Adding to my arraylist causes ConcurrentModificationException even with iterator

Question

Before adding works fine with the normal for each loop, but removing isn't so I used an iterator. Now every 3rd item I add to my arrayList gives me a ConcurrentModificationException. What seems to be the problem?

private ArrayList<Marker> mMarkers which is instantiated in onCreate

Here I have a list view on my onCreateView

mListView.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener() {
        @Override
        public boolean onItemLongClick(AdapterView<?> parent, View view, int position, long id) {
            final Events event = mEventsParseQueryAdapter.getItem(position);

            GoogleMap googleMap = mSupportMapFragment.getMap();

            if (mMarkers.isEmpty()) {
                mMarkers.add(googleMap.addMarker(new MarkerOptions()
                        .position(getLatLngFromGeoPoint(event.getLocation()))));
            } else {
                Iterator<Marker> markerIterator = mMarkers.iterator();

                //noinspection WhileLoopReplaceableByForEach
                while (markerIterator.hasNext()) {
                    Marker marker = markerIterator.next();
                    if (marker.getPosition().equals(getLatLngFromGeoPoint(event.getLocation()))) {
                        return false;
                    } else {
                        mMarkers.add(googleMap.addMarker(new MarkerOptions()
                                .position(getLatLngFromGeoPoint(event.getLocation()))));
                    }
                }
            }
            return true;
        }
    });

and here's my marker on click listener which pops a dialog and have an option to remove the marker

googleMap.setOnMarkerClickListener(new GoogleMap.OnMarkerClickListener() {
        @Override
        public boolean onMarkerClick(final Marker marker) {

            googleMap.animateCamera(CameraUpdateFactory.newLatLngZoom(marker.getPosition(), CONSTANT.ZOOM_LEVEL));

            AlertDialog.Builder dialog = new AlertDialog.Builder(getActivity());
            dialog.setNegativeButton(R.string.checkout, new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int id) {
                    mMarkers.remove(marker);
                    marker.remove();
                }
            }).setPositiveButton(R.string.frames, new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                }
            });
            dialog.create().show();

            return true;
        }
    });

tinker · Accepted Answer · 2015-04-12 11:54:00Z

2

You cannot modify a list while iterating over it, no matter whether you use for-each or an iterator. Read more on the exception page: http://docs.oracle.com/javase/7/docs/api/java/util/ConcurrentModificationException.html

answered Apr 12, 2015 at 11:54

tinker

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Zielu · Accepted Answer · 2015-04-12 11:53:59Z

1

mMarkers.add(googleMap.addMarker(new MarkerOptions()

is called inside your iteration loop (while (markerIterator.hasNext())), so you are adding element at the time of traversing the collection which gives you the error.

Actually you probably wanted the :

           } else {
                mMarkers.add(googleMap.addMarker(new MarkerOptions()
                        .position(getLatLngFromGeoPoint(event.getLocation()))));
            }

branch not inside the loop but rahter outside the whole while loop (when no existing marker was found a new one is created and added.

answered Apr 12, 2015 at 11:53

Zielu

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Karan · Accepted Answer · 2015-04-12 12:02:54Z

0

You are parsing/adding or reading/writing on the list concurrently using a fail-fast iterator(markerIterator in your casE). If the iterator will detect such behavior in the code it will throw this exception.

it is not generally permissible for one thread to modify a Collection while another thread is iterating over it

Hence your read/write operations should be disassociated, time vise.

answered Apr 12, 2015 at 12:02

Karan

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