How to Avoid ConcurrentModificationException in that case?

Question

I understand that when I try to modify (add in that case) the list I got ConcurrentModificationException, but what is the best solution to fix that?

for (Map.Entry<String, Child> entry : children.entrySet() {
     childEvent.child = entry.getValue();
     if (childEvent.getDate() != null && childEvent.getDate().equals(selectedDate)) {
         if(this.selectedDayevents.isEmpty()) {
             // List                                
             this.selectedDayevents.add(childEvent);
          }
         for (CareDay selectedCareDay : this.selectedDayevents) {
             // Here I have to combine data in some cases...
         }
     }  
}

Collect the changes in an other `Map`, then use `Map.addAll` after you did loop over the entries. — Johannes Kuhn, Oct 22 '19 at 06:11
But I need to add data inside the second for-loop to CareDay-objects so I think that it is not the solution to collect every changed object to third List,Map etc and then I have to loop those again (I think) — Sami, Oct 22 '19 at 06:28
If you need to modify list during iterating - use this list `Iterator` to `add` and `remove` elements or create new list and add/remove to/from it. — Michał Krzywański, Oct 22 '19 at 06:29
Please show the failing code in your [mcve]. Apparently the `add` statement you're showing is not the one that is failing? In that case you should remove it to make the example *minimal* , and please add the statement this *is* failing. Also please indent the code correctly - it's hardly possible to read it now. — Erwin Bolwidt, Oct 22 '19 at 06:30

score 0 · Answer 1 · answered Oct 22 '19 at 07:20

One simple way around this problem is to iterate over a copy of the entry set:

for (Map.Entry<String, Child> entry : new HashSet<>(children.entrySet())) {
    // same code
}

If your map is not too big and you’re not doing it very often, you won’t notice a difference in performance.

score -1 · Answer 2 · answered Oct 22 '19 at 06:42

-1

If your requirement is to enable concurrent access to the collection, then you should explore java concurrency APIs and especially ConcurrentHashMap or ConcurrentSkipListMap for your case.

answered Oct 22 '19 at 06:42

Elisha Ebenezer

57
6

The OP didn't mention anything related to concurrent access. If you want to ask the OP a question, please use comments when you have enough reputation to do so. – Erwin Bolwidt Oct 22 '19 at 06:46
Erwin, my answer clearly starts with "If your requirement is...", hence the necessary conditional nature of the answer is explicitly mentioned. – Elisha Ebenezer Oct 22 '19 at 07:16

How to Avoid ConcurrentModificationException in that case?

2 Answers2