使用Android Studio开发天气应用程序时遇到问题。 doInBackGround（）方法需要一个return语句，但是当我返回forecastJsonStr时，它会显示错误（第181行）。它说我需要返回String []而不是String值。第283行询问了类似的return语句。它表示缺少return语句。
import android.net.Uri;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListAdapter;
import android.widget.ListView;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
//import java.sql.Time;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import android.text.format.Time;
import static com.github.afrin14.sunshine.R.id.container;
public class ForecastFragment extends Fragment {
public ForecastFragment() {
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setHasOptionsMenu(true);
}
@Override
public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
inflater.inflate(R.menu.forecastfragment, menu);
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if (id == R.id.action_refresh) {
FetchWeatherTask weatherTask = new FetchWeatherTask();
weatherTask.execute("94043");
return true;
}
return super.onOptionsItemSelected(item);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup
container, Bundle savedInstanceState) {
String[] forecastArray = {
"Today - Sunny - 88/63",
"Tomorrow - Foggy - 70/40",
"Wed - Cloudy - 72/63",
"Thurs - Asteroids - 75/65",
"Fri - Heavy Rain - 65/56",
"Sat - HELP TRAPPED IN WEATHERSTATION 60/51",
"Sun - Sunny - 80/68",
};
List<String> weekForecast = new ArrayList<String>
(Arrays.asList(forecastArray));
ListAdapter mForecastAdapter = new ArrayAdapter<String>(
getActivity(),
R.layout.list_item_forecast,
R.id.list_item_forecast_textview,
weekForecast);
View rootView = inflater.inflate(R.layout.fragment_main,
container, false);
ListView listView = (ListView)
rootView.findViewById(R.id.listview_forecast);
listView.setAdapter(mForecastAdapter);
return rootView;
}
public class FetchWeatherTask extends AsyncTask<String, Void,
String[]> {
private final String LOG_TAG =
FetchWeatherTask.class.getSimpleName();
@Override
protected String[] doInBackground(String... 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 forecastJsonStr = null;
String forecastsonStr = 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
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();
URL url = new URL("http://api.openweathermap.org/data/2.5/forecast/daily?q=94043&mode=json&units=metric&cnt=7");
Log.v(LOG_TAG, "Buit 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;
}
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, numDays);
}catch(JSONException e){
Log.e(LOG_TAG, e.getMessage(), e);
e.printStackTrace();
}
return forecastJsonStr;
}
/* 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();
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;
}
return resultStrs;
}
@Override
protected String[] doInBackground(String... params){
if(params.length == 0){
return null;
}
}
}
}
答案 0 :(得分：0)
AsyncTask的第一个通用名称是定义doInBackground方法中返回的内容。如果要返回String而不是String数组（String []）而不是将async任务更改为extend：
AsyncTask<String, Void, String>
并将doInBackground方法更改为返回String。如果您不希望它返回任何内容，请将其更改为“Void”。
在你的第二个 AsyncTask doInBackground方法中（你为什么还有两个？）问题是当你的if子句为true时该方法只返回一些东西（null），否则什么都不返回。你也应该在else的情况下返回一些东西。
要完全了解泛型和返回类型的内容，我建议您阅读AsyncTask docummentation
答案 1 :(得分：0)
您已使用返回类型String数组初始化Async任务。
public class FetchWeatherTask extends AsyncTask<String, Void, String[]>
将如下所示的初始化更改为String以使其正常工作。
public class FetchWeatherTask extends AsyncTask<String, Void, String>
更改下面的方法
@Override
protected String doInBackground(String... params)
现在你不会得到这个错误。希望它有所帮助：）
答案 2 :(得分：0)
您必须调用postExecute方法才能从http调用中获取结果。我已经完全编辑了你的代码。这将按预期工作。
您的代码应该是这样的。
public class ForecastFragment extends Fragment {
public ForecastFragment() {
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setHasOptionsMenu(true);
}
@Override
public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
inflater.inflate(R.menu.forecastfragment, menu);
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
int id = item.getItemId();
if (id == R.id.action_refresh) {
FetchWeatherTask weatherTask = new FetchWeatherTask();
weatherTask.execute("94043");
return true;
}
return super.onOptionsItemSelected(item);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup
container, Bundle savedInstanceState) {
String[] forecastArray = {
"Today - Sunny - 88/63",
"Tomorrow - Foggy - 70/40",
"Wed - Cloudy - 72/63",
"Thurs - Asteroids - 75/65",
"Fri - Heavy Rain - 65/56",
"Sat - HELP TRAPPED IN WEATHERSTATION 60/51",
"Sun - Sunny - 80/68",
};
List<String> weekForecast = new ArrayList<String>
(Arrays.asList(forecastArray));
ListAdapter mForecastAdapter = new ArrayAdapter<String>(
getActivity(),
R.layout.list_item_forecast,
R.id.list_item_forecast_textview,
weekForecast);
View rootView = inflater.inflate(R.layout.fragment_main,
container, false);
ListView listView = (ListView)
rootView.findViewById(R.id.listview_forecast);
listView.setAdapter(mForecastAdapter);
return rootView;
}
public class FetchWeatherTask extends AsyncTask<String, Void, String> {
private final String LOG_TAG =
FetchWeatherTask.class.getSimpleName();
private int numDays = 7;
@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";
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();
URL url = new URL("http://api.openweathermap.org/data/2.5/forecast/daily?q=94043&mode=json&units=metric&cnt=7");
Log.v(LOG_TAG, "Buit URI " + builtUri.toString());
urlConnection = (HttpURLConnection) url.openConnection();
urlConnection.setRequestMethod("GET");
urlConnection.connect();
InputStream inputStream = urlConnection.getInputStream();
StringBuilder builder = new StringBuilder();
if (inputStream == null)
return null;
reader = new BufferedReader(new InputStreamReader(inputStream));
String line;
while ((line = reader.readLine()) != null) {
builder.append(line).append("\n");
}
if (builder.length() == 0)
return null;
forecastJsonStr = builder.toString();
Log.v(LOG_TAG, "Forecast JSON String: " + forecastJsonStr);
} catch (IOException e) {
Log.e(LOG_TAG, "Error ", e);
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);
}
}
}
return forecastJsonStr;
}
@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
try{
String[] weatherDataFromJson = getWeatherDataFromJson(s);
// do whatever you want with the output.
}catch(JSONException e){
Log.e(LOG_TAG, e.getMessage(), e);
e.printStackTrace();
}
}
private String getReadableDateString(long time) {
SimpleDateFormat shortenedDateFormat = new SimpleDateFormat("EEE MMM dd");
return shortenedDateFormat.format(time);
}
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;
}
private String[] getWeatherDataFromJson(String forecastJsonStr) 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);
Time dayTime = new Time();
dayTime.setToNow();
int julianStartDay = Time.getJulianDay(System.currentTimeMillis(),
dayTime.gmtoff);
dayTime = new Time();
String[] resultStrs = new String[numDays];
for (int i = 0; i < weatherArray.length(); i++) {
String day;
String description;
String highAndLow;
JSONObject dayForecast = weatherArray.getJSONObject(i);
long dateTime;
dateTime = dayTime.setJulianDay(julianStartDay + i);
day = getReadableDateString(dateTime);
JSONObject weatherObject =
dayForecast.getJSONArray(OWM_WEATHER).getJSONObject(0);
description = weatherObject.getString(OWM_DESCRIPTION);
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;
}
return resultStrs;
}
}
}
希望这有帮助：）