In case of list when we traverse it with first loop
Iterator itr = set.iterator();
while (itr.hasNext()) {
System.out.println(itr.next());
}
The cursor value and size will become same.Cursor contains value for total no of elements traversed and inside hashNext() method for list traversal contains code as:
public boolean hasNext() {
return cursor != size;
}
So after first while loop cursor == size.But after removing element from list size becomes (originalSize-1).So for next while loop it goes inside while and inside itr.next() method it checks for modcount modification and throws ConcurrentModificationException.
In case of Set it checks for next != null for every itr.hasnext() call.And after traversing first while loop next becomes null.Removing element from set does not affect next value as null and itr.hasNext will return next == null as true and hence it does not go inside while loop to check modcount modification.And hence it does not throw ConcurrentModification Exception.
