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
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Claude Sonnet 5
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# java-interview-lab
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Runnable companion code for ankurm.com's Java interview-prep article series. Every number,
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stack trace, and console line quoted in those articles comes from a file committed somewhere
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under this repository — nothing is retyped or paraphrased from memory.
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One module per article. This repository currently holds:
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| Module | Article | Status |
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|---|---|---|
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| [`collections`](collections/) | Top 40 Java Collections Interview Questions (HashMap Internals to Fail-Fast Iterators) | this commit |
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## `collections` module
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HashMap/HashSet/ArrayList internals, verified against the real JDK 25 `java.util.HashMap`
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source (not against its javadoc prose) and reproduced with real, runnable code.
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### Versions
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| Component | Version | Verified against |
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|---|---|---|
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| JDK | **25.0.4.1+1** (Temurin, LTS) | `java -version`; `java.util.HashMap` source extracted from this JDK's own `lib/src.zip` |
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| JUnit | **5.11.0** | `org.junit.jupiter:junit-jupiter` on Maven Central |
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| Maven | 3.9.11 | `mvn -version` |
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The three constants this article leans on hardest (`TREEIFY_THRESHOLD = 8`,
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`UNTREEIFY_THRESHOLD = 6`, `MIN_TREEIFY_CAPACITY = 64`) were read directly out of
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`java.util.HashMap`'s source for this exact JDK build, not copied from a blog post or an older
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javadoc. See `collections/src/main/java/.../BucketInspector.java` for the reflective dump that
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proves them at runtime.
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### Quickstart
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```bash
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cd collections
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export JAVA_HOME=/path/to/jdk-25 # must be JDK 25 or newer
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export PATH="$JAVA_HOME/bin:$PATH"
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mvn test # runs everything, including the HashMap-internals demos
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```
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The first `mvn` invocation in a fresh environment must run **online** (not `-o`) once, so the
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compiler/surefire plugins populate the local repository cache; `-o` works for every run after
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that. This is a Maven quirk, not specific to this module.
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Run one scenario at a time with `./scripts/run.sh <name>` (`add-opens-fail`, `add-opens-ok`,
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`treeify`, `cme`, `eqhash`, `bucket` — see the script for what each prints). Regenerate
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every committed transcript in `docs/output/` with `./scripts/run-all.sh`.
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**No HTTP endpoints in this module** — every demo is a plain Java `main()` method or a
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JUnit test; there is nothing to start as a server. The diagnostic tool is
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`BucketInspector.dump(HashMap)`, a reflective helper, not an endpoint.
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### The one flag that matters: `--add-opens`
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`BucketInspector` reflects into `java.util.HashMap`'s private `table` field to show the real
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bucket layout. Since Java 9, that throws `InaccessibleObjectException` unless the JVM is
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started with:
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```
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--add-opens java.base/java.util=ALL-UNNAMED
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```
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`pom.xml`'s surefire configuration sets this for `mvn test` automatically. Running
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`ReflectionAddOpensDemo` directly with plain `java -cp ...` reproduces the crash on purpose
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— see `docs/output/01-reflection-without-add-opens.txt`.
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### Source layout
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```
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collections/
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├── pom.xml
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├── scripts/
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│ ├── run.sh one named scenario at a time
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│ └── run-all.sh regenerates every file in docs/output/
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├── src/main/java/com/ankurm/interviewlab/collections/
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│ ├── BucketInspector.java reflective bucket/bin dump (needs --add-opens)
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│ ├── CollidingKey.java hashCode()==1 for every instance - forces one bucket
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│ ├── ReflectionAddOpensDemo.java standalone entry point for the --add-opens demo
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│ └── eqhash/
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│ ├── EqualsOnlyPoint.java equals() overridden, hashCode() is NOT - the broken case
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│ ├── CorrectPoint.java both overridden consistently - the fixed case
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│ └── MutableKey.java a key whose hash-relevant field can change post-insertion
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├── src/test/java/com/ankurm/interviewlab/collections/
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│ ├── support/Transcript.java writes docs/output/NN-*.txt while the test asserts the same numbers
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│ ├── BucketLayoutTest.java
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│ ├── TreeifyTriggerTest.java
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│ ├── ConcurrentModificationTest.java
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│ └── EqualsHashCodeContractTest.java
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└── docs/output/*.txt captured real output - see index below
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```
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### Index of captured output (`docs/output/`)
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| File | What it proves |
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|---|---|
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| `01-reflection-without-add-opens.txt` | `InaccessibleObjectException`, uncaught, crashing the JVM — real stack trace |
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| `02-reflection-with-add-opens.txt` | Same code, with the flag: a real bucket dump of a 2-entry `HashMap` |
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| `03-bucket-layout-small-map.txt` | Real bucket indices for 6 string keys at default capacity 16 |
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| `04-treeify-trigger.txt` | Capacity doubling 16→32→64 purely from `treeifyBin()`'s own resize calls, then a real `Node`→`TreeNode` conversion at insert #11 |
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| `05-cme-classic-reproduction.txt` | A real `ConcurrentModificationException`, full stack trace, from `for`-each + `Collection.remove()` |
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| `06-cme-silent-non-reproduction.txt` | The documented case where the SAME kind of mid-loop removal does **not** throw |
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| `07-cme-concurrenthashmap-no-throw.txt` | `ConcurrentHashMap`'s weakly-consistent iterator mutated during iteration, never throws |
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| `08-iterator-remove-safe.txt` | `Iterator.remove()` used correctly — no exception, correct result |
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| `09-equals-hashcode-broken-contract.txt` | `equals()` without `hashCode()`: a real `HashSet` failing to deduplicate two equal objects |
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| `10-mutable-key-lost-entry.txt` | A key mutated after insertion: the entry is unreachable by lookup but still physically present |
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All ten are regenerated by one command (`./scripts/run-all.sh`); eight of the ten (03 through 10)
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are generated by a JUnit test that also asserts the same numbers it prints, so a figure in the
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article that stops being true fails the build, not just the next blog audit.
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## License
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MIT — see [LICENSE](LICENSE).
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