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Add collections module: HashMap bucket layout, treeification, and a real ConcurrentModificationException reproduction

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Companion code for the ankurm.com post "Top 40 Java Collections Interview Questions
(HashMap Internals to Fail-Fast Iterators)". Verified against JDK 25's own HashMap
source (TREEIFY_THRESHOLD=8, UNTREEIFY_THRESHOLD=6, MIN_TREEIFY_CAPACITY=64), with
every claim backed by a committed docs/output/*.txt transcript from a real run.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01YXCrLgRKFgCh9RHKW8xaqJ
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asmhatre
2026-10-01 04:27:27 +00:00
co-authored by Claude Sonnet 5
commit d4b3a63615
27 changed files with 1085 additions and 0 deletions
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collections/docs/output/06-cme-silent-non-reproduction.txt
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The fail-fast contract is 'best-effort', not guaranteed: a case that does NOT throw
===================================================================================
list before: [10, 20] (size=2)
elements the loop actually saw before ending: [10]
list after the loop: [20]
ConcurrentModificationException thrown: false
Why: after removing 10, size becomes 1 and the iterator's cursor is already 1
(it advanced to 1 when next() returned 10). ArrayList.Itr.hasNext() is just
'return cursor != size;' - no modCount check - so hasNext() sees 1 != 1, returns
false, and the loop ends normally. next() is the only method that checks
modCount, and it is never called again. The element 20 is silently never visited.