Iterating a linkedlist

Question

I am working on an application which has some legacy code. Here, there is a linkedlist and the code iterates that linklist using an iterator in a while loop.

 LinkedList ll = grammarSection.getSectionsAsLinkList();
 Iterator iter = ll.iterator();
 int i=0;
 while (iter.hasNext()) {
 1. GrammarSection agrammarSection = (GrammarSection) iter.next();
 2. grammarLineWithMatches = m_grammarLineMatcher.getMatch(agrammarSection, p_line);
 3. if (grammarLineWithMatches != null) { //condition a 
 4. if (getPeek(ll)!=agrammarSection)
 5. ll.addFirst(ll.remove(i)); //changing the linkedlist Line5
 return grammarLineWithMatches;
 }
 i++;
 }

In the while loop, if condition a is true, then the linkedlist is modified as in line5. However, in this case, the next method on line1 throws a ConcurrentModificationException. How to add and delete the linkedlist without getting any ConcurrentModificationException

• coobird: possible duplicate of how to remove ConcurrentModificationException
• Bozho: not exact duplicate in this case.

Answer 1

You can't change the collection you are currently iterating. You can:

• create a copy of it and iterate the copy instead
• don't use iterator - loop from 0 to list.size(). But with LinkedList this is not efficient.

If it was only about removal, you can use iter.remove(), but you also have addFirst(..)

Answer 2

The short answer is: you can't.

The JDK's List implementations are designed to be modified by the iterator, to preserve order of iteration (there's no way to tell whether an arbitrary list change will do this, so the iterator assumes the worst).

The solution, in your case, is to create a new LinkedList. As you iterate through, either add the iterated elements to the end or beginning of the new list. Then throw away the old.

• asgs: ..NOT to be modified by the iterator?
• Anon: @asgs - no, they are designed to be modified by the iterator. They are not designed to be modified both directly and via the iterator.
• asgs: yea i was referring to the second part. Thanks for the confirmation.
• joe776: Not to be modified while iterating, except by the current iterator. The iterator is allowed to remove the last returned element.

Answer 3

Iterators are fail-fast iterators so it may throw ConcurrentModificationException in your case.

The simplest solution would be;

instead of deleting items from list while iterating, add the ones that are going to be deleted to a new list and after the loop, you can remove all those using the removeAll method of list.

or vice versa, keep the ones you need to retain and assign the new list to the old one