2 questions about enums and the closed/withdrawn JEP 301

[David Alayachew]

Hello Brian,

Thank you for your response!

> F-bounds are weird-looking at first:
>
>      abstract class Enum<T extends Enum<T>> { ... }
>
> in part because it's not always obvious
> what "T" means at first. We're used to
> seeing type variables that represent
> element types (List<T>), or result types
> (Supplier<T>), but this T really means
> "subclass".  If it were written
>
>      abstract class Enum<Subclass extends Enum<Subclass>>
> { Subclass[]
values(); }
>
> it would already be clearer.

This clarified the F bounds fully, thank you. Thinking of a type parameter
as a sort of subclass is something I never thought of before. I see how it
applies and works here though.

> You might first try writing it as
>
>      abstract class Enum<Subclass> { ... }
>
> but this permits extension in unexpected
> ways:
>
>      class FooEnum extends Enum<String> { ... }
>
> You could try refining it as
>
>      abstract class Enum<Subclass extends Enum> { ... }
>
> but this still permits
>
>      class FooEnum extends Enum<BarEnum> { ... }
>
> What we're trying to express is that the
> type argument of Enum *is* the class
> being declared.  Which is where the
> f-bound comes in:
>
>      abstract class Enum<Subclass
>      extends Enum<Subclass>> { Subclass[]
values(); }
>
> which can only be satisfied by a
> declaration of the form
>
>      class X extends Enum<X> { ... }

I had had a discussion with someone long ago [1], arguing over whether or
not T extends Something<T> is meaningfully different from T extends
Something<?>. I now see the difference. It communicates what the only type
permitted should be.

> The first few times you look at it, it seems
> weird and magic, and then at some point
> it seems obvious :)

Yeah, I can definitely feel the gears turning lol.

Let me ask though, is there a formal name for these type of relationships?
I have seen in other discussions things like L types or Q types on
Valhalla. Maybe they are entirely unrelated. Moreso my question is, is
there some formal name that captures this type of relationship or logic?

For example, if I want to learn integrals, I would need to first understand
derivatives, how they play with that E shaped summation thing, then learn
antiderivatives, and then I can learn about integrals. However, if I look
up Calculus, I get a nice overhead view of each of these components,
allowing me to learn one after the other until I can understand. Googling
calculus is all I need to be able to learn everything I need to understand
integrals.

Is there a similar blanket term that covers this type of logic? I
understand f bounded types now, but I want to learn about other
relationships like this and don't really know the parent term they fall
under.

Thank you for your time and help!
David Alayachew

[1] = https://stackoverflow.com/a/70504547
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