Difference between fail-fast vs fail-safe
The terms “fail-fast” and “fail-safe” are associated with concurrent programming and refer to different strategies for handling concurrent modifications to data structures:
1. Fail-Fast
- Explanation: In a fail-fast approach, if a data structure is modified concurrently while it is being iterated over, the iterator detects the concurrent modification and immediately throws a ConcurrentModificationException. The goal of fail-fast iterators is to quickly notify the programmer about potential issues with concurrent modifications to prevent further unexpected behavior.
- Â Usage: Fail-fast iterators are commonly used in environments where detecting concurrent modifications is critical for maintaining data integrity, such as in single-threaded or lightly concurrent applications fail-fast vs fail-safe.
2. Fail-Safe
- Explanation: In a fail-safe approach, data structures are designed to handle concurrent modifications gracefully, typically by making a copy of the data structure before iterating over it. This ensures that modifications made by one thread do not interfere with the iteration process of another thread. Fail-safe iterators do not throw ConcurrentModificationException but may not reflect the most up-to-date state of the data structure.
- Â Usage: Fail-safe iterators are commonly used in heavily concurrent environments where it is essential to avoid thread interference during iteration, even at the expense of potentially iterating over stale data fail-fast vs fail-safe.

Table of Contents
java
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
public class FailFastExample {
public static void main(String[] args) {
List<Integer> list = new ArrayList<>();
list.add(1);
list.add(2);
list.add(3);
Iterator<Integer> iterator = list.iterator();
// Concurrent modification (adding element while iterating)
while (iterator.hasNext()) {
System.out.println(iterator.next());
list.add(4); // ConcurrentModificationException will be thrown here
}
}
}
fail-fast Example
In this fail-fast example, we attempt to add an element to the list while iterating over it using an iterator. This concurrent modification triggers a ConcurrentModificationException, causing the program to fail-fast and notify the programmer about the fail-fast vs fail-safe concurrent modification issue.
java
import java.util.Iterator;
import java.util.concurrent.ConcurrentHashMap;
public class FailSafeExample {
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 (adding element while iterating)
while (iterator.hasNext()) {
String key = iterator.next();
System.out.println(key + ": " + map.get(key));
map.put("D", 4); // No ConcurrentModificationException will be thrown
}
}
}
fail-safe Example
In this fail-safe example, we use a ConcurrentHashMap, which provides a fail-safe iterator. Even though we modify the map by adding an element (“D”) while iterating over its keys, no ConcurrentModificationException is thrown. The iteration process continues gracefully, and the new element is not included in the iteration. This demonstrates the fail-safe behavior of the ConcurrentHashMap iterator.