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

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.