Understanding Fail-Fast Iterators and ConcurrentModificationException in Java Collections

When the underlying collection is modified during iteration, fail‑fast iterators may throw a ConcurrentModificationException, quickly failing instead of allowing nondeterministic behavior later.

The exception mainly occurs in two situations: (1) the collection is altered via its own methods while a fail‑fast iterator is iterating it in the same thread, and (2) one thread modifies the collection’s structure while another thread is iterating it with a fail‑fast iterator.

Example that triggers the exception (modifying the list inside the loop):

package com.renzhikeji.demo;

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

/**
 * @author 认知科技技术团队
 * 微信公众号：认知科技技术团队
 */
public class JdkDemo {
    public static void main(String[] args) throws InterruptedException {
        List<Integer> list = new ArrayList<>(List.of(1, 2, 3, 5, 6, 7, 8));
        Iterator<Integer> iterator = list.iterator();
        while (iterator.hasNext()) {
            Integer a = iterator.next();
            System.out.println(a);
            list.add(10); // modifies the collection during iteration
        }
    }
}

Another erroneous example where the collection’s remove method is called directly inside the loop:

package com.renzhikeji.demo;

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

/**
 * @author 认知科技技术团队
 * 微信公众号：认知科技技术团队
 */
public class JdkDemo {
    public static void main(String[] args) throws InterruptedException {
        List<Integer> list = new ArrayList<>(List.of(1, 2, 3, 5, 6, 7, 8));
        Iterator<Integer> iterator = list.iterator();
        while (iterator.hasNext()) {
            Integer a = iterator.next();
            System.out.println(a);
            list.remove((Object)7); // direct removal causes exception
        }
    }
}

Correct approach using the iterator’s remove() method after a condition check:

package com.renzhikeji.demo;

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

/**
 * @author 认知科技技术团队
 * 微信公众号：认知科技技术团队
 */
public class JdkDemo {
    public static void main(String[] args) throws InterruptedException {
        List<Integer> list = new ArrayList<>(List.of(1, 2, 3, 5, 6, 7, 8));
        Iterator<Integer> iterator = list.iterator();
        while (iterator.hasNext()) {
            Integer a = iterator.next();
            System.out.println(a);
            if (a == 7) {
                iterator.remove(); // safe removal via iterator
            }
        }
        System.out.println(list);
    }
}

To handle concurrent modifications, two strategies are recommended: (1) avoid modifying the collection while iterating—use synchronization to prevent concurrent access or always use iterator.remove() instead of Collection.remove(); (2) replace fail‑fast iterators with fail‑safe ones from concurrent collections such as ConcurrentHashMap or CopyOnWriteArrayList, which do not throw this exception.