ConcurrentModificationException on isEmpty() method of ArrayList

Question

I have the following code:

Map<String, List<String>> map;
for(String k : map.keySet()){
   List<String> list = map.get(k);
   boolean empty = list.isEmpty();//CME
   if(!empty && somecheck(k, ...)){
      list.clear();
   }
}

And I'm getting ConcurrentModificationException in isEmpty() method. List is an ArrayList. There is no other threads modifying list, because it was created in this method before (and the all map too). The only place modifying list is clear(), but it called after isEmpty() and loop cannot execute on one list twice.

I'm using java 1.7

java.util.ConcurrentModificationException
    at java.util.ArrayList$SubList.checkForComodification(ArrayList.java:1169)
    at java.util.ArrayList$SubList.size(ArrayList.java:998)
    at java.util.AbstractCollection.isEmpty(AbstractCollection.java:86)

what is "if(list)" check supposed to do and how does it compile? Please provide stackoverflow.com/help/mcve — Oleg Estekhin
– Oleg Estekhin, Commented May 31, 2017 at 10:00
@ΦXocę웃Пepeúpaツ As i said, map created in this method. — TEXHIK
– TEXHIK, Commented May 31, 2017 at 10:05

BarrySW19 · Accepted Answer · 2017-05-31 10:07:40Z

6

From the stacktrace you have given it appears the exception is being thrown in the sub-class implementing the SubList functionality - I assume the lists in your map are actually sublists of another list?

Presumably what is happening is that you are modifying the underlying list after you have created the sublist view (remember, a sublist is just a view onto another list - it does not take an independent copy).

Instead of putting sublists into the map, try taking a copy instead, for example:

map.put(key, new ArrayList(originalList.subList(start, end));

answered May 31, 2017 at 10:07

BarrySW19

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2 Comments

Sam Over a year ago

Good sleuthing!

Holger Over a year ago

Perhaps, the list values in that map are all sublists of the same source list, so when clear() is invoked on one of them during in the loop, calling isEmpty() on one of the others in the next iteration will fail…

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