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.

Via: http://stackoverflow.com/questions/602636/concurrentmodificationexception-and-a-hashmap/602660#602660

ConcurrentModificationException and a HashMap