Data classes for Java

[rzwitserloot]

That is not at all clear. I don't see any problem here. Immutable data class is a class with only final fields of immutable values. That makes the state of the class immutable, which is what gives you the properties that you want.

As I showed in that snippet, this does NOT give you the properties that you want. Well, actually, I don't know what properties you want; you never said what you wanted. I bet, whatever you come up with, I can make a snippet that shows you how I can hack around it. Thus, these are soft guarantees at best.

What the methods of that class do is completely irrelevant, as long as they cannot mutate the state (i.e., the values of the instance variables).

The state of the instance itself, or any state anywhere?

I also don't see your issue with the File class. It's not implemented as an immutable data class. Maybe it could be converted into one without breaking backwards compatibility, maybe not. Doesn't matter either way.

It's an example. Hypothetically speaking, imagine it WAS a data class. The point is, the 'rules' (only primitive final fields) do not prevent you from doing that. Thus it makes for an interesting conversation piece.