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

equals-hashcode/output/01-contract.txt

@@ -0,0 +1,17 @@
+== Reflexive / null ==
+p.equals(p)    = true
+p.equals(null) = false
+
+== Symmetry broken by instanceof + extra field ==
+plain.equals(red) = true
+red.equals(plain) = false
+List.of(plain).contains(red) = false
+List.of(red).contains(plain) = true
+
+== Transitivity broken by the lenient fix ==
+r.equals(plain) = true
+plain.equals(b) = true
+r.equals(b)     = false
+
+== Consistency broken by a changing equals() ==
+f.equals(f2) x4 = true false true false