遍历hasmap并使用键值

Question

I'm trying to iterate through a hashmap and and use the key during my iteration. I decided to use it inside an Async Task because the iteration was blocking my UI thread.

My hashmap has LatLng as the Key and a Marker as the Object, when I get the key during Iteration and pass it to a new LatLng that works fine but when I try to use this new LatLng I get a java.util.ConcurrentModificationException exception. Strangely this doesn't happen when I use the code without the AsyncTask

Here is my code

    class AddCityMarker extends AsyncTask<Integer, Integer, String> {

    protected String doInBackground(Integer... counter) {           

        //CityMarker cMarker;
        LatLng marker_loc;

        List<String> city_markers = new ArrayList<String>();
        if(displayed.size() > 0){

            Iterator<HashMap.Entry<LatLng, Marker>> myIterator = displayed.entrySet().iterator();
            while(myIterator.hasNext()) {
                //cMarker = new CityMarker();
                HashMap.Entry<LatLng, Marker> entry = myIterator.next();                    
                marker_loc = entry.getKey();

                Log.i("ZOOM", "Key = " + marker_loc + ", Value = " + entry.getValue());

                List<Address> addresses = null;                     
                try {
                  //This is where I get the error
                    addresses = gcode.getFromLocation(marker_loc.latitude, marker_loc.longitude, 1);
                } catch (IOException e) {
                    e.printStackTrace();
                }

                if(addresses.size() > 0) {  
                    if(!city_markers.contains(addresses.get(0).getLocality())){
                        city_markers.add(addresses.get(0).getLocality());
                        //map.addMarker(new MarkerOptions().position(marker_loc).snippet("CITY"));
                    }                           
                }
            }                   
        }


        return null;
    }

    protected void onPostExecute(String jsonResult) {

        try {   

            if(isClubMarkers){
                map.clear();
                isClubMarkers = false;
            }
            displayed.clear();

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Answer 1

From Android 1.6 to Android HoneyComb, the async tasks are running in parallel mode. That means like different threads. Some times in your case, you might be calling the async task more than one time and they may try to access the same HashMap for the itteration. You can avoid this exception by using "ConcurrentHashMap" instead of HashMap. But the proper way is to identify whether the async task is calling more than once and try to avoid it if its there. This can also happen if you tries to operate with the Hashmap when the Async task itterates though the same. If thats the case use "ConcurrentHashMap" which is thread safe.