java.util.ConcurrentModificationException after removing an element of a list

Question

The line ITSPoI allowedPoi = j.next() provides the following error after executing allowedPoIs.remove(k):

java.util.ConcurrentModificationException at java.util.ArrayList$Itr.checkForComodification(Unknown Source)

private Map<String,Integer> findClosest(Route tempRoute, List<ITSPoI> allowedPoIs, List<ITSPoI> tabulist) {  
    double cost,mincost = 999999999; 
    Map<String,Integer> closest = new HashMap<String,Integer>();

    for (int i=0; i<tempRoute.getPOIs().size(); i++)        {
        int k = 0;
        for(Iterator<ITSPoI> j = allowedPoIs.iterator(); j.hasNext(); )  {

            ITSPoI allowedPoi = j.next();
            if (!intabu(allowedPoi,tabulist))
            {
                try 
                {
                    cost = _cvrtw.getCostMatrix().getCost(tempRoute.getPOI(i).getNodeId(),allowedPoi.getNodeId());

                    if (cost<mincost){
                        mincost = cost;
                        closest.put("index",i);
                        closest.put("poi",k);
                        allowedPoIs.remove(k);

                    }
                } 
                catch (Exception e) 
                {
                    e.printStackTrace();
                }
            }
            k++; 
            } 
        }
    return closest; 
    }

it is not generally permissible for one thread to modify a Collection while another thread is iterating over it. — Ashok
– Ashok, Commented Oct 1, 2013 at 7:41
@Ashok - or while the same thread is iterating over it, which is a more common problem (and the issue here). — Andrzej Doyle
– Andrzej Doyle, Commented Oct 1, 2013 at 7:46
enough to replace Array with Vector or List myList = Collections.synchronizedList (myList); — Maxim Shoustin
– Maxim Shoustin, Commented Oct 1, 2013 at 7:52

Maxim Shoustin · Accepted Answer · 2013-10-01 07:49:21Z

You can't remove element from allowedPoIs when you run in loop. Remove it from iterator like:

j.remove();

instead allowedPoIs.remove(k);

I would write this method with while to make it clearer:

private Map<String,Integer> findClosest(Route tempRoute, List<ITSPoI> allowedPoIs, List<ITSPoI> tabulist) {  
    double cost,mincost = 999999999; 
    Map<String,Integer> closest = new HashMap<String,Integer>();

           Iterator<ITSPoI> iter;

    for (int i=0; i<tempRoute.getPOIs().size(); i++)        {
        int k = 0;

        iter = allowedPoIs.iterator();

        while(iter.hasNext()){
            ITSPoI allowedPoi = iter.next();
            if (!intabu(allowedPoi,tabulist))
            {
                try 
                {
                    cost = _cvrtw.getCostMatrix().getCost(tempRoute.getPOI(i).getNodeId(),allowedPoi.getNodeId());

                    if (cost<mincost){
                        mincost = cost;
                        closest.put("index",i);
                        closest.put("poi",k);                           
                        iter.remove(); // fix
                    }
                } 
                catch (Exception e) 
                {
                    e.printStackTrace();
                }
            }
            k++; 
        }       

    }
    return closest; 
}

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