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);
            }
        }
    }

Pyranja · Accepted Answer · 2012-06-08 10:28:05Z

3

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.

answered Jun 8, 2012 at 10:28

Pyranja

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1 Comment

Kevin Kamer Over a year ago

so it should be itrVak.remove();? but when i do that it trows Exception in thread "AWT-EventQueue-0" java.lang.IllegalStateException

Prince John Wesley · Accepted Answer · 2012-06-08 11:22:38Z

2

While iterating, you should use Iterator instance for removing object:

itre.remove();

edited Jun 8, 2012 at 11:22

answered Jun 8, 2012 at 10:25

Prince John Wesley

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6 Comments

Kevin Kamer Over a year ago

But must be removed from vakken not from the iterator. So if its is an iterator and i remove it there it will also be removed from vakken(arrayList)

Prince John Wesley Over a year ago

@KevinKamer: it will be removed from the original instance.

Kevin Kamer Over a year ago

an iterator doesnt let me remove an object. only remove what he is processing. But it trows Exception in thread "AWT-EventQueue-0" java.lang.IllegalStateException

Prince John Wesley Over a year ago

it will remove the current item. use right iterator instance.

UVM Over a year ago

@PrinceJohnWesley, I think there is no method like remove() with arguement in Iterator Class.You can call like this: itre.remove();

|

UVM · Accepted Answer · 2012-06-08 10:57:43Z

1

You can try like this:

itre.remove();

answered Jun 8, 2012 at 10:57

UVM

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Comments

ngesh · Accepted Answer · 2012-06-08 10:27:01Z

0

ITERATOR never lets you modify when you are iterating.. you need to use loops instead.. this happens coz you are using the Iterator, same time other thread is modifying the list...

answered Jun 8, 2012 at 10:27

ngesh

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