ConcurrentModificationException for ArrayList [duplicate]

Question

I have the following piece of code:

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

When I try to run it, I get ConcurrentModificationException, can anyone explain why it happens, even if the code is running in same thread? And how could I avoid it?

An explanation of this exception is that the iterator of the ArrayList is a fail-fast iterator; i.e. it will fail (throw exception) when it detects that its collection in the mean-time has been modified. In comparison to fail-safe iterators which don't throw concurrent modification exceptions (e.g. on collections ConcurrentHashMap and CopyOnWriteArrayList) — Mike Argyriou
– Mike Argyriou, Commented May 28, 2014 at 12:05

Konrad Garus · Accepted Answer · 2010-07-06 09:13:46Z

181

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();
    }
}

answered Jul 6, 2010 at 9:13

Konrad Garus

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2 Comments

Hugo Gresse Over a year ago

the foreach syntax of java actually use Iterator, some IDE will report this solution and propose to replace with the foreach (for(MyListener listener : MyListenerList))

Konrad Garus Over a year ago

@HugoGresse Yes, but this is the opposite direction. Iterator exposes remove that is safe for its iteration, something that foreach "loses".

Edward Dale · Accepted Answer · 2010-07-06 09:11:48Z

29

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
  }
}

answered Jul 6, 2010 at 9:11

Edward Dale

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Comments

froman · Accepted Answer · 2012-12-20 00:27:55Z

24

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.

answered Dec 20, 2012 at 0:27

froman

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Comments

idiotgenius · Accepted Answer · 2010-07-06 13:29:34Z

10

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

try java.util.concurrent.CopyOnWriteArrayList.class

answered Jul 6, 2010 at 13:29

idiotgenius

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1 Comment

idiotgenius Over a year ago

I had a same problem with HashMap, fixed with another implemention of Map interface. You should test it yourself.I don't know detail about CopyOnWriteArrayList

bragboy · Accepted Answer · 2016-03-07 14:51:25Z

8

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();
}

edited Mar 7, 2016 at 14:51

answered Jul 6, 2010 at 9:06

bragboy

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Comments

Suganthan Madhavan Pillai · Accepted Answer · 2013-11-12 04:02:56Z

4

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

answered Nov 12, 2013 at 4:02

Suganthan Madhavan Pillai

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