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Java's for each loop [message #1858692]

Sun, 16 April 2023 21:49

bretny relly

Messages: 14
Registered: March 2023

Junior Member

What is causing my for-each loop to throw a "ConcurrentModificationException" error when I try to modify the collection within the loop? How can I modify the collection safely?

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4));
        Iterator<Integer> iterator = numbers.iterator();
        while (iterator.hasNext()) {
            int number = iterator.next();
            if (number % 2 == 0) {
                iterator.remove();
            }
        }
        System.out.println(numbers);
    }
}

In general, it's advisable to avoid altering a collection while it's being iterated over. If changes are required, consider utilising a synchronised collection or another thread-safe collection to avoid concurrent modification, as described in this article.
So what happens if I try to change the collection within the loop? How can I safely modify the collection?

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