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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
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collections/docs/output/10-mutable-key-lost-entry.txt
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Mutating a key's hash-relevant field after insertion: the entry is still THERE, just unfindable
===============================================================================================
map.put(key(tag=42), ...) then key.setTag(99)
containsKey(new MutableKey(42)) before mutation : true
containsKey(new MutableKey(42)) after mutation : false (looks for old hash's bucket - key no longer hashes there)
containsKey(new MutableKey(99)) after mutation : false (looks in the NEW hash's bucket - key was never filed there either)
map.size() is still : 1 (the entry was never removed!)
the mutated key object IS still found by direct iteration over keySet(): true