hashmap-concurrenthashmap: companion code for the HashMap vs ConcurrentHashMap interview guide

Adds JMH put/get throughput benchmarks (synchronizedMap vs ConcurrentHashMap, swept at 1/2/4/8 threads), a computeIfAbsent recursion trap demo (CHM tolerates recursion into a different key but throws IllegalStateException on the same key; plain HashMap throws ConcurrentModificationException even single-threaded), and a null-handling demo (CHM rejects null keys/values on put() and null keys on get() too). 7/7 JUnit tests pass; all output/ transcripts captured from real runs. Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01FhzLY5p6okFva3qsnsRyvM
2026-09-30 09:29:06 +00:00
parent af615cc17c
commit 4a74153d87
16 changed files with 790 additions and 0 deletions
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+$ java NullHandlingDemo
+
+=== HashMap: null key and null value are both fine ===
+hm.get(null) = nullKeyValue
+hm.get("k1") = null
+
+=== ConcurrentHashMap: null key throws on put() ===
+chm.put(null, "x") threw NullPointerException
+
+=== ConcurrentHashMap: null value throws on put() ===
+chm.put("k1", null) threw NullPointerException
+
+=== ConcurrentHashMap: get(null) ALSO throws, not just put() ===
+chm.get(null) threw: Cannot invoke "Object.hashCode()" because "key" is null