Getting ConcurrentException when working with list

Question

I am working with below code :

List<String> layerDataList = new ArrayList<String>();
if (layerDataList.isEmpty()) {
  layerDataList.add(layerData);
}

Iterator<String> lir = layerDataList.iterator(); // Iterator created

while (lir.hasNext()) {
  String layerD = lir.next();
  if (layerD != layerData) {
    layerDataList.add(layerData);
  }
}

I got exception : java.util.ArrayList$Itr.checkForComodification(Unknown Source)

What is the issue?

Also, let's just throw in [How do I compare strings in Java?](http://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java) for good measure. — Andy Turner, Feb 05 '16 at 09:58
you have to use methods on `Iterator` and `ListIterator` to add and remove elements from list, don't do it directly on `List` itself. [See question](http://stackoverflow.com/questions/223918/iterating-through-a-list-avoiding-concurrentmodificationexception-when-removing) — Sabir Khan, Feb 05 '16 at 10:05
read about Fail-Safe and Fail-Fast in collection. It's great if you will google it. — Musaddique S, Feb 05 '16 at 10:31
Why are you asking the same question again? Do you assume that the correct answer will change? — Hovercraft Full Of Eels, Feb 07 '16 at 05:46

Zbynek Vyskovsky - kvr000 · Accepted Answer · 2016-02-05T09:57:48.417

3

You can't traverse through the list using Iterator and modify the list with List-level add/remove methods at the same time.

To add an element in such situations you can use ListIterator and its method add.

edited Feb 05 '16 at 09:57

answered Feb 05 '16 at 09:54

Zbynek Vyskovsky - kvr000

18,186
3
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43

Actually he *can* **remove** – Andremoniy Feb 05 '16 at 09:55
@Andremoniy but only with the `Iterator`'s remove method, not the `List`'s. – Andy Turner Feb 05 '16 at 09:57
@AndyTurner Obviously. This wasn't clarified in the answer – Andremoniy Feb 05 '16 at 09:59

score 1 · Answer 2 · answered Feb 05 '16 at 09:58

Whenever you modify a Collection, you invalidate all the Iterators that depend on it. Instead, you could use the iterator itself (assuming you define it as a ListIterator, or something down those lines) to modify the collection:

ListIterator<String> lir = layerDataList.iterator();
while (lir.hasNext()) {
  String layerD = lir.next();
  if (layerD != layerData) {
    lir.add(layerData); // Here!
  }
}

score 0 · Answer 3 · answered Feb 05 '16 at 09:58

The documentation says:

The iterators returned by this ArrayList's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

To add while iterating to a list you can try like this:

ListIterator<String> lir = layerDataList.iterator();
while (lir.hasNext()) {
  String layerD = lir.next();
  if (layerD != layerData) {
    lir.add(layerData); 
  }
}

Getting ConcurrentException when working with list

3 Answers3

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