equals-hashcode: equals/hashCode contract demos, missing hashCode and constant-hash costs, mutable keys, records, JPA entity id trap, 13 contract tests

Co-Authored-By: Claude Sonnet 5.5 <[email protected]> Claude-Session: https://claude.ai/code/session_017ic4b66uwsWr9B1ouyrzUh
2026-11-12 20:08:35 +05:30
parent 319697c345
commit 682b7b067c
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+== what the JDK uses ==
+"hello".hashCode()            = 99162322
+31*h + c over "hello"          = 99162322
+List.of(1,2,3).hashCode()      = 30817
+Arrays.hashCode({1,2,3})       = 30817
+Objects.hash(1,2,3)            = 30817
+Set.of(1,2,3).hashCode()       = 6  (sum of elements)
+Map.of(1,2).hashCode()         = 3  (key^value per entry, summed)
+
+== Objects.hash(...) traps ==
+Objects.hash("a") vs "a".hashCode() = 128 vs 97
+Objects.hash(1, 2) == Objects.hash(2, 1) ? false
+Objects.hash() (no args)       = 1
+Objects.hash((Object) null)    = 31
+
+== Arrays as fields ==
+x.equals(y) = false, Arrays.equals = true
+x.hashCode() == y.hashCode() ? false
+Arrays.hashCode equal ? true
+
+== equal hashCodes do not mean equal objects ==
+"Aa".hashCode() = 2112, "BB".hashCode() = 2112, equal strings? false