java.util.ConcurrentModificationException

This post shows show to solve the problem of java.util.ConcurrentModificationException for ArrayList.

The error message looks like the following:

Exception in thread "main" java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
	at java.util.ArrayList$Itr.next(Unknown Source)
	...
	...

The Problem

You may want to iterate through an ArrayList, and delete some element under some certain condition. For example, the following code looks reasonable:

List<String> list = new ArrayList<String>();
list.add("A");
list.add("B");
list.add("C");
 
for (String s : list) {
    if (s.equals("B")) {
        list.remove(s);
    }
}

Running the code actually will result in the following exception:
java-util-concurrentModificationException

Solution 1

Iterator can be used to solve this problem. Iterators allow the caller to remove elements from the underlying collection during the iteration.

Iterator<String> iter = list.iterator();
while(iter.hasNext()){
    String str = iter.next();
    if( str.equals("B")){
        iter.remove();
    }
}

Solution 2

Instead of ArrayList, CopyOnWriteArrayList can be used to solve the problem. CopyOnWriteArrayList is a thread-safe variant of ArrayList in which all mutative operations (add, set, and so on) are implemented by making a fresh copy of the underlying array.

List<String> list = new CopyOnWriteArrayList<String>();
list.add("A");
list.add("B");
list.add("C");
 
for (String s : list) {
    if (s.equals("B")) {
        list.remove(s);
    }
}

This is the same for other data structures, such as HashSet, LinkedList, etc.

Category >> Collections  
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  • abdulkerim

    I solved with CopyOnWriteArrayList. thanks so much

  • XeNeX

    This is not true:
    “The above code is fine, because they do not use array as the underlining data structure.”
    In each of collection is a counter which indicate if collection has been changed (it structure – added or removed element not replaced). If structure of collection will change we will get exception 🙂

  • Bibhudutta Pradhan

    I added 2 more elements into the LinkedList and let the remaining codes unchanged.
    It did throw “java.util.ConcurrentModificationException”
    why ?

  • Akram Ahmad

    Your suggestion to use “CopyOnWriteArrayList” is brilliantly helpful. I’m exploring it’s use, and it is looking good so far 🙂 Plus I need to crack open my well-worn copy of Brian Goetz’s classic work on concurrency (JCIP) for a refresher…. Thank you!