Why is a ConcurrentModificationException thrown and how to debug it

Question

I am using a Collection (a HashMap used indirectly by the JPA, it so happens), but apparently randomly the code throws a ConcurrentModificati

Answer 1

This is not a synchronization problem. This will occur if the underlying collection that is being iterated over is modified by anything other than the Iterator itself.

Iterator it = map.entrySet().iterator();
while (it.hasNext())
{
   Entry item = it.next();
   map.remove(item.getKey());
}

This will throw a ConcurrentModificationException when the it.hasNext() is called the second time.

The correct approach would be

   Iterator it = map.entrySet().iterator();
   while (it.hasNext())
   {
      Entry item = it.next();
      it.remove();
   }

Assuming this iterator supports the remove() operation.

Answer 2

I ran into this exception when try to remove x last items from list. myList.subList(lastIndex, myList.size()).clear(); was the only solution that worked for me.