ConcurrentModificationException for ArrayList [duplicate]

Answer 1

Like the other answers say, you can't remove an item from a collection you're iterating over. You can get around this by explicitly using an Iterator and removing the item there.

Iterator<Item> iter = list.iterator();
while(iter.hasNext()) {
  Item blah = iter.next();
  if(...) {
    iter.remove(); // Removes the 'current' item
  }
}

Answer 2

You can't remove from list if you're browsing it with "for each" loop. You can use Iterator. Replace:

for (DrugStrength aDrugStrength : aDrugStrengthList) {
    if (!aDrugStrength.isValidDrugDescription()) {
        aDrugStrengthList.remove(aDrugStrength);
    }
}

With:

for (Iterator<DrugStrength> it = aDrugStrengthList.iterator(); it.hasNext(); ) {
    DrugStrength aDrugStrength = it.next();
    if (!aDrugStrength.isValidDrugDescription()) {
        it.remove();
    }
}

Answer 3

While iterating through the loop, you are trying to change the List value in the remove() operation. This will result in ConcurrentModificationException.

Follow the below code, which will achieve what you want and yet will not throw any exceptions

private String toString(List aDrugStrengthList) {
        StringBuilder str = new StringBuilder();
    List removalList = new ArrayList();
    for (DrugStrength aDrugStrength : aDrugStrengthList) {
        if (!aDrugStrength.isValidDrugDescription()) {
            removalList.add(aDrugStrength);
        }
    }
    aDrugStrengthList.removeAll(removalList);
    str.append(aDrugStrengthList);
    if (str.indexOf("]") != -1) {
        str.insert(str.lastIndexOf("]"), "\n          " );
    }
    return str.toString();
}

Answer 4

I like a reverse order for loop such as:

int size = list.size();
for (int i = size - 1; i >= 0; i--) {
    if(remove){
        list.remove(i);
    }
}

because it doesn't require learning any new data structures or classes.

Answer 5

We can use concurrent collection classes to avoid ConcurrentModificationException while iterating over a collection, for example CopyOnWriteArrayList instead of ArrayList.

Check this post for ConcurrentHashMap

http://www.journaldev.com/122/hashmap-vs-concurrenthashmap-%E2%80%93-example-and-exploring-iterator

Answer 6

there should has a concurrent implemention of List interface supporting such operation.

try java.util.concurrent.CopyOnWriteArrayList.class