Question about Value Classes and inheritance

[David Alayachew]

To answer your question specifically Rémi, I figured this would be useful
to prevent things like misuse. For example, the JEP shows how BigInteger
can't be a value class due to its internal use of mutability. Well,
something like enforcing value classes might have prevented that. Or maybe
not, I am ignorant about that example specifically.

But hopefully you see my point? It feels like there a lot of validation
opportunities being left on the table. Value classes aren't just faster,
they are safer. So, being able to assert that safety for all child classes
seems nice.

Or maybe I am wrong, and Value classes aren't inherently safer. Finality
feels safe to me, and finality by default feels safer. Idk.

Truthfully, I'm operating more on feelings here than genuine experience.
I'll have something more concrete (or see where I'm wrong) to say once the
new EA comes out.

On Tue, Sep 23, 2025, 8:31 AM Remi Forax <forax at univ-mlv.fr> wrote:

>
>
> ------------------------------
>
> *From: *"David Alayachew" <davidalayachew at gmail.com>
> *To: *"valhalla-dev" <valhalla-dev at openjdk.java.net>
> *Sent: *Tuesday, September 23, 2025 2:19:40 PM
> *Subject: *Question about Value Classes and inheritance
>
> Hello @valhalla-dev <valhalla-dev at openjdk.java.net>,
>
> I read through JEP 401, and the subsection about subtyping surprised me.
>
> It appears that Abstract Value Classes can permit both Value child classes
> and Identity child classes. The example of BigInteger helps explain why.
>
> Ok, but there doesn't seem to be any way to say "only value classes can
> extend me!"
>
> Why is that?
>
> And to be clear, I am not bothered by Abstract Value Classes permitting
> Identity children, that's fine.
>
>
> I'm glad this is fine by you because it's important for backward
> compatibility, it means that you can declare an existing abstract class has
> "value enable", without requiring sub-classes to be changed.
>
> I'm moreso confused by lack of ability to say "only value children".
>
>
> Declaring a class has a value class is really close to be an
> implementation detail (it mainly depends on if your code abuse of == or
> not),
> so why do you want such ability ?
>
>
>
> Thank you for your time and consideration.
> David Alayachew
>
>
> regards,
> Rémi
>
>
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