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

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
2026-10-01 04:27:27 +00:00
commit d4b3a63615
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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" "$f"
+done
+
+echo "== 03-10: JUnit-driven transcripts (BucketLayoutTest, TreeifyTriggerTest, ConcurrentModificationTest, EqualsHashCodeContractTest) =="
+mvn -q -B test
+
+echo "== done - docs/output now contains: =="
+ls -1 docs/output
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+#!/usr/bin/env bash
+# Runs one named scenario and prints it to stdout (does not touch docs/output - use
+# run-all.sh to regenerate the committed transcripts). Examples:
+#
+#   ./scripts/run.sh add-opens-fail      # reflection WITHOUT --add-opens (crashes on purpose)
+#   ./scripts/run.sh add-opens-ok        # reflection WITH --add-opens
+#   ./scripts/run.sh treeify             # run just the treeification test, verbose
+#   ./scripts/run.sh cme                 # run just the ConcurrentModificationException tests
+set -eu
+cd "$(dirname "$0")/.."
+
+scenario="${1:?usage: run.sh <add-opens-fail|add-opens-ok|treeify|cme|eqhash|bucket>}"
+
+mvn -q -B compile
+
+case "$scenario" in
+  add-opens-fail)
+    java -cp target/classes com.ankurm.interviewlab.collections.ReflectionAddOpensDemo ;;
+  add-opens-ok)
+    java --add-opens java.base/java.util=ALL-UNNAMED -cp target/classes \
+      com.ankurm.interviewlab.collections.ReflectionAddOpensDemo ;;
+  treeify)
+    mvn -B test -Dtest=TreeifyTriggerTest ;;
+  cme)
+    mvn -B test -Dtest=ConcurrentModificationTest ;;
+  eqhash)
+    mvn -B test -Dtest=EqualsHashCodeContractTest ;;
+  bucket)
+    mvn -B test -Dtest=BucketLayoutTest ;;
+  *)
+    echo "unknown scenario: $scenario" >&2; exit 1 ;;
+esac