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;
}

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);
          }
      }
  }
  

  }
  }