Java tutorial
package a122016.rr.com.alertme; /** * Created by rachitrawat on 12/26/2016. */ /* * Copyright (C) 2016 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. */ import android.text.TextUtils; import android.util.Log; 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.MalformedURLException; import java.net.URL; import java.nio.charset.Charset; import java.util.ArrayList; /** * Helper methods related to requesting and receiving PoliceStations data from server. */ public final class QueryUtilsPoliceStation { /** * Tag for the log messages */ private static final String LOG_TAG = QueryUtilsPoliceStation.class.getSimpleName(); /** * Create a private constructor because no one should ever create a {@link QueryUtilsPoliceStation} object. * This class is only meant to hold static variables and methods, which can be accessed * directly from the class name QueryUtils (and an object instance of QueryUtils is not needed). */ private QueryUtilsPoliceStation() { } /** * Query the server and return appropriate PoliceStations object */ public static ArrayList<PoliceStation> fetchPoliceStationsData(String requestUrl) { // Create URL object URL url = createUrl(requestUrl); // Perform HTTP request to the URL and receive a JSON response back String jsonResponse = null; try { jsonResponse = makeHttpRequest(url); } catch (IOException e) { Log.e(LOG_TAG, "Problem making the HTTP request.", e); } // Extract relevant fields from the JSON response and create a {@link PoliceStations} object ArrayList<PoliceStation> PoliceStationsList = extractFeatureFromJson(jsonResponse); // Return the {@link PoliceStations} object return PoliceStationsList; } /** * Returns new URL object from the given string URL. */ private static URL createUrl(String stringUrl) { URL url = null; try { url = new URL(stringUrl); } catch (MalformedURLException e) { Log.e(LOG_TAG, "Problem building the URL ", e); } return url; } /** * Make an HTTP request to the given URL and return a String as the response. */ private static String makeHttpRequest(URL url) throws IOException { String jsonResponse = ""; // If the URL is null, then return early. if (url == null) { return jsonResponse; } HttpURLConnection urlConnection = null; InputStream inputStream = null; try { urlConnection = (HttpURLConnection) url.openConnection(); urlConnection.setReadTimeout(10000 /* milliseconds */); urlConnection.setConnectTimeout(15000 /* milliseconds */); urlConnection.setRequestMethod("GET"); urlConnection.connect(); // If the request was successful (response code 200), // then read the input stream and parse the response. if (urlConnection.getResponseCode() == 200) { inputStream = urlConnection.getInputStream(); jsonResponse = readFromStream(inputStream); } else { Log.e(LOG_TAG, "Error response code: " + urlConnection.getResponseCode()); } } catch (IOException e) { Log.e(LOG_TAG, "Problem retrieving the PoliceStations JSON results.", e); } finally { if (urlConnection != null) { urlConnection.disconnect(); } if (inputStream != null) { // Closing the input stream could throw an IOException, which is why // the makeHttpRequest(URL url) method signature specifies than an IOException // could be thrown. inputStream.close(); } } return jsonResponse; } /** * Convert the {@link InputStream} into a String which contains the * whole JSON response from the server. */ private static String readFromStream(InputStream inputStream) throws IOException { StringBuilder output = new StringBuilder(); if (inputStream != null) { InputStreamReader inputStreamReader = new InputStreamReader(inputStream, Charset.forName("UTF-8")); BufferedReader reader = new BufferedReader(inputStreamReader); String line = reader.readLine(); while (line != null) { output.append(line); line = reader.readLine(); } } return output.toString(); } /** * Return a {@link PoliceStation} object that has been built up from * parsing the given JSON response. */ private static ArrayList<PoliceStation> extractFeatureFromJson(String PoliceStationsJSON) { // If the JSON string is empty or null, then return early. if (TextUtils.isEmpty(PoliceStationsJSON)) { return null; } // Create an array list of PoliceStations ArrayList<PoliceStation> PoliceStationsList = new ArrayList<PoliceStation>(); // Try to parse the JSON response string. If there's a problem with the way the JSON // is formatted, a JSONException exception object will be thrown. // Catch the exception so the app doesn't crash, and print the error message to the logs. try { // Create a JSONArray from the JSON response string JSONArray PoliceStationArray = new JSONArray(PoliceStationsJSON); // For each PoliceStation in the PoliceStationArray, create an {@link PoliceStation} object for (int i = 0; i < PoliceStationArray.length(); i++) { // Get a single PoliceStation at position i within the list of PoliceStations JSONObject currentPoliceStation = PoliceStationArray.getJSONObject(i); // Extract the value for the key called "Name" String name = currentPoliceStation.getString("Name"); // Extract the value for the key called "Number" String number = currentPoliceStation.getString("Number"); double latitude = currentPoliceStation.getDouble("Latitude"); double longitude = currentPoliceStation.getDouble("Longitude"); // Create a new {@link PoliceStation} object PoliceStation PoliceStation = new PoliceStation(name, number, latitude, longitude); // Add the new {@link PoliceStation} to the list of PoliceStations. PoliceStationsList.add(PoliceStation); } } catch (JSONException e) { // If an error is thrown when executing any of the above statements in the "try" block, // catch the exception here, so the app doesn't crash. Print a log message // with the message from the exception. Log.e("QueryUtilsPoliceStation", "Problem parsing the PoliceStations JSON results", e); } // Return the array list return PoliceStationsList; } }