fail fast iterator in java

A fail-fast iterator is an iterator implementation in Java that detects and throws a `ConcurrentModificationException` if the underlying collection is structurally modified while the iteration is in progress. This mechanism ensures that the iterator fails quickly and reliably when concurrent modification of the collection is detected, rather than risking unpredictable behavior or data corruption.

Characteristics of Fail-Fast Iterators:

• 1. Detection of Concurrent Modification:
  • Fail-fast iterators detect concurrent modification of the underlying collection by maintaining an internal modification count or a version number.
  • Before each iteration operation (such as `next()` or `hasNext()`), the iterator checks whether the modification count of the collection has changed since the iterator was created or since the last operation.
• 2. Throwing ConcurrentModificationException:
  • If the iterator detects a concurrent modification of the collection during iteration, it immediately throws a `ConcurrentModificationException`.
  • This exception indicates that the collection has been structurally modified unexpectedly while the iteration was in progress.
• 3. Ensuring Consistency:
  • Fail-fast iterators prioritize consistency and correctness by ensuring that the iteration reflects the state of the collection at the time of iterator creation.
  • They do not provide any guarantees about the behavior of the iterator or the underlying collection if concurrent modification occurs.

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

public class FailFastIteratorExample {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(2);
        list.add(3);

        // Create an iterator
        Iterator<Integer> iterator = list.iterator();

        // Iterate over the list
        while (iterator.hasNext()) {
            Integer element = iterator.next();
            System.out.println(element);

            // Concurrent modification during iteration
            if (element == 2) {
                list.remove(element); // Throws ConcurrentModificationException
            }
        }
    }
}
```

In this example, a fail-fast iterator is used to iterate over an `ArrayList`. During the iteration, the element `2` is removed from the list, causing a concurrent modification of the collection. As a result, the iterator immediately detects the modification and throws a `ConcurrentModificationException`. This behavior ensures that the inconsistency caused by concurrent modification is detected and reported promptly.