Fail Fast vs Fail Safe Iterators in Java: A Comprehensive Guide

Naveen Metta
4 min readFeb 15, 2024
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Iterators play a crucial role in Java programming when it comes to traversing through collections. Among the various types of iterators, “Fail-Fast” and “Fail-Safe” are two distinct approaches. In this article, we will delve into the concepts of Fail-Fast and Fail-Safe iterators in Java, exploring their definitions, use cases, and providing practical code examples. Additionally, we will discuss scenarios where each type of iterator is most suitable, helping developers make informed decisions in their coding practices.

Fail-Fast Iterator:

A Fail-Fast iterator is designed to throw a ConcurrentModificationException if the collection is modified while the iterator is traversing it. This approach ensures that the iterator immediately becomes aware of any structural modification to the underlying collection.

Code Example — Fail-Fast Iterator:

List<String> failFastList = new ArrayList<>(Arrays.asList("Java", "Spring", "React"));

Iterator<String> iterator = failFastList.iterator();
while (iterator.hasNext()) {
String element =;
failFastList.add("NewElement"); // ConcurrentModificationException thrown here

Fail-Fast Iterator:
In this example, the Fail-Safe iterator is used with a CopyOnWriteArrayList. The iterator works on a snapshot of the original collection, allowing modifications without affecting the ongoing iteration.

import java.util.Iterator;
import java.util.concurrent.CopyOnWriteArrayList;

public class FailSafeIteratorExample {
public static void main(String[] args) {
// Creating a Fail-Safe collection (CopyOnWriteArrayList)
CopyOnWriteArrayList<String> failSafeList = new CopyOnWriteArrayList<>(List.of("Java", "Spring", "React"));

// Creating Fail-Safe Iterator
Iterator<String> iterator = failSafeList.iterator();

// Modifying the collection using Fail-Safe Iterator
while (iterator.hasNext()) {
String element =;

// Adding a new element to the collection (No ConcurrentModificationException thrown)


Fail-Fast iterators are typically more efficient but throw exceptions when concurrent modifications are detected.

Fail-Safe iterators avoid exceptions but might not reflect the latest changes in the collection.

Scenarios and Use Cases:

Concurrency and Thread Safety:

Fail-Fast iterators are more suitable in scenarios where strict concurrency control is required. They quickly identify modifications and prevent potential data inconsistencies.

Fail-Safe iterators, with their snapshot or copy-based approach, are ideal in multi-threaded environments where simultaneous modifications need to be supported without compromising the integrity of ongoing iterations.
Performance Considerations:

Fail-Fast iterators are generally more performance-oriented since they work directly with the underlying collection. However, the cost of throwing exceptions in case of modifications should be considered.

Fail-Safe iterators might incur additional memory overhead due to the creation of a copy or snapshot. The performance impact is often negligible in read-intensive scenarios but can be a concern in write-intensive applications.

Application Requirements:

In scenarios where real-time responsiveness is critical, Fail-Fast iterators help identify and handle modifications immediately. This is beneficial in applications where quick feedback is essential.

Fail-Safe iterators are preferable when a consistent view of the collection is more important than immediate detection of modifications. Applications that can tolerate a slight delay in reflecting changes may benefit from this approach.

Data Volume and Iteration Frequency:

Fail-Fast iterators are more efficient for large datasets where minimizing iteration time is crucial. They excel when the collection is modified infrequently compared to the number of iterations.
Fail-Safe iterators, with their copy-based approach, become more beneficial when modifications are frequent, and the cost of creating copies is amortized across multiple iterations.

Trade-Offs Between Fail-Fast and Fail-Safe:

Robustness vs. Performance:

Fail-Fast iterators prioritize robustness by immediately detecting modifications, but this comes at the cost of potential performance overhead, especially in scenarios with frequent modifications.

Fail-Safe iterators emphasize performance by allowing concurrent modifications but may sacrifice immediate awareness of changes in favor of consistent and predictable iteration.

Concurrency Control:

Fail-Fast iterators are a natural choice when strict concurrency control is a requirement. They act as a safeguard against unintentional modifications, helping maintain data integrity.
Fail-Safe iterators provide a more lenient approach, allowing concurrent modifications without throwing exceptions. This can be advantageous in scenarios where high concurrency and performance are critical.
Application Resilience:

Fail-Fast iterators, due to their immediate failure on detection of modifications, might lead to application disruptions. Graceful error handling and fallback mechanisms are essential to maintain application resilience.

Fail-Safe iterators, while allowing concurrent modifications, necessitate careful consideration of potential inconsistencies in the view presented to the user. Applications should be designed to handle delayed updates gracefully.

By comprehensively exploring the features, advantages, and use cases of both Fail-Fast and Fail-Safe iterators, developers can make informed decisions that align with the goals and constraints of their projects. Whether prioritizing performance, concurrency control, or consistency, the choice between these iterators should be driven by the unique requirements of the application at hand.

Developers should consider the nuances and trade-offs associated with each type of iterator to implement effective and resilient solutions. A nuanced understanding of Fail-Fast and Fail-Safe iterators will undoubtedly contribute to more robust and efficient code, enabling Java developers to navigate the intricacies of collection traversal with confidence.

In conclusion, the choice between Fail-Fast and Fail-Safe iterators is not a one-size-fits-all decision. Instead, it depends on the specific demands and characteristics of the application in question. As Java developers continue to evolve and adapt their coding practices, the knowledge of these iterators will serve as a valuable tool in creating reliable, high-performance applications.



Naveen Metta

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