What’s the difference between fail-fast and fail-safe iterators in Java Collections?

🚀 Java Interview Questions – Episode 10

💡 What’s the difference between fail-fast and fail-safe iterators in Java Collections?

When iterating over a collection, modifying it at the same time can lead to different behaviors depending on the type of iterator used:

⚠️ Fail-Fast Iterator

➡️ Immediately throws a ConcurrentModificationException if the collection is modified outside the iterator during iteration.

➡️ Used in: ArrayList, HashMap, HashSet

🔍 Works on the original collection, not a copy.

🛡️ Fail-Safe Iterator

➡️ Allows modification of the collection during iteration without throwing exceptions.

➡️ Used in: ConcurrentHashMap, CopyOnWriteArrayList

🔍 Operates on a clone or snapshot of the collection.

🧠 Summary:

Fail-Fast = fast but not safe for concurrent changes

Fail-Safe = safe but potentially less performant

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