RFR: 8272137: Make Collection and Optional classes streamable

[Alan Snyder]

Ah, if only one could define a type alias Streamable<T> = Supplier<Stream<T>>...

> On Aug 15, 2021, at 8:42 PM, Tagir F.Valeev <tvaleev at openjdk.java.net> wrote:
> 
> On Mon, 9 Aug 2021 12:28:23 GMT, CC007 <github.com+5381337+CC007 at openjdk.org> wrote:
> 
>> create Streamable and ParallelStreamable interface and use them in Collection and Optional
> 
> Mostly agreed with Brian. Judging from 7 years of using Stream API, I can say that this abstraction would not solve any real problem. If you need a way to create many identical streams on demand, just accept `Supplier<Stream<T>>`. This allows more flexibility for clients. They can not only supply `myCollection::stream` or `myOptional::stream` but also `() -> Arrays.stream(myArray)`, `() -> IntStream.range(...).boxed()`, `() -> myCollection.stream().filter(something)` or whatever else. A dedicated `Streamable` interface is too limited and will require adapters in many cases but you can already adapt anything to `Supplier<Stream<T>>`. People already use `Supplier<Stream<T>>` idiom pretty often, so creating a new `Streamable` interface would add an API mess: some people would stick with `Supplier` and others would migrate to `Streamable`. So I vote to reject this PR.
> 
> I said "mostly" because I think that PR is a good starting point for discussion. It's much easier to explain which enhancement you are proposing if you already present some code. And we are already at corelibs-dev, as PR comments are mirrored there, and for some people, it's more comfortable to discuss via GitHub interface, as you don't have to subscribe and get tons of unrelated e-mails, you can concentrate on a single discussion only. So in my opinion, it's completely ok to write code and create a PR before the discussion, even if it's likely to be thrown away.
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> PR: https://git.openjdk.java.net/jdk/pull/5050
>