[pkulak]

Not sure what you're getting at. Immutable means exactly that. If you hand out an object, then do a copy change, that change isn't reflected in the object you gave to another method/thread/fiber/etc. Immutability doesn't mean application state never changes; it means that a single reference will always point to memory that hasn't changed.

[kaba0]

Parent means basically the difference between identity and primitive/value classes. In Haskell, you’ve only got the latter (maybe you can manage something with lower level controls exposed), that is in a non-haskell syntax new Point(3,2) == new Point(3,2), even though in memory the two object is different.

“Problem” is with records, that they are only shallowly immutable. record Rect(List<Point> corners) will readily hand out a “pointer” to a mutable list. It can be solved of course by storing only immutable objects.

What parent may have failed to get from grandparent comment is that the latter likely meant it under the hood, transparently to the user. That is, new Point(3,2) != new Point(3,2) but the JVM can make the object reference the same data, because the field itself is final. Thus a copy can be optimized at the JVM level, while still having identity.