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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
This commit is contained in:
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/scripts/run-all.sh
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#!/usr/bin/env bash
# Regenerates every file in docs/output/ from scratch. Run from the module root
# (java-interview-lab/collections).
#
# Needs JDK 25 (LTS) on PATH as `java`/`javac`, and a `mvn` that can reach Maven Central the
# first time (surefire/compiler plugins must be cached locally before -o works).
set -eu
cd "$(dirname "$0")/.."
echo "== compiling =="
mvn -q -B compile
echo "== 01/02: reflection into HashMap internals, with and without --add-opens =="
mkdir -p docs/output
java -cp target/classes com.ankurm.interviewlab.collections.ReflectionAddOpensDemo \
> docs/output/01-reflection-without-add-opens.txt 2>&1 || true
java --add-opens java.base/java.util=ALL-UNNAMED -cp target/classes \
com.ankurm.interviewlab.collections.ReflectionAddOpensDemo \
> docs/output/02-reflection-with-add-opens.txt 2>&1
# Strip the sandbox's own proxy/trust-store banner (JAVA_TOOL_OPTIONS) - it is environment
# noise, not part of either demonstration, and should not be in a committed transcript.
for f in docs/output/01-reflection-without-add-opens.txt docs/output/02-reflection-with-add-opens.txt; do
grep -v '^Picked up JAVA_TOOL_OPTIONS' "$f" > "$f.tmp" && mv "$f.tmp" "$f"
done
echo "== 03-10: JUnit-driven transcripts (BucketLayoutTest, TreeifyTriggerTest, ConcurrentModificationTest, EqualsHashCodeContractTest) =="
mvn -q -B test
echo "== done - docs/output now contains: =="
ls -1 docs/output