[External] : Re: JEP draft: Implicit Classes and Enhanced Main Methods (Preview)

Ron Pressler ron.pressler at oracle.com
Fri Feb 17 10:11:20 UTC 2023

On 16 Feb 2023, at 21:41, forax at univ-mlv.fr<mailto:forax at univ-mlv.fr> wrote:

I still think that fields should not be allowed inside an implicit class, because when you remove the class declaration a field and a local variable are too similar and because an implicit class has no user defined constructor.

I think your general point has some merit — I’ll get to that later — but first let me address the concrete points you raise.

Here is a series of examples showing how confusing it can be.

How would any of those be made easier to understand by the presence of a class declaration when you don’t know what a class is?

By the way, we should certainly look into making some error messages — especially those encountered by beginners — easier to understand.

Also conceptually, being able to define fields without constructors is problematic, because you are bypassing the the notion of encapsulation.

Encapsulation from what? Encapsulation is a programming-in-the-large notion, but even at the technical level, an implicit class is well encapsulated by virtue of it being unnamed (and the default access remains package).

Implicit class instance fields are more complex that usual class fields because of the lack of constructors.

I’m not sure I understand the relationship you make to constructors (BTW, you can define initializers).

Python and JS, both also first language have a notion of shared variables that can be introduced before object fields. Clojure and ML, both functional languages, also have a similar notion of shared variables. Even Haskell has constants. Surely you’d agree that at least final fields — constants — are necessary to do any kind of nice programming?

Teaching using a simpler model is great but not if as a student you have to unlearn something previously introduced.

I wholeheartedly agree, but what is the thing that needs to be unlearned?

But now back to where I think your general point has merit. I think final fields are a must, but one could certainly argue that non-final fields are not. You can certainly do a lot of programming without them. But I think that allowing final fields and disallowing non-final fields *in Java* would be weird, because to designate something as final you need extra syntax, so we’d reject syntactically simpler, valid, code and accept more complex one. Moreover, there are things that are easier to do and tech with mutable fields.

However, there’s the matter of static, which you used in your examples but didn’t explicitly discuss. Because a an implicit class is effectively a singleton (plus, the class cannot be referenced by other classes), there is no useful difference between an instance field and a static field, so I think we should entertain the notion of disallowing static members — fields, methods, or even member classes (although things that are implicitly static, such as records would obviously be allowed).

One argument against that may be is that if an experienced Java programmer has an existing small program that they want to make prettier by turning into an implicit class — implicit classes are mainly motivated by learners, but they’re not *just* for them — then the process would be made harder by disallowing static members.

In short, I think we must allow fields, but we can think about disallowing (explicitly) static members altogether.

— Ron

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/amber-spec-experts/attachments/20230217/3a92544d/attachment-0001.htm>

More information about the amber-spec-experts mailing list