在JSON上使用JSON來處理Lat Long

Question

我正在嘗試從API獲取JSON對象，以對Google地圖上的每個標記使用Lat Long。

我可以讀入JSON，因為我已經能夠將它作為字符串放入列表視圖中，但是似乎無法將其解析為雙精度，每次都會崩潰。

Java的：

package com.fly.plane;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import com.fly.plane.R;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.MapFragment;
import com.google.android.gms.maps.Projection;
import com.google.android.gms.maps.SupportMapFragment;

import com.google.android.gms.maps.model.BitmapDescriptorFactory;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.Marker;
import com.google.android.gms.maps.model.MarkerOptions;


import android.R.array;
import android.app.ListActivity;
import android.app.ProgressDialog;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Point;
import android.location.Location;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Handler;
import android.os.SystemClock;
import android.support.v4.app.FragmentActivity;
import android.util.Log;
import android.view.View;
import android.view.animation.Interpolator;
import android.view.animation.LinearInterpolator;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;

public class MyMapActivity extends ListActivity {

    private ProgressDialog pDialog;

    // URL to get data JSON
    private static String url = "http://edmundgentle.com/snippets/flights/api.php";

    // JSON Node speeds
    private static final String TAG_data = "data";
    private static final String TAG_BEARING = "bearing";
    private static final String TAG_speed = "speed";
    private static final String TAG_ARR = "arr";
    private static final String TAG_DEP = "dep";
    private static final String TAG_DEP_LAT = "lat";
    private static final String TAG_DEP_LON = "lon";

    // data JSONArray
    JSONArray data = null;

    // Hashmap for ListView
    ArrayList<HashMap<String, String>> contactList;

    // Hashmap for ListView
    ArrayList<Double> ct;

    //final Handler handler;
    private GoogleMap mMap; 
    public static final LatLng dest(int i){
        final double latitude = -34.6 * i;
        final double longitude = 34.23 * i;
        return new LatLng(latitude, longitude);
    }
    public double latt = -15.48169437461;
    public double lng = -15.48169437461;
    public ArrayList<Integer> dLat;

    public String dlat;
    public String dlon;
    public String alat;
    public String alon;

    public boolean wait = true;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_my_map);

        contactList = new ArrayList<HashMap<String, String>>();

        ct = new ArrayList<Double>();

        //ListView lv = getListView();

      //create markers  
        new Getdata().execute();

        }

    }

    /**
     * Async task class to get json by making HTTP call
     * */
    private class Getdata extends AsyncTask<Void, Void, Void> {

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            // Showing progress dialog
            pDialog = new ProgressDialog(MyMapActivity.this);
            pDialog.setMessage("Please wait...");
            pDialog.setCancelable(false);
            pDialog.show();

        }

        @Override
        protected Void doInBackground(Void... arg0) {
            // Creating service handler class instance
            HTTPHandler sh = new HTTPHandler();

            // Making a request to url and getting response
            String jsonStr = sh.makeServiceCall(url, HTTPHandler.GET);

            Log.d("Response: ", "> " + jsonStr);
            boolean limit = false;
            int j = 0;
            if (jsonStr != null  ||  limit == false) {
                j++;
                //if (j > 20)limit = true;
                try {
                    JSONObject jsonObj = new JSONObject(jsonStr);

                    // Getting JSON Array node
                    data = jsonObj.getJSONArray(TAG_data);

                    // looping through All data
                    for (int i = 0; i < data.length(); i++) {
                        JSONObject c = data.getJSONObject(i);

                        // departure node is JSON Object
                        JSONObject dep = c.getJSONObject(TAG_DEP);
                        //String dtime = dep.getString(TAG_DEP_TIME);
                        String dlat = dep.getString(TAG_DEP_LAT);
                        String dlon = dep.getString(TAG_DEP_LON);

                        // arrival node is JSON Object
                        JSONObject arr = c.getJSONObject(TAG_ARR);

                        // tmp hashmap for single contact
                        HashMap<String, String> contact = new HashMap<String, String>();

                        contact.put(TAG_DEP_LAT, dlat );
                        contact.put(TAG_DEP_LON,dlon);                  

                        // adding contact to contact list
                        contactList.add(contact);

                        if (i== data.length()){
                            wait = false;
                        }
                    }
                } catch (JSONException e) {
                    e.printStackTrace();
                }
            } else {
                Log.e("ServiceHandler", "Couldn't get any data from the url");
            }

            return null;
        }

        @Override
        protected void onPostExecute(Void result) {
            super.onPostExecute(result);
            // Dismiss the progress dialog
            if (pDialog.isShowing())
                pDialog.dismiss();            
            // use plane api for latlon

            mMap = ((MapFragment) getFragmentManager().findFragmentById(R.id.map)).getMap();
            List<Marker> markers = new ArrayList<Marker>();
            for (int i = 0; i < 1 ; i++)
            {           
                // Now parse it in double

                double latitude = Double.parseDouble(dlat);
                double longitude = Double.parseDouble(dlon);

                //double latitude = 34 + i;
                //double longitude = 56 + i;

                    Marker marker =  mMap.addMarker(new MarkerOptions()
                    .position(new LatLng(latitude, longitude))
                    //.position(new LatLng(latitude, longitude))
                    .title("Hello world")
                    .icon(BitmapDescriptorFactory.fromResource(R.drawable.planemarker)));
                    markers.add(marker);            

                }  

            //setListAdapter(adapter);
        }

    }


}

這是我收到的消息：

02-17 14:58:06.635: D/dalvikvm(23289): GC_FOR_ALLOC freed 54K, 24% free 23518K/30936K, paused 24ms, total 24ms
02-17 14:58:06.645: I/dalvikvm-heap(23289): Grow heap (frag case) to 30.616MB for 4194320-byte allocation
02-17 14:58:07.726: D/dalvikvm(23289): GC_FOR_ALLOC freed 4204K, 23% free 23921K/30932K, paused 33ms, total 33ms
02-17 14:58:07.736: I/dalvikvm-heap(23289): Grow heap (frag case) to 31.010MB for 4194320-byte allocation
02-17 14:58:08.517: D/dalvikvm(23289): GC_FOR_ALLOC freed 4293K, 23% free 23831K/30932K, paused 29ms, total 30ms
02-17 14:58:08.527: I/dalvikvm-heap(23289): Grow heap (frag case) to 30.922MB for 4194320-byte allocation
02-17 14:58:09.227: D/dalvikvm(23289): GC_FOR_ALLOC freed 4194K, 23% free 23868K/30932K, paused 27ms, total 27ms
02-17 14:58:09.237: I/dalvikvm-heap(23289): Grow heap (frag case) to 30.958MB for 4194320-byte allocation
02-17 14:58:09.968: D/dalvikvm(23289): GC_FOR_ALLOC freed 4149K, 23% free 23884K/30932K, paused 25ms, total 26ms
02-17 14:58:09.968: I/dalvikvm-heap(23289): Grow heap (frag case) to 30.973MB for 4194320-byte allocation
02-17 14:58:10.118: D/dalvikvm(23289): GC_FOR_ALLOC freed 4377K, 24% free 23626K/30932K, paused 25ms, total 25ms
02-17 14:58:10.128: I/dalvikvm-heap(23289): Grow heap (frag case) to 30.721MB for 4194320-byte allocation
02-17 14:58:10.439: D/dalvikvm(23289): GC_FOR_ALLOC freed 4508K, 24% free 23642K/30932K, paused 24ms, total 25ms
02-17 14:58:13.532: D/dalvikvm(23289): GC_FOR_ALLOC freed 2705K, 24% free 23782K/30932K, paused 26ms, total 27ms
02-17 14:58:25.974: D/dalvikvm(23289): GC_FOR_ALLOC freed 1949K, 20% free 24954K/30932K, paused 37ms, total 38ms
02-17 14:58:32.340: I/dalvikvm(23289): threadid=3: reacting to signal 3
02-17 14:58:32.430: I/dalvikvm(23289): Wrote stack traces to '/data/anr/traces.txt'
02-17 14:58:39.787: D/dalvikvm(23289): GC_FOR_ALLOC freed 1959K, 17% free 26506K/31692K, paused 51ms, total 51ms
02-17 14:58:54.081: D/dalvikvm(23289): GC_FOR_ALLOC freed 2235K, 17% free 28302K/33764K, paused 67ms, total 69ms
02-17 14:59:12.779: D/AndroidRuntime(23289): Shutting down VM
02-17 14:59:12.779: W/dalvikvm(23289): threadid=1: thread exiting with uncaught exception (group=0x41890898)
02-17 14:59:12.829: E/AndroidRuntime(23289): FATAL EXCEPTION: main
02-17 14:59:12.829: E/AndroidRuntime(23289): java.lang.NullPointerException
02-17 14:59:12.829: E/AndroidRuntime(23289):    at java.lang.StringToReal.parseDouble(StringToReal.java:244)
02-17 14:59:12.829: E/AndroidRuntime(23289):    at java.lang.Double.parseDouble(Double.java:295)
02-17 14:59:12.829: E/AndroidRuntime(23289):    at com.fly.plane.MyMapActivity$Getdata.onPostExecute(MyMapActivity.java:222)
02-17 14:59:12.829: E/AndroidRuntime(23289):    at com.fly.plane.MyMapActivity$Getdata.onPostExecute(MyMapActivity.java:1)
02-17 14:59:12.829: E/AndroidRuntime(23289):    at android.os.AsyncTask.finish(AsyncTask.java:631)
02-17 14:59:12.829: E/AndroidRuntime(23289):    at android.os.AsyncTask.access$600(AsyncTask.java:177)
02-17 14:59:12.829: E/AndroidRuntime(23289):    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
02-17 14:59:12.829: E/AndroidRuntime(23289):    at android.os.Handler.dispatchMessage(Handler.java:99)
02-17 14:59:12.829: E/AndroidRuntime(23289):    at android.os.Looper.loop(Looper.java:137)
02-17 14:59:12.829: E/AndroidRuntime(23289):    at android.app.ActivityThread.main(ActivityThread.java:5419)
02-17 14:59:12.829: E/AndroidRuntime(23289):    at java.lang.reflect.Method.invokeNative(Native Method)
02-17 14:59:12.829: E/AndroidRuntime(23289):    at java.lang.reflect.Method.invoke(Method.java:525)
02-17 14:59:12.829: E/AndroidRuntime(23289):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1187)
02-17 14:59:12.829: E/AndroidRuntime(23289):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1003)
02-17 14:59:12.829: E/AndroidRuntime(23289):    at dalvik.system.NativeStart.main(Native Method)

我想我已經知道了

            double latitude = Double.parseDouble(dlat);
            double longitude = Double.parseDouble(dlon);

似乎引起問題，好像我將它們更改為設定值一樣，效果很好。

我會花上幾天的時間，無法弄清為什么它不起作用，對我的幫助不勝感激！

干杯菲爾。

Answer 1

在doInBackground方法中，您可以重新創建dlat和dlong字符串。 它們與您在代碼頂部創建的全局dlat和dlong變量無關。

 String dlat = dep.getString(TAG_DEP_LAT);
 String dlon = dep.getString(TAG_DEP_LON);

但是在onPostExecute內部，您嘗試訪問在開始時創建的全局變量，但看起來好像從未初始化過它們。

  double latitude = Double.parseDouble(dlat);
  double longitude = Double.parseDouble(dlon);

那就是為什么你得到一個空指針。

編輯：快速修復-嘗試替換（在doInBackground方法內）

String dlat = dep.getString(TAG_DEP_LAT);
String dlon = dep.getString(TAG_DEP_LON);

通過

dlat = dep.getString(TAG_DEP_LAT);
dlon = dep.getString(TAG_DEP_LON);