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A fail-fast iterator needs to compare the map’s current structural state with the state it observed when it was created. A single Boolean cannot reliably represent that relationship, especially when a map has multiple live iterators or more than one change. The usual design is a map-level modCount and a separate expectedModCount snapshot in each iterator. This detects some unexpected changes; it does not make the map thread-safe.
What a fail-fast iterator is meant to detect
In Java’s HashMap, iterators over the collection views are documented to throw ConcurrentModificationException if the map is structurally modified after an iterator is created, except when the change is made through that iterator’s own remove() method. “Concurrent” in the exception’s name does not mean that multiple threads are required: one thread can trigger it by modifying the map through another reference while iterating.
Oracle defines structural modification as adding or deleting mappings. Replacing the value associated with a key that is already present is not structural under the Java SE 26 HashMap API. A custom map should make its own rule explicit: count changes that invalidate the traversal’s view of the structure, such as a new mapping, a successful deletion, or a reorganization that makes the iterator’s traversal state stale.
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A Boolean can report that a change happened, but it does not record which version of the map an iterator saw. If the flag is cleared after one iterator checks it, another live iterator can miss the change. If the flag is never cleared, it cannot distinguish an old change from a new one or tell whether the iterator began before or after the change.
Multiple iterators make the problem clear. Each one needs to remember its own starting state. A shared flag cannot independently represent those snapshots, while a counter can: every relevant structural change advances the map’s version, and every iterator stores the version it observed at creation.
| Design | Per-iterator observed state | Successive changes | Multiple live iterators | Iterator-owned removal |
|---|---|---|---|---|
| Shared Boolean flag | No; all iterators see the same flag | Does not distinguish one change from several | Clearing it for one iterator can hide a change from another | Cannot naturally resynchronize only the iterator that removed an item |
| Map counter plus iterator snapshot | Yes; each iterator stores its own expected count | Each counted change advances the version | Each iterator compares its snapshot independently | The iterator that removes an item can update its own snapshot |
How the counter and snapshot work
OpenJDK’s HashMap implementation uses a map-level modCount. An iterator initializes its own expectedModCount from that value and checks for a mismatch while advancing to a node. The relationship is simple:
Rank #2
map.structuralChange():
map.modCount += 1
iterator created:
iterator.expectedModCount = map.modCount
iterator.next():
if iterator.expectedModCount != map.modCount:
throw ConcurrentModificationException
return nextEntry
iterator.remove():
removeCurrentEntry()
iterator.expectedModCount = map.modCountThis is illustrative pseudocode, not a claim about a particular custom implementation. The key is that the iterator compares the current map version to its private snapshot, rather than relying on a shared “something changed” signal.
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Which operations should advance the count?
Increment the counter when a successful operation changes the structure in a way that invalidates active traversals. For Java HashMap semantics, that includes adding a new mapping or deleting one, but not replacing the value of an existing key. OpenJDK also treats certain internal structural reorganizations, such as rehashing, as modifications. Whether a custom map needs to count a resize depends on whether that operation invalidates the iterator’s traversal state.
Do not advance the counter for an operation that failed to change the structure. For example, attempting to remove a key that is not present should not count as a structural change. Keep this policy consistent across every mutation path, including helpers that may alter buckets or links internally.
How to handle removal through the iterator
An iterator’s own remove() is the controlled exception to fail-fast detection. In OpenJDK’s implementation, after the iterator successfully removes its current entry, it refreshes its expectedModCount to match the map. That resynchronizes only the iterator that performed the authorized mutation; other iterators retain their earlier snapshots and can detect the change when they next check.
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The usual iterator state rules still apply: removal is invalid before a successful next(), and a second removal is invalid until another item has been returned. Check both the current-entry state and the modification count when implementing this method. If the map’s removal fails, do not refresh the snapshot as though a successful mutation occurred.
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Where to check for a mismatch
Check at points where the iterator consumes or advances the underlying structure. OpenJDK’s HashMap iterator checks the counts in nextNode(). Its hasNext() checks whether another node exists but does not perform that same modification-count check, so a custom iterator should not assume that every method must detect a change immediately. Define and document the checks your iterator actually performs.
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If the map supports split traversal or bulk traversal, those paths need their own review. OpenJDK’s HashMap source carries expected modification state in its spliterators and checks during traversal as well. A counter used only by the ordinary iterator does not automatically protect alternative traversal APIs.
Fail-fast is a diagnostic, not a concurrency guarantee
Oracle’s Java SE 26 API warns that fail-fast behavior cannot be guaranteed in the presence of unsynchronized concurrent modification and says the exception should be used only to detect bugs, not as a correctness mechanism. The counter is not a lock, does not establish memory visibility between threads, and cannot guarantee that a race will be reported. Its finite-width representation can also theoretically wrap around, so a matching value is not mathematical proof that no change occurred.
If multiple threads can structurally modify and traverse a map, use external synchronization or choose a collection designed for concurrent access. Treat ConcurrentModificationException as a useful warning when it occurs—not as protection that makes unsafe access safe.
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