Difference Between Fail-Fast and Fail-Safe Iterators in Java

Ramesh Fadatare

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Apr 5, 2025

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What is a Fail-Fast Iterator?

A Fail-Fast Iterator is an iterator that immediately fails (throws an exception) if it detects any structural modification of the collection after the iterator was created.

Key Characteristics

Fail-Fast Example with ArrayList

import java.util.*;

public class FailFastExample {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        names.add("Amit");
        names.add("Sneha");
        names.add("Rahul");

        Iterator<String> iterator = names.iterator();
        while (iterator.hasNext()) {
            String name = iterator.next();
            if (name.equals("Sneha")) {
                names.remove(name); // Modifying the collection during iteration
            }
        }
    }
}

Output:

Exception in thread "main" java.util.ConcurrentModificationException

⚠️ Why the Error?

• The iterator detects that the list was structurally modified (remove() was called).
• Java’s fail-fast mechanism throws ConcurrentModificationException to prevent unpredictable behavior.

What is a Fail-Safe Iterator?

A Fail-Safe Iterator is an iterator that does not throw an exception if the collection is modified during iteration. Instead, it iterates over a copy of the collection.

Key Characteristics

Fail-Safe Example with ConcurrentHashMap

import java.util.concurrent.ConcurrentHashMap;

public class FailSafeExample {
    public static void main(String[] args) {
        ConcurrentHashMap<Integer, String> map = new ConcurrentHashMap<>();
        map.put(1, "Amit")…