Immutability was never incompatible with OOP, just the opposite in fact. Even Alan Kay often criticized languages like C++ and Java for encouraging the use of setters and, thus, “turning objects back into data structures”.
C# is still one of my favorite languages (even though I use F# most of the time now), but I do admire Java for making it significantly more painful to write mutable rather than immutable classes; it's too bad that fact was lost on so many programmers.
Kudos for sharing the Rich Hickey video; it's one of my favorites of all time.
Yes, the same applies to my beloved C#, but that language was much less hostile to immutability. Indeed, the prettier mutator syntax was even positioned as a feature once upon a time.
To be clear, I'm the guy that insists on defining classes as either abstract or sealed, and almost always marks fields as readonly. But, I'm okay with the kind bounded mutability that you mentioned; clients of a `Foo` instance have to treat it as immutable.
Here is how I do OOP:
* I make classes to hide state, and hidden state is the same as being stateless.
* As I learn more about the problem, I start subdividing classes into smaller classes (not necessarily via inheritance).
* So, as my understanding of the problem increases, the number of class division increases, and by the pigeonhole principle, the amount of state approaches zero.
C# is still one of my favorite languages (even though I use F# most of the time now), but I do admire Java for making it significantly more painful to write mutable rather than immutable classes; it's too bad that fact was lost on so many programmers.
Kudos for sharing the Rich Hickey video; it's one of my favorites of all time.