java.util.ConcurrentModificationException android after remove elements from array list

Question

I have the folloing code in my android app:

/**
 * callback executed after fetching the data.
 */
public void OnPointsFetch(ArrayList<Shop> result) {

    toggleLoader(false);

    this.shops = result;

    if(activeFilter == Constants.POINTS_FILTER_AVAILABLE){
        for(Shop s : result){
            if(s.getClientPoints().getPointsAvailable() == 0){
                this.shops.remove(s);
            }
        }
    }
    else{
        for(Shop s : result){
            if(s.getClientPoints().getPointsSpent() == 0){
                this.shops.remove(s);
            }   
        }
    }


    ptsListAdapter.setCollection(this.shops);
    ptsListAdapter.setFilter(this.activeFilter);

}

This method is called on the result of an async task. I need to remove some elements of the collection before passing to the list adapter.

    11-23 17:39:59.760: E/AndroidRuntime(19777): java.util.ConcurrentModificationException
11-23 17:39:59.760: E/AndroidRuntime(19777):    at java.util.ArrayList$ArrayListIterator.next(ArrayList.java:569)

assylias · Accepted Answer · 2012-11-23 17:48:06Z

57

You can't remove items from a list while iterating over it. You need to use an iterator and its remove method:

for(Iterator<Shop> it = result.iterator(); it.hasNext();) {
    Shop s = it.next();
    if(s.getClientPoints().getPointsSpent() == 0) {
        it.remove();
    }   
}

answered Nov 23, 2012 at 17:48

assylias

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Community · Accepted Answer · 2020-06-20 09:12:55Z

2

You get this error typically when

You modify the collection directly while it is iterating over the collection

or even worse when
one threads modifies the collection, while another iterates over it.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Nov 23, 2012 at 17:50

AlexWien

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1 Comment

assylias Over a year ago

He would get that exception with one thread too (he is actually probably only running one thread).

dotrc · Accepted Answer · 2014-10-20 11:30:27Z

1

Not sure if the accepted answer would work, as internally it would be trying to again modify the same list. A cleaner approach would be to maintain a 'deletion' list, and keep adding elements to that list within the loop. Once we are ready with the deletion list, they can be removed after the loop. This should work in all cases where we do not need the deleted element to be reprocessed. If yes, then the existing deletion list can be checked for presence of that element.

    List<String> list = new ArrayList<String>();
    List<String> listRemove = new ArrayList<String>();

    list.add("1");
    list.add("2");
    list.add("3");
    list.add("4");
    list.add("5");
    list.add("6");
    list.add("7");
    list.add("8");

    System.out.println("list : " + list);

    for (String i : list) {
        if (i.equals("2")) {
            listRemove.add(i);
        }
    }
    list.removeAll(listRemove);
    System.out.println("updated list: " + list);

answered Oct 20, 2014 at 11:30

dotrc

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1 Comment

Stony Over a year ago

No, it can reduce the possibility. ArrayList is not thread-safe. If multiple thread modify the the same arrayList, the issue will also happen.

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