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

equals-hashcode/output/02-missing-hashcode.txt

@@ -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

equals-hashcode/output/03-mutable-key.txt

@@ -0,0 +1,14 @@
+before: get = 100, contains = true
+
+after acc.owner = "ankur":
+map.get(acc)             = null
+set.contains(acc)        = false
+map.containsKey(acc)     = false
+map.size()               = 1
+iteration still sees it  = [Account(ankur)]
+map.remove(acc)          = null
+map.size() after remove  = 1
+
+set.add(acc) again -> size = 2 [Account(ankur), Account(ankur)]  (one object, two entries)
+
+immutable key: map.get("asha") = 1

equals-hashcode/output/04-jdk-symmetry.txt

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+date.equals(ts) = true
+ts.equals(date) = false
+date.getTime() == ts.getTime() = true

equals-hashcode/output/05-records.txt

@@ -0,0 +1,23 @@
+== generated equals/hashCode ==
+a.equals(b)                = true
+a.hashCode() == b.hashCode() = true
+new HashSet of both, size  = 1
+a == b                     = false
+
+== array component: reference equality ==
+w1.equals(w2) = false
+same array shared: true
+
+== mutable List component: equals works, hashCode can change ==
+hashCode before = 4714, after list.add(3) = 34537
+record in HashSet, list mutated -> contains = false
+
+== doubles compare with Double.compare semantics ==
+NaN record equals NaN record = true
+Double.NaN == Double.NaN     = false
+0.0 record equals -0.0 record = false
+0.0 == -0.0                   = true
+
+== overriding equals in a record ==
+t1.equals(t2) = true
+toString still shows both fields: Tagged[isbn=1, title=Effective Java 3e]

equals-hashcode/output/06-entity-id.txt

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+== id-based equals/hashCode, id assigned on save ==
+two unsaved entities equal?  true  (both ids null)
+set size after adding two different unsaved entities = 1
+add, then save(): set.contains(c) = false
+
+== constant hashCode ==
+add, then save(): set.contains(d) = true
+
+== natural key, immutable ==
+add, then save(): set.contains(e) = true

equals-hashcode/output/07-hashcode-recipes.txt

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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

equals-hashcode/output/08-tests.txt

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+[INFO] Tests run: 13, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.205 s -- in com.ankurm.equalshashcode.EqualsHashCodeTest
+[INFO] Tests run: 13, Failures: 0, Errors: 0, Skipped: 0
+[INFO] BUILD SUCCESS