Fail-Fast vs Fail-Safe Iterators in Java

Iterators in Java behave differently when the underlying collection is modified during iteration. This difference creates two important types:

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Fail-Fast Iterators
Fail-Safe Iterators

Understanding these behaviors is very important for interviews and real-world multithreaded applications.

Fail-Fast Iterators

A Fail-Fast Iterator immediately throws a ConcurrentModificationException if the collection is structurally modified during iteration (other than using iterator.remove()).

Found In (Non-Concurrent Collections)

Fail-fast iterators are used in normal Java collections:

ArrayList
LinkedList
HashSet
LinkedHashSet
TreeSet
HashMap (via keySet(), values(), entrySet())
LinkedHashMap
TreeMap
Vector (modern iterator)

These belong to the java.util package.

How Fail-Fast Works Internally

Fail-fast iterators use two variables:

modCount → tracks structural changes in the collection
expectedModCount → value stored by iterator at the start

When iterating:

If (modCount != expectedModCount)
    throw ConcurrentModificationException

So any structural modification during iteration breaks synchronization and throws an exception.

Fail-Fast Example (Throws Exception)

import java.util.*;

public class Main {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("A");
        list.add("B");
        list.add("C");

        for (String s : list) {
            if (s.equals("B")) {
                list.remove(s);  // Structural modification
            }
        }
    }
}

Output:

Exception in thread "main" java.util.ConcurrentModificationException

How to Use Fail-Fast Safely

Use iterator.remove(), not list.remove():

Iterator<String> it = list.iterator();

while (it.hasNext()) {
    String s = it.next();
    if (s.equals("B")) {
        it.remove(); // Safe
    }
}

This avoids fail-fast exception because removal happens through the iterator itself.

Characteristics of Fail-Fast

Throws ConcurrentModificationException
Faster (no copy is created)
Not thread-safe
Unsafe during concurrent modifications
Used in almost all non-concurrent collections

2. Fail-Safe Iterators

A Fail-Safe Iterator does not throw exceptions even if the collection is modified during iteration.

Why?

Because it works on a copy (snapshot) of the underlying collection.

Found In (Concurrent Collections)

Fail-safe iterators come from the java.util.concurrent package:

ConcurrentHashMap
CopyOnWriteArrayList
CopyOnWriteArraySet
ConcurrentSkipListMap
ConcurrentSkipListSet

These collections are designed for multithreading.

How Fail-Safe Works Internally

Creates a clone or snapshot copy of the collection
Iterator works on the copy, not the original
Original collection can be safely modified during iteration
Does not throw ConcurrentModificationException
Changes made during iteration are NOT visible to the iterator

Fail-Safe Example (No Exception)

import java.util.concurrent.*;

public class Main {
    public static void main(String[] args) {
        CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>();
        list.add("A");
        list.add("B");

        for (String s : list) {
            list.add("C");  // No exception
        }

        System.out.println(list);
    }
}

Output:

[A, B, C, C]

Characteristics of Fail-Safe

No ConcurrentModificationException
Safe in multi-threaded environments
Allows modification during iteration
Slower due to copying
The iterator does not reflect new changes
Thread-safe