How to Safely Remove Elements from an ArrayList During Iteration?

Avoiding Concurrent Modification Exception While Removing Elements from ArrayList During Iteration

Modifying a collection while iterating over it using the advanced for loop (for-each loop) can lead to the infamous ConcurrentModificationException. This occurs when an attempt is made to remove an element from an ArrayList during iteration.

Cause:

The for loop creates a copy of the list internally and iterates over this copy. When an element is removed from the original list, the size of the copy is immediately reduced, which results in the index of the current element being invalidated.

Solution:

To avoid this issue, use an iterator along with the remove() method.

Iterator Approach:

An iterator is an object that iterates over the elements of a collection. It has methods like hasNext, which checks if there are more elements, and next, which returns the next element. Using an iterator allows for safe removal during iteration.

Iterator<String> iter = myArrayList.iterator();

while (iter.hasNext()) {
    String str = iter.next();

    if (someCondition)
        iter.remove();
}

In this approach, the iterator maintains its internal state, ensuring that the index remains valid even after an element is removed. The iter.remove() method safely removes the current element from the original list.

Note: This approach is more efficient than using a traditional for loop with an index because it does not require maintaining a separate index variable and updating it after each removal.

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