Stuck with "java.util.ConcurrentModificationException"

Question

Here is my code:

// eventList is a LinkedList

public void run() {

    Iterator<Event> it = eventList.iterator();
    int size = eventList.size();

    while(size > 0) {
        while(it.hasNext()) {
            Event e = it.next(); //flaged line

            if(e.ready()) {
                System.out.println(e);
                e.action();
                eventList.remove(e);
                --size;
            }
        }
    }
}

The error java.util.ConcurrentModificationException is thrown at the flag lined (Event e = it.next();). Do you see a mistake in my code that makes obvious the reason of that exception to be thrown?

possible duplicate of loop on list with remove

McDowell
– McDowell

2011-05-09 12:09:15 +00:00
Commented May 9, 2011 at 12:09 — McDowell
– McDowell, Commented May 9, 2011 at 12:09

Joachim Sauer · Accepted Answer · 2011-04-13 07:32:56Z

17

Your are modifying eventList while using eventList.remove() while iterating over it. You must not do this, or the Iterator becomes unusable.

Simply replace eventList.remove(e) with it.remove() and it should be fine.

Also, you can easily run into an endless loop if one of your events isn't ready in the first run because it.hasNext() will never return true once it returned false, but size won't be modified either. One solution would be to move the whole Iterator it = ... line inside the first while loop.

I'd also modify the outer while loop to use while (!e.isEmpty()) instead of trying to track the size of eventList manually.

answered Apr 13, 2011 at 7:32

Joachim Sauer

309k59 gold badges568 silver badges624 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Riccardo Cossu Over a year ago

+1 for this answer; just a note, this problem is described in the docs for that exception: download.oracle.com/javase/6/docs/api/java/util/…

artaxerxe Over a year ago

Thank you. I think that the problem was that I looked to much time at code:) Thank you.

rsp · Accepted Answer · 2011-04-13 07:33:52Z

1

You should remove the element through the iterator, otherwise the iterator gets reset because the underlying collection changed.

answered Apr 13, 2011 at 7:33

rsp

23.4k6 gold badges59 silver badges72 bronze badges

Collectives™ on Stack Overflow

Stuck with "java.util.ConcurrentModificationException"

2 Answers 2

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related