Question

I am trying to write a very simple method to remove duplicates in a LinkedList:

I try to do this without using additional buffer, so I maintain two iterators on the linked list, one does the normal iteration, and another iterates through all prior nodes to check for dupes (as indicated in CareerCup); however, the compiler tells me there is a CME even though I am calling itr1.remove():

public static void RemoveWithoutBuffer(LinkedList l) {
    ListIterator itr1 = l.listIterator();   
    int count1 = 0;
    int count2 = 0;
    while (itr1.hasNext()) {
        Object next = itr1.next();
        count1++;
        count2 = 0;
        ListIterator itr2 = l.listIterator();
        while (itr2.hasNext()) {
            count2++;
            if (count2 == count1)
                break;
            if (itr2.next() == next){
                itr1.remove();
            }
        }

    }
}

Another simpler solution of this problem with the aid of hashset is easy as follows, and no exception reported:

    public static void Remove(LinkedList l) {
    HashSet set = new HashSet();
    ListIterator itr = l.listIterator();
    while (itr.hasNext()) {
        Object next = itr.next();
        if (set.contains(next))
            itr.remove();
        else
            set.add(next);
    }
}

Is it because when I am iterating through itr2 I cannot modify on itr1? Is there a way to fix this? Thank you guys.

Answer 1

In the first case yes – you’re altering the collection’s contents via iterator2, while iterator1 is not aware about the changes. In the second case HashSet/HashMap don’t allow removing elements while iterating through them.

You can add removed elements to another collection, and removeAll them after an iteration. E.g.

    List toRemove = new ArrayList();
    for (Object next : collection) {
        if (someCondition) toRemove.add(next);
    }
    collection.removeAll(toRemove);

I hope it helps.

P.S. more details on how to remove elements from list, concerning algorithm complexity you can read here Removing ArrayList object issue