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
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+$ java ComputeIfAbsentRecursionDemo
+
+=== ConcurrentHashMap: computeIfAbsent recursing into a DIFFERENT key ===
+no exception, result = 99, map = {1=99, 2=99}
+
+=== HashMap: computeIfAbsent recursing into a DIFFERENT key (single-threaded!) ===
+threw java.util.ConcurrentModificationException: null
+
+=== ConcurrentHashMap: computeIfAbsent recursing into the SAME key ===
+threw java.lang.IllegalStateException: Recursive update