How to Resolve ConcurrentModificationException When Iterating Over a List in Java?

What causes ConcurrentModificationException while iterating over a List in Java, and how can I fix it?

List<String> list = new ArrayList<>(Arrays.asList("A", "B", "C"));

for (String item : list) {
    if (item.equals("B")) {
        list.remove(item); // This line will cause ConcurrentModificationException
    }
}

The ConcurrentModificationException in Java occurs when a collection is modified while iterating over it using an iterator or enhanced for-loop. This exception is a part of the Java Collections Framework and is designed to prevent inconsistent states.

List<String> list = new ArrayList<>(Arrays.asList("A", "B", "C"));
Iterator<String> iterator = list.iterator();

while (iterator.hasNext()) {
    String item = iterator.next();
    if (item.equals("B")) {
        iterator.remove(); // Safe way to remove item
    }
}

Causes

• Modifying a list while traversing it using a `for-each` loop or an iterator.
• Using one collection in one thread while modifying it in another thread without proper synchronization.

Solutions

• Use `Iterator.remove()` method while iterating through the collection.
• Use a temporary list to collect items to be removed and delete them after the loop.
• Utilize concurrent collections like `CopyOnWriteArrayList` for thread-safe modifications.

Mistake: Using `list.remove(item)` within a for-each loop.

Solution: Use `iterator.remove()` to safely remove items during iteration.

Mistake: Not checking for thread safety when using multiple threads.

Solution: Consider using concurrent collections like `ConcurrentHashMap` or `CopyOnWriteArrayList`.