Remove an object from a list - ConcurrentModificationException

Question

I have this code:

public static void main(String[] args) {
    User user = new User("user1","user1",1l);
    User user1 = new User("user2","user2",2l);
    User user2 = new User("user3","user3",3l);

    List<User> list = new ArrayList<User>();
    list.add(user);
    list.add(user1);
    list.add(user2);

    for(User user3 : list){
        System.out.println(user3.getName());
        if(user3.getName().equals("user1")){
            list.remove(user3);
        }
    }
}

when executing this code , I get the following error:

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

How can I avoid it?

score 2 · Answer 1 · answered Feb 23 '14 at 19:29

2

Use Iterator to iterate the list and then use Iterator.remove() to remove the particular element

answered Feb 23 '14 at 19:29

Kick

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This is not entirely true. – Rohit Jain Feb 23 '14 at 19:30
I have updated the answer.Thanks for the valuable comments – Kick Feb 23 '14 at 19:34

score 1 · Accepted Answer · answered Feb 23 '14 at 19:37

When you are iterating through a list (collection) using the for-each loop, you should not try to modify the collection as it is undefined behaviour( even though internally it uses Iterator).

The best way to handle removal of elements while iterating through a collection is by using an Iterator.

Iterator<User> it = list.iterator();

while (it.hasNext()) {

  User user = it.next();
  if(user.getName().equals("user1")){
        it.remove();
   }

 }

Remove an object from a list - ConcurrentModificationException

2 Answers2