[External] : Re: EG meeting, 2021-11-17

[Brian Goetz]

No, I’m talking more broadly.

    abstract class A implements PureObject {
        int a;
    }

    abstract class B extends A {
        int b;
    }

    pure class C extends B {
        int c;
    }

Now C is a final, pure class with fields a, b, and c.  A and B are abstract superclasses of C.

There’d be details to work out, but this is not an impossible lift.  The question is whether the return on complexity is there or not.


On Nov 18, 2021, at 5:58 PM, Remi Forax <forax at univ-mlv.fr<mailto:forax at univ-mlv.fr>> wrote:



________________________________
From: "Brian Goetz" <brian.goetz at oracle.com<mailto:brian.goetz at oracle.com>>
To: "Kevin Bourrillion" <kevinb at google.com<mailto:kevinb at google.com>>
Cc: "Dan Heidinga" <heidinga at redhat.com<mailto:heidinga at redhat.com>>, "daniel smith" <daniel.smith at oracle.com<mailto:daniel.smith at oracle.com>>, "valhalla-spec-experts" <valhalla-spec-experts at openjdk.java.net<mailto:valhalla-spec-experts at openjdk.java.net>>
Sent: Jeudi 18 Novembre 2021 23:34:51
Subject: Re: EG meeting, 2021-11-17
I think it is reasonable to consider allowing bucket two classes to be abstract.  They could be extended by other classes which would either be abstract or final. The intermediate types are polymorphic but the terminal type is monomorphic.

A similar argument works for records.

I suppose you are talking about empty (no field) abstract classes.
We need that for j.l.Object, j.l.Number or j.l.Record.

From a user POV, it's not very different from an interface with default methods.

Rémi


Sent from my iPad

On Nov 18, 2021, at 5:27 PM, Kevin Bourrillion <kevinb at google.com<mailto:kevinb at google.com>> wrote:

On Wed, Nov 17, 2021 at 7:05 PM Dan Heidinga <heidinga at redhat.com<mailto:heidinga at redhat.com>> wrote:

Let me turn the question around: What do we gain by allowing
subclassing of B2 classes?

I'm not claiming it's much. I'm just coming into this from a different direction.

In my experience most immutable (or stateless) classes have no real interest in exposing identity, but just get defaulted into it. Any dependency on the distinction between one instance and another that equals() it would be a probable bug.

When B2 exists I see myself advocating that a developer's first instinct should be to make new classes in B2 except when they need something from B1 like mutability (and perhaps subclassability belongs in this list too!). As far as I can tell, this makes sense whether there are even any performance benefits at all, and the performance benefits just make it a lot more motivating to do what is already probably technically best anyway.

Now, if subclassability legitimately belongs in that list of B1-forcing-factors, that'll be fine, I just hadn't fully thought it through and was implicitly treating it like an open question, which probably made my initial question in this subthread confusing.



--
Kevin Bourrillion | Java Librarian | Google, Inc. |kevinb at google.com<mailto:kevinb at google.com>