Jan 30

How to fix ConcurrentModificationException?

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The general cause of a ConcurrentModificationException is from trying to remove an instance from a collection that is being iterated over as in the following example.


Iterator i = mylist.iterator();
while (i.hasNext())
{
   Object o = i.next();
   if (something)
   {
       // following line will throw ConcurrentModificationException
       mylist.remove(o);
   }
}

The correct way is to instead use the Iterator’s remove() method as shown in the following snippet:


Iterator i = mylist.iterator();
while (i.hasNext())
{
   Object o = i.next();
   if (something)
   {
       i.remove();
   }
}

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6 Responses to “How to fix ConcurrentModificationException?”

  1. Sergi Says:
    May 27th, 2011 at 4:16 am

    Thank you very much! This solved a problem I had that was driving me crazy.

  2. Raymond Says:
    June 21st, 2011 at 6:54 pm

    Thank you, you just save my day !!!

  3. Mike Says:
    August 6th, 2011 at 5:03 am

    Like the others, thanks, you’ve saved me a headache!

  4. Nick Says:
    August 30th, 2011 at 3:01 am

    Thanks for the explanation!!!
    In my case I decided to do clone(), so to avoid this, because my program runs in two threads.

  5. devi Says:
    January 13th, 2012 at 5:12 pm

    Thanks a lot…

  6. Robby Says:
    February 27th, 2012 at 8:07 pm

    Thx, worked like a charm 🙂

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