Concurrent Modification Exception

Question

How do I resolve ConcurrentModificationException from the below program . I need a list where first element is "Znk", and then the sorted listed following it.

I understand, I am getting this because I am adding and removing in the same iteration . But how do I resolve this and get the desired output.

public class ListSwapIndex {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        ArrayList<String> swapIndex = new ArrayList<String>();
        ArrayList<String> swapIndextemp = new ArrayList<String>();
        swapIndex.add("Ank");
        swapIndex.add("Znk");
        swapIndex.add("Bnk");
        swapIndex.add("Dnk");
        swapIndex.add("Enk");
        swapIndex.add("Lnk");

        for (String string : swapIndex) {
            if(string.equals("Znk")){
                swapIndextemp.add(string);
                swapIndex.remove(string);
                }           
        }
        swapIndextemp.addAll(swapIndex);
        System.out.println(swapIndextemp);

    }

}

No, it's because you are modifying the array concurrently. Use a vector or a synchronized arraylist instead. — Ioan, Jul 16 '16 at 18:45
See [this](http://stackoverflow.com/questions/18448671/how-to-avoid-concurrentmodificationexception-while-removing-elements-from-arr) possible duplicate "Can't archive with enhanced for loop" — Malatesh, Jul 16 '16 at 18:53

Sergey Kalinichenko · Accepted Answer · 2016-07-16T18:51:44.450

You are not allowed to modify a collection concurrently with iterating it. Java protects from this by checking the collection being iterated, and failing quickly when a modification is found.

Using ListIterator<T> instead of iterating with for-each loop fixes the problem, because list iterator of ArrayList allows deletions:

for (ListIterator<String> iter=swapIndex.listIterator(); iter.hasNext() ; ) {
    String current = iter.next();
    if(current.equals("Znk")){
        swapIndextemp.add(string);
        iter.remove();
    }
}

Note, however, that this approach is suboptimal, because removal from an array list is an O(n) operation, resulting in O(n²) overall performance. You would be better off iterating the list twice - once to put all "Znk"s at the front, and once more to put the rest of the items after it. This gives you an overall performance of O(n).

Thanks ..this works for me .Also , your explanation helps in increasing the performance — themaster, Jul 16 '16 at 18:58

Concurrent Modification Exception

1 Answers1