How to Resolve ConcurrentModificationException with ArrayList in Java For-Loop?

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Summary: Learn how to resolve the `ConcurrentModificationException` that occurs when using an `ArrayList` in a for-loop in Java, with practical examples and best practices.
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How to Resolve ConcurrentModificationException with ArrayList in Java For-Loop?

Encountering a ConcurrentModificationException while working with an ArrayList in a for-loop is a common issue Java developers face. This exception indicates that the structure of the list has been modified (e.g., elements added or removed) during its traversal. In this post, we’ll explore why this happens and how to effectively resolve it with practical examples and best practices.

What is ConcurrentModificationException?

The ConcurrentModificationException is thrown by methods that detect concurrent modification of an object when such modification is not permissible. Specifically, it often occurs when a collection (like an ArrayList) is structurally modified while an iteration is being performed on it.

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Why does this happen?

The enhanced for-loop (for-each loop) in Java uses an underlying iterator to traverse the list. When the ArrayList is modified structurally (element added or removed), the iterator’s state becomes inconsistent, causing the ConcurrentModificationException.

How to Resolve the Exception?

Use Iterator’s remove() Method

Instead of directly modifying the ArrayList, use the iterator’s remove() method to avoid ConcurrentModificationException.

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Use CopyOnWriteArrayList

Another solution is to use CopyOnWriteArrayList, which is a thread-safe variant of ArrayList. It creates a new copy of the list on every modification, thus avoiding the exception.

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Traditional For-Loop with Index Traversal

You can avoid using iterators altogether by using a traditional for-loop with an index to modify the list.

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Use Streams and Filters (Java 8 and above)

If you’re using Java 8 or later, you can utilize streams and filters for more concise and idiomatic code. However, keep in mind that this method creates a new list.

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Best Practices

Choosing the Right Approach: The choice between an iterator, CopyOnWriteArrayList, or a traditional for-loop should be based on your specific use case and performance considerations.

Code Readability: Strive to keep code clear and maintainable. Overly complex solutions could lead to bugs and maintenance issues.

By understanding the reasons behind ConcurrentModificationException and applying these techniques, Java developers can effectively handle and avoid this common issue when working with ArrayList in for-loops.