我试图通过在公共汽车行驶的所有公共汽车站之间绘制方向路线来在我的地图上绘制公共汽车路线。
我有一个列表（markerPoints），其中包含所有公共汽车站的 LatLng 对象，并尝试为所有点运行一个循环，其中我为每个点接收到循环当前位置的公共汽车站之间的方向路线和下一个。
问题是运行此代码时有时会出现 Nullpointerexeption 错误。它通常在我第一次运行时运行，然后在下次运行时崩溃。但是，如果我更改循环使其只运行一次，因此只获得一个方向路线，它正在工作......
谁能帮我找到解决问题的方法，或者指出我做错了什么？
我的方法与循环调用异步方法来接收方向路由
public void DrawRoutePath(){
//Start on "Torget" because of issues with route path in city center
for (int i = 0; i < (markerPoints.size()-1); i++) {
LatLng origin = markerPoints.get(i);
LatLng dest = markerPoints.get(i+1);
// Getting URL to the Google Directions API
String url = getDirectionsUrl(origin, dest);
DownloadTask task = new DownloadTask();
// Start downloading json data from Google Directions API
task.execute(url);
}
}
这些是我获取异步方向的方法：
private String getDirectionsUrl(LatLng origin,LatLng dest){
// Origin of route
String str_origin = "origin="+origin.latitude+","+origin.longitude;
// Destination of route
String str_dest = "destination="+dest.latitude+","+dest.longitude;
// Sensor enabled
String sensor = "sensor=false";
// Building the parameters to the web service
String parameters = str_origin+"&"+str_dest+"&"+sensor;
// Output format
String output = "json";
// Building the url to the web service
String url = "https://maps.googleapis.com/maps/api/directions/"+output+"?"+parameters;
return url;
}
/** A method to download json data from url */
private String downloadUrl(String strUrl) throws IOException{
String data = "";
InputStream iStream = null;
HttpURLConnection urlConnection = null;
try{
URL url = new URL(strUrl);
// Creating an http connection to communicate with url
urlConnection = (HttpURLConnection) url.openConnection();
// Connecting to url
urlConnection.connect();
// Reading data from url
iStream = urlConnection.getInputStream();
BufferedReader br = new BufferedReader(new InputStreamReader(iStream));
StringBuffer sb = new StringBuffer();
String line = "";
while( ( line = br.readLine()) != null){
sb.append(line);
}
data = sb.toString();
br.close();
}catch(Exception e){
Log.d("Exception while downloading url", e.toString());
}finally{
iStream.close();
urlConnection.disconnect();
}
return data;
}
// Fetches data from url passed
private class DownloadTask extends AsyncTask<String, Void, String>{
// Downloading data in non-ui thread
@Override
protected String doInBackground(String... url) {
// For storing data from web service
String data = "";
try{
// Fetching the data from web service
data = downloadUrl(url[0]);
}catch(Exception e){
Log.d("Background Task",e.toString());
}
return data;
}
// Executes in UI thread, after the execution of
// doInBackground()
@Override
protected void onPostExecute(String result) {
super.onPostExecute(result);
ParserTask parserTask = new ParserTask();
// Invokes the thread for parsing the JSON data
parserTask.execute(result);
}
}
/** A class to parse the Google Places in JSON format */
private class ParserTask extends AsyncTask<String, Integer, List<List<HashMap<String,String>>> >{
// Parsing the data in non-ui thread
@Override
protected List<List<HashMap<String, String>>> doInBackground(String... jsonData) {
JSONObject jObject;
List<List<HashMap<String, String>>> routes = null;
try{
jObject = new JSONObject(jsonData[0]);
DirectionsJSONParser parser = new DirectionsJSONParser();
// Starts parsing data
routes = parser.parse(jObject);
}catch(Exception e){
e.printStackTrace();
}
return routes;
}
// Executes in UI thread, after the parsing process
@Override
protected void onPostExecute(List<List<HashMap<String, String>>> result) {
ArrayList<LatLng> points = null;
PolylineOptions lineOptions = null;
MarkerOptions markerOptions = new MarkerOptions();
// Traversing through all the routes
for(int i=0;i<result.size();i++){
points = new ArrayList<LatLng>();
lineOptions = new PolylineOptions();
// Fetching i-th route
List<HashMap<String, String>> path = result.get(i);
// Fetching all the points in i-th route
for(int j=0;j<path.size();j++){
HashMap<String,String> point = path.get(j);
double lat = Double.parseDouble(point.get("lat"));
double lng = Double.parseDouble(point.get("lng"));
LatLng position = new LatLng(lat, lng);
points.add(position);
}
// Adding all the points in the route to LineOptions
lineOptions.addAll(points);
lineOptions.width(5);
lineOptions.color(Color.RED);
}
// Drawing polyline in the Google Map for the i-th route
mMap.addPolyline(lineOptions);
}
}
来自 logcat 的错误日志
04-22 20:03:18.911: E/AndroidRuntime(10471): FATAL EXCEPTION: main
04-22 20:03:18.911: E/AndroidRuntime(10471): java.lang.NullPointerException
04-22 20:03:18.911: E/AndroidRuntime(10471): at maps.z.ac.<init>(Unknown Source)
04-22 20:03:18.911: E/AndroidRuntime(10471): at maps.z.bi.a(Unknown Source)
04-22 20:03:18.911: E/AndroidRuntime(10471): at maps.z.bi.b(Unknown Source)
04-22 20:03:18.911: E/AndroidRuntime(10471): at maps.z.ag.addPolyline(Unknown Source)
04-22 20:03:18.911: E/AndroidRuntime(10471): at com.google.android.gms.maps.internal.IGoogleMapDelegate$Stub.onTransact(IGoogleMapDelegate.java:137)
04-22 20:03:18.911: E/AndroidRuntime(10471): at android.os.Binder.transact(Binder.java:326)
04-22 20:03:18.911: E/AndroidRuntime(10471): at com.google.android.gms.maps.internal.IGoogleMapDelegate$a$a.addPolyline(Unknown Source)
04-22 20:03:18.911: E/AndroidRuntime(10471): at com.google.android.gms.maps.GoogleMap.addPolyline(Unknown Source)
04-22 20:03:18.911: E/AndroidRuntime(10471): at com.mso.master.MyMapFragment$ParserTask.onPostExecute(MyMapFragment.java:469)
04-22 20:03:18.911: E/AndroidRuntime(10471): at com.mso.master.MyMapFragment$ParserTask.onPostExecute(MyMapFragment.java:1)
04-22 20:03:18.911: E/AndroidRuntime(10471): at android.os.AsyncTask.finish(AsyncTask.java:631)
04-22 20:03:18.911: E/AndroidRuntime(10471): at android.os.AsyncTask.access$600(AsyncTask.java:177)
04-22 20:03:18.911: E/AndroidRuntime(10471): at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
04-22 20:03:18.911: E/AndroidRuntime(10471): at android.os.Handler.dispatchMessage(Handler.java:99)
04-22 20:03:18.911: E/AndroidRuntime(10471): at android.os.Looper.loop(Looper.java:137)
04-22 20:03:18.911: E/AndroidRuntime(10471): at android.app.ActivityThread.main(ActivityThread.java:4931)
04-22 20:03:18.911: E/AndroidRuntime(10471): at java.lang.reflect.Method.invokeNative(Native Method)
04-22 20:03:18.911: E/AndroidRuntime(10471): at java.lang.reflect.Method.invoke(Method.java:511)
04-22 20:03:18.911: E/AndroidRuntime(10471): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
04-22 20:03:18.911: E/AndroidRuntime(10471): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:558)
04-22 20:03:18.911: E/AndroidRuntime(10471): at dalvik.system.NativeStart.main(Native Method)