ArrayList remove error

Question

I'm working on a project for school but i'm a little stuck right now

My problem is that i have an arrayList of Squares Each Square has a value(from 0 to 100). Its starting value is 9999 so i can check if its is checked. If a square is checked i want it to be removed from the arrayList. So after a while there will be no Squares left.

there is a little bit of code where the first value is set so thats why i check if the value is 9999.

But i get an error. One that i havent seen before.

Exception in thread "AWT-EventQueue-0" java.util.ConcurrentModificationException

Vak = Square

this is my code:

while (!vakken.isEmpty()) { // check if empty
        Iterator itrVak = vakken.iterator();
        while (itrVak.hasNext()) {
            Vak vak = (Vak) itrVak.next(); // here is get the error
            if (vak.getValue() != 9999) {// check if square value is 9999
                Collection checkVakken = vak.getNeighbour().values();
                Iterator itre = checkVakken.iterator();
                while (itre.hasNext()) {
                    Vak nextVak = (Vak) itre.next();
                    if (nextVak != null) {
                        if (nextVak.getValue() == 9999) {
                            nextVak.setValue(vak.getValue() + 1); // set value by its neighbour
                            vakken.remove(vak);
                            checkvakken.add(vak);
                        }
                    }
                }
            } else {
                vakken.remove(vak);
                checkvakken.add(vak);
            }
        }
    }

Answer 1

You are removing elements from the collection while you are iterating it. As the iterator may produce unpredictable results in this situation, it fails fast throwing the exception you encountered.

You may only alter a collection through the iterator's methods while traversing it. There should be remove method on the iterator itself, that removes the current element and keeps the iterator intact.