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
22 changed files with 692 additions and 0 deletions
@@ -0,0 +1,15 @@
+== equals() without hashCode() ==
+a.equals(b)                 = true
+a.hashCode() == b.hashCode() = false
+HashSet size after adding a then b = 2
+HashSet.contains(new equal key)    = false
+HashMap.get(equal key)             = null
+
+== with hashCode() ==
+HashSet size after adding c then d = 1
+HashSet.contains(new equal key)    = true
+
+== a legal but useless hashCode(): return 42 ==
+keys                                  = 2000
+equals() calls for 2000 gets, constant hash = 2002999
+equals() calls for 2000 gets, good hash     = 2000