java.util.ConcurrentModificationException thrown when adding to List

Question

When I run this, I get a java.util.ConcurrentModificationException despite me using iterator.remove();

it's obviously me adding the number 6 in the loop. Does this happen because the iterator "doesn't know" it's there and is there anyway to fix it?

public static void main(String args[]){

    List<String> list = new ArrayList<>();

    list.add("1");
    list.add("2");
    list.add("3");
    list.add("4");
    list.add("5");

    for(Iterator<String> it = list.iterator();it.hasNext();){
        String value = it.next();

        if(value.equals("4")) {
            it.remove();
            list.add("6");
        }

        System.out.println("List Value:"+value);
    }
}

Georg Plaz · Accepted Answer · 2021-06-28 11:12:08Z

The ConcurrentModificationException is thrown when calling String value = it.next();. But the actual culprit is list.add("6");. You mustn't modify a Collection while iterating over it directly. You are using it.remove(); which is fine, but not list.add("6");.

While you can solve the problem with Streams, I will first offer a solution with Iterators, as this is a good first step for understanding the problem.

You need a ListIterator<String> if you want to add and remove during iteration:

for (ListIterator<String> it = list.listIterator(); it.hasNext();){
    String value = it.next();

    if (value.equals("4")) {
        it.remove();
        it.add("6");
    }

    System.out.println("List Value: " + value);
}

This should do the trick!

A solution using Streams:

List<String> newList = list.stream()
        .map(s -> s.equals("4") ? "6" : s)
        .collect(Collectors.toList());

Here we create a Stream from your List. We map all values to themselves, only "4" gets mapped to "6" and then we collect it back into a List. But caution, newList is immutable!

Georg Plaz · Accepted Answer · 2019-10-28 15:06:04Z

0

java.util.ConcurrentModificationException is a very common exception when working with java collection classes. Java Collection classes are fail-fast, which means if the Collection will be changed while some thread is traversing over it using iterator, the iterator.next() will throw ConcurrentModificationException. Concurrent modification exception can come in case of multithreaded as well as single threaded java programming environment.

edited Oct 28, 2019 at 15:06

Georg Plaz

6,0185 gold badges43 silver badges66 bronze badges

answered Apr 23, 2018 at 0:51

Deepesh kumar Gupta

8962 gold badges12 silver badges31 bronze badges

Collectives™ on Stack Overflow

java.util.ConcurrentModificationException thrown when adding to List

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related