concurrentmodificationexception while using iterator in java

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Understanding and Preventing ConcurrentModificationException with Iterators in Java

The `ConcurrentModificationException` (CME) in Java is a common and often frustrating exception encountered when working with collections (like `ArrayList`, `LinkedList`, `HashMap`, etc.) and their iterators. It signals that a collection has been structurally modified while an iterator is in use, without using the iterator's own modification methods. This violation of contract leads to undefined behavior and, therefore, Java throws this exception to alert the developer.

Let's break down the issue, understand the underlying mechanism, and explore strategies to prevent and handle `ConcurrentModificationException` in your Java code.

*1. What is a Structural Modification?*

A structural modification refers to adding or removing elements directly from a collection while an iterator is active on that collection. It can also include operations like clearing the entire collection. Changes to the content of elements within the collection (e.g., changing a value of an object stored in a list) are not considered structural modifications as long as the collection's size and structure remain the same.

*Examples of Structural Modifications:*

Adding an element to a `List` using `list.add(element)`.
Removing an element from a `List` using `list.remove(element)` or `list.remove(index)`.
Clearing a `List` using `list.clear()`.
Adding or removing key-value pairs from a `Map` using `map.put(key, value)` or `map.remove(key)`.

*2. The Root Cause: Fail-Fast Iterators*

Java's collection iterators are typically designed to be "fail-fast." This means that instead of risking unpredictable or incorrect behavior caused by concurrent modifications, they attempt to detect such modifications and throw a `ConcurrentModificationException` at the first sign of trouble.

How do they detect this? Most (but not all) iterators maintain a "modification count" or "version" of the und ...

#Java
#ConcurrentModificationException
#Iterator