immutable: List.of vs unmodifiableList vs copyOf - null handling, views vs copies, JMH wrap/copy overhead, a Guava comparison, and the record defensive-copy trap

immutable/output/02-views-vs-copies.txt

@@ -0,0 +1,21 @@
+=== unmodifiableList is a VIEW: changes to the backing list show through ===
+backing = [a, b, c]
+view    = [a, b, c]
+after backing.add("d"):
+backing = [a, b, c, d]
+view    = [a, b, c, d]  <- changed, with no code touching 'view' directly
+
+=== List.copyOf is a SNAPSHOT: changes to the source do NOT show through ===
+source = [x, y, z]
+copy   = [x, y, z]
+after source.add("w"):
+source = [x, y, z, w]
+copy   = [x, y, z]  <- unchanged, it was a real copy at the moment copyOf() ran
+
+=== List.copyOf has a documented optimization: copying an already-immutable list is a no-op ===
+List.copyOf(List.of(...)) returns the SAME instance: true
+List.copyOf(new ArrayList<>(...)) returns a DIFFERENT instance: true
+
+=== Map and Set follow the same pattern as List ===
+unmodMapView after mutating backing map = {one=1, two=2}  (view: changed)
+mapCopy after mutating the ORIGINAL map  = {one=1}  (copy: unchanged)