Briefest summary of today's Valhalla+nullness picture

[Brian Goetz]

> 1. If your class doesn't do Identity Things, make it a value class. 
> The VM can do smarter things now.

Which also makes your class safer too!

> 2. `==` isn't really an Identity Thing: it will just continue to mean 
> "observably identical*" as it always has. System.hashCode() remains 
> defined in terms of `==`. But actual identity dependence will fail 
> (when feasible, at compile time).

Some readers might retort that the cost of this is `==` went from 
ultra-super-cheap to maybe-but-who-knows.  We might counter-retort that 
such micro-performance concerns are usually a distraction.

> 3. Any type based on that class can be marked as nullable `?` or 
> non-null `!`. Conceptually (if not literally), `Foo!` is a subtype of 
> `Foo?`.

Where by "conceptually" you are appealing to "is-a".

> 4. Nullness enforcement is "best-effort" and better than nothing. *If* 
> you want the gory details of what's enforced you can dig into them. 
> There's still room for third-party nullness analysis tools to help.
>
> 5. If a value class has a do-nothing constructor, and you're fine with 
> non-null variables of that type being initialized to that value, add 
> `implicit` to the constructor. The VM can do more smarter things now.
>
> 6. Consider adding `non-atomic` to the class; the VM can do even morer 
> smarter things now. The downside is that racy code (already risky) 
> might fail in worse ways; you decide if that worries you or not.

And specifically, if your class has cross-field invariants, it should 
worry you.

> 7. The above holds for the 8 primitive types too; yes, they are still 
> special, but mostly in /additional/ ways, rather than exceptions to 
> the usual rules. `int` and `Integer!` are now nearly synonymous.
>
> And we can finally all retire the term "primitive class" now at last. :-)

Praise be.

> While that list is meant to have asterisks and just be "good enough 
> for most people most of the time", it might need some important 
> corrections nevertheless, so, I'm glad to hear them.
>
>
> * yes float/double are weird
>
> -- 
> Kevin Bourrillion | Java/Kotlin Ecosystem Team | Google, 
> Inc. |kevinb at google.com
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