jBox2d querying aabb

Newbie & Debugging Questions
#1

Hey again
I must admit I’m feeling pretty dumb - it seems like every couple of minutes I run into another issue with jBox2d, this one is to do with querying the world object’s AABBs and it’s giving me an annoying exception:


Exception in thread "AWT-EventQueue-0" java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
	at java.util.ArrayList$Itr.next(ArrayList.java:851)
	at morningside.physics.PhysicsManager.queryWorld(PhysicsManager.java:332)
	at morningside.physics.PhysicsManager.processCommand(PhysicsManager.java:295)
	at morningside.main.Console.execute(Console.java:148)

The way I query the AABB is by using two arraylists that I empty after each query:


 public ArrayList<Body> queryWorld(float x1, float y1, float x2, float y2) {
        callbackList.clear();
        bodyList.clear();

        queryAABB = new AABB(new Vec2(x1, y1), new Vec2(x2, y2));
        gWorld.queryAABB(new QueryCallback() {
            @Override
            public boolean reportFixture(Fixture fixture) {
                callbackList.add(fixture);
                return true;
            }
        }, queryAABB);

        for (Fixture f : callbackList) {
            if (!bodyList.contains(f.getBody())) { //the error occurs here
                callbackList.add(f);
            }
        }

        return bodyList;
    }

The error seems to be occurring at the line that I commented - it’s when i try to access the list I’ve populated via the callback methods after I call the query. I’m just not entirely sure what causes this error - is there a way for QueryAABB to block somehow? This is the only place I access that list as of now so I’m sort of stumped.

If you need any more info ask me, again thank you all for your help.

#2

You cannot add to “callbackList” (line 16 in your code listing) while you iterate over it (line 14).

#3 

 public ArrayList<Body> queryWorld(float x1, float y1, float x2, float y2) {
        callbackList.clear();
        bodyList.clear();

        queryAABB = new AABB(new Vec2(x1, y1), new Vec2(x2, y2));
        gWorld.queryAABB(new QueryCallback() {
            @Override
            public boolean reportFixture(Fixture fixture) {
                callbackList.add(fixture);
                return true;
            }
        }, queryAABB);

+        List<Fixture> temp = new ArrayList<>();

        for (Fixture f : callbackList) {
            if (!bodyList.contains(f.getBody())) { //the error occurs here
+                temp.add(f);
            }
        }

+       callbackList.addAll(temp);

        return bodyList;
    }
#4

ah, now that makes too much sense for me to have thought of that by myself 8)
thank you very much