Fail-fast vs Fail-safe Iterator

• Fail-fast iterator throws an exception when the collection is modified
• Fail-safe iterator works on a copy and does not throw an exception

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Core Difference

Point	Fail-fast	Fail-safe
Behavior on modification	Throws exception	Continues safely
Exception thrown	ConcurrentModificationException	No exception
Works on	Original collection	Copy of collection
Thread safety	❌ Not thread-safe	✅ Thread-safe
Performance	Faster	Slightly slower
Memory usage	Low	Higher
Examples	ArrayList, HashMap	ConcurrentHashMap, CopyOnWriteArrayList

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Fail-fast Iterator

A fail-fast iterator immediately fails if the collection is structurally modified during iteration.

Characteristics

• Detects concurrent modification
• Throws ConcurrentModificationException
• Helps identify bugs early

Fail-fast iterators use modCount to detect structural modification.

What is a structural modification?

• add()
• remove()
• clear()
• put() (for maps)

How fail-fast works

1. Iterator stores current modCount as expectedModCount
2. On every next() / hasNext()

3. JVM checks:

    if (modCount != expectedModCount)
    throw ConcurrentModificationException
   

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Fail-safe Iterator

A fail-safe iterator iterates over a snapshot copy of the collection.

Characteristics

• No exception on modification
• Works safely in multi-threaded environments
• Changes may not be reflected during iteration

How ConcurrentHashMap Avoids the Exception

No modCount checks

• It does not track a global modification count
• So no comparison → no exception

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Fail-fast iterators throw an exception on concurrent modification, whereas fail-safe iterators work on a copy and continue safely.