What is the difference between fail-fast and fail-safe iterators in Java?
Jul 09, 2025 am 12:09 AM
Fail-fast iterators throw ConcurrentModificationException when structural changes occur during iteration, while fail-safe iterators operate on a snapshot and do not throw exceptions. 1. Fail-fast detects modifications via a counter and throws exceptions unless changes are made through the iterator’s remove method, as seen in ArrayList. 2. Fail-safe, like in CopyOnWriteArrayList, iterates over a copy, avoiding exceptions but potentially showing stale data. 3. Use fail-fast for single-threaded environments to catch bugs early, and fail-safe in multi-threaded contexts to prevent concurrency issues, keeping in mind memory and performance trade-offs.
In Java, the difference between fail-fast and fail-safe iterators mainly comes down to how they handle structural modifications during iteration—like adding or removing elements while looping through a collection.
Here’s a straightforward breakdown of what each one does and when it matters.
What Fail-Fast Iterators Do
Fail-fast iterators throw a ConcurrentModificationException if they detect that the underlying collection has been structurally modified after the iterator was created (except through the iterator's own remove method).
This behavior is common in collections from the java.util package like ArrayList, HashMap, and others.
How it works:
- Each iterator keeps a modification counter.
- If the collection is changed outside the iterator (e.g., by another thread or even in the same thread before the iteration completes), the counter changes.
- When a mismatch is detected, the exception is thrown immediately.
Example situation:
List<String> list = new ArrayList<>(Arrays.asList("a", "b", "c"));
for (String s : list) {
if (s.equals("b")) {
list.remove(s); // This will throw ConcurrentModificationException
}
}What to do instead:
- Use the iterator's own
remove()method if you need to delete elements during iteration. - Or switch to a concurrent collection if working with multiple threads.
What Fail-Safe Iterators Do
Fail-safe iterators don't throw exceptions if the collection is modified during iteration. They work on a copy of the original collection or data structure, so changes to the original don’t affect the iterator.
These are typically found in collections from java.util.concurrent or utilities like CopyOnWriteArrayList.
How it works:
- The iterator operates on a snapshot of the collection at the time the iterator was created.
- Modifications to the original collection won’t be visible to the iterator, and vice versa.
Example situation:
List<String> list = new CopyOnWriteArrayList<>(Arrays.asList("a", "b", "c"));
for (String s : list) {
if (s.equals("b")) {
list.remove(s); // No exception, but this won't affect the current iteration
}
}Important to note:
- You won’t get real-time updates from the collection.
- Memory and performance can be affected because copies are made.
When to Use Each Type
You’ll usually choose based on your use case and environment:
-
Use fail-fast when:
- You want bugs to surface early.
- You're working in single-threaded environments.
- You expect modifications to be rare during iteration.
-
Use fail-safe when:
- You’re dealing with multi-threaded code.
- You can tolerate stale data.
- You want to avoid exceptions due to concurrent modification.
Also keep in mind:
- Some concurrent collections still have weakly consistent iterators that may show partial changes.
- Always check documentation for specific behaviors.
That’s the core of the difference. It’s not too complicated once you see how each type responds to changes under the hood.
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