有正确更新所有内容，这就是为什么我搞砸了。
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();