Helper methods for JSON parsing for Udacity's Developing Android Apps. Sunshine app, Lesson 2: https://www.udacity.com/course/viewer#!/c-ud853/l-1469948762/e-1630778644/m-1630778645

UdacityAndroid-JSONHelper.java

/* 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;
    }
 
    return resultStrs;
}

Oh my.. that for loop would be a one-liner in Python ^^ It's a pain going back to Java for Android...

One question: What happens if the provided numDays is less than the number of elements in the weatherArray? Wouldn't that lead to an array index out of bounds exception when trying to store the last results in resultsStr? Why provide number of days at all, when it's already implicitly included in the JSON String?

My thoughts exactly. Why do we need number of days? May be later.. I don't know.. But programming for future need is not good programming practice.

Here is my code for ForecastFragment.java , I don't know where is the problem
/*

• Copyright (C) 2014 The Android Open Source Project
  *
• Licensed under the Apache License, Version 2.0 (the "License");
• you may not use this file except in compliance with the License.
• You may obtain a copy of the License at
  *

•  http://www.apache.org/licenses/LICENSE-2.0
  
• Unless required by applicable law or agreed to in writing, software
• distributed under the License is distributed on an "AS IS" BASIS,
• WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
• See the License for the specific language governing permissions and
• limitations under the License.
  */

package com.example.enig.sunshine.app;
import android.net.Uri;
import android.os.AsyncTask;
import android.os.Bundle;
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.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.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Date;
import java.util.List;

/**

• Encapsulates fetching the forecast and displaying it as a {@link ListView} layout.
  */
  public class ForecastFragment extends Fragment {
  private ArrayAdapter mForecastAdapter;
  public ForecastFragment() {
  }

  @OverRide
  public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  // Add this line in order for this fragment to handle menu events.
  setHasOptionsMenu(true);
  }

  @OverRide
  public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
  inflater.inflate(R.menu.forecastfragment, 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) {
  FetchWeatherTask weatherTask=new FetchWeatherTask();
  weatherTask.execute("94043");
  return true;
  }
  return super.onOptionsItemSelected(item);
  }

  @OverRide
  public View onCreateView(LayoutInflater inflater, ViewGroup container,
  Bundle savedInstanceState) {

  // Create some dummy data for the ListView.  Here's a sample weekly forecast
  String[] data = {
          "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 - 20/7"
  };
  List<String> weekForecast = new ArrayList<String>(Arrays.asList(data));
  
  // Now that we have some dummy forecast data, create an ArrayAdapter.
  // The ArrayAdapter will take data from a source (like our dummy forecast) and
  // use it to populate the ListView it's attached to.
  ArrayAdapter<String> forecastAdapter =
          new ArrayAdapter<String>(
                  getActivity(), // The current context (this activity)
                  R.layout.list_item_forecast, // The name of the layout ID.
                  R.id.list_item_forecast_textview, // The ID of the textview to populate.
                  weekForecast);
  
  View rootView = inflater.inflate(R.layout.fragment_main, container, false);
  
  // Get a reference to the ListView, and attach this adapter to it.
  ListView listView = (ListView) rootView.findViewById(R.id.listview_forecast);
  listView.setAdapter(forecastAdapter);
  
  return rootView;
  

  }

  public class FetchWeatherTask extends AsyncTask<String, Void, String[]> {

  private final String LOG_TAG = FetchWeatherTask.class.getSimpleName();
  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) {
      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 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(builtUri.toString());
          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;
          }
          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 null;
  }
  @Override
  protected void onPostExecute(String[] result){
      if(result !=null){
          mForecastAdapter.clear();
          for(String dayForecastStr:result){
              mForecastAdapter.add(dayForecastStr);
          }
      }
  }
  

  }
  }