Java中的ConcurrentModificationException即使没有迭代

Question

Can anyone help me out with this code? It throws a ConcurrentModificationException . I do know that while iterating a collection and simultaneously adding or removing elements from it, we get a ConcurrentModificationException . But I am not iterating in the first place.

import java.util.*;
class Test {
public static void main(String[] args){
ArrayList<String> arr = new ArrayList<String>();
ListIterator<String> itr = arr.listIterator();
arr.add("Hello");
arr.add("hi");
System.out.println(itr.next());
}
}

Answer 1

When you call itr.next() , you are iterating the list (just not completely). A foreach loop in Java is really just syntactic sugar for:

Iterator<T> it = list.iterator();
while (it.hasNext()) {
    T curr = it.next();
}

When modifying the collection, all existing iterators are invalidated, whether you've used them or not yet. You must obtain a new iterator after modifying the list.

Answer 2

The iterators of ArrayList are failfast means if you are trying to iterate and add/remove elements at the same time, the Iterator or ListIterator will throw ConcurrentModificationException.

itr.next() // this is iterating(going over the ArrayList)

arr.add("Hello");//This statement is adding to the ArrayList during iteration
arr.add("hi");//Again this statement is adding to ArrayList during iteration

This causes the ConcurrentModificationException.

If you want to iterate as well as modify(add/remove) the collection simulateously use CopyOnWriteArrayList,ConcurrentHashMap. The iterators of CopyOnWriteArrayList,ConcurrentHashMap are failsafe.