RFR: 8272137: Make Collection and Optional classes streamable

[CC007]

On Mon, 16 Aug 2021 03:39:02 GMT, Tagir F. Valeev <tvaleev 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.

It is fine that the pull request is closed for now, but I agree with @amaembo that if this conversation is indeed mirrored there, then it shouldn't be a problem to discuss the proposal here, along with the proposed changes.

I actually worked out a usecase (See: https://github.com/CC007/InterfaceSegregationDemo), where I partially reimagined the Collection interface stack and its neighboring interfaces/classes. I also added a `Demo` class to show how I imagined to use these interfaces and to show how the granularity of these interfaces can be useful.

The fact that I have to create copies of interfaces and delegates for implementations makes me feel like something is lacking in the collection stack and that for this usecase `Supplier<E>` wouldn't cut it, but feel free to fork and refactor my code to prove me wrong.

PS: while creating the Demo class, I noticed that it couldn't be analyzed if the return value of a method with type `<T extends Streamable<String> & Addable<String> & Iterable<String> & RandomAccess>` could be safely cast to a new local variable with type `<SA extends Streamable<String> & Addable<String>` and that you can't declare variables of type `<? extends Streamable<String> & Addable<String>` (it won't let you use the `&` when using the `?`), but I digress.

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PR: https://git.openjdk.java.net/jdk/pull/5050