fail safe iterator in java

In Java, a fail-safe iterator is an iterator that does not throw a `ConcurrentModificationException` if the underlying collection is modified during iteration. Instead, it operates on a snapshot of the collection taken at the time of iterator creation. Any modifications made to the collection after the iterator is created are not visible to the iterator.

Characteristics of Fail-Safe Iterators:

• 1. No Concurrent Modification Detection:
  • Fail-safe iterators do not detect concurrent modifications of the underlying collection during iteration.
  • They operate on a copy of the collection’s state at the time of iterator creation, ensuring that any modifications made to the collection after iterator creation do not affect the ongoing iteration.
• 2. Safety from Concurrent Modifications:
  • Fail-safe iterators provide safety from concurrent modifications by working on a snapshot of the collection.
  • They ensure that the iteration proceeds smoothly without interference from modifications made by other threads or concurrent operations.
• 3. No `ConcurrentModificationException`:
  • Fail-safe iterators do not throw a `ConcurrentModificationException` even if the collection is modified during iteration.
  • This behavior contrasts with fail-fast iterators, which immediately detect and throw an exception upon concurrent modification.
• 4. Consistent Iteration State:
  • Fail-safe iterators maintain a consistent iteration state regardless of modifications made to the collection after iterator creation.
  • They provide a predictable and stable iteration behavior, ensuring that elements seen during iteration remain unchanged.

```java
import java.util.Iterator;
import java.util.concurrent.ConcurrentHashMap;

public class FailSafeIteratorExample {
    public static void main(String[] args) {
        ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
        map.put("A", 1);
        map.put("B", 2);
        map.put("C", 3);

        Iterator<String> iterator = map.keySet().iterator();

        // Concurrent modification after iterator creation
        map.put("D", 4);

        // Iteration using fail-safe iterator
        while (iterator.hasNext()) {
            String key = iterator.next();
            System.out.println(key + ": " + map.get(key));
        }
    }
}
```

In this example, a `ConcurrentHashMap` is modified (an element is added) after the iterator for its key set is created but before the iteration completes. However, the iterator does not throw a `ConcurrentModificationException`. Instead, it operates on a snapshot of the key set taken at the time of iterator creation, ensuring a fail-safe iteration. The newly added element (“D”) is not included in the iteration.