Introduce MonoFromOptionalSwitchIfEmpty and OptionalMapMonoJust Refaster rules

[werli]

I've spotted the following interesting pattern:

Optional.of(someNullableValue())
  .map(Mono::just)
  .orElse(someMono());

The mix between Optional and Mono API is unnecessary.

These rules aim to rewrite these to:

Mono.justOrEmpty(someNullableValue())
  .switchIfEmpty(someMono());

---

Suggested commit message:

Introduce `MonoFromOptionalSwitchIfEmpty` and `OptionalMapMonoJust` Refaster rules
(#384)

[github-actions]

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[Stephan202]

Thanks for opening a PR!

Added one commit :)

Tweaked suggested commit message (a bit long, but alas):

Introduce `OptionalMapMonoJust` and `MonoFromOptionalSwitchIfEmpty` Refaster rules
(#384)

[Stephan202]

The Mono::justOrEmpty -> Mono::just case can be its own "precursor rule", following our approach to defining the smallest improvement per Refaster rule. Will push a proposal.

[werli]

Good one, thanks. I didn't see that initially.

[github-actions]

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[rickie]

Got one question.

Besides that, no remarks. LGTM!

W.r.t. suggested commit message, LGTM as well.

[rickie]

I know this is our usual way of defining the Javadoc.
However, it couldn't hurt to mention the exact reason either as @Description or instead of over more contrived alternatives. Not mixing the Optional and Mono API is a good argument and nice to document here as well.

We could IMO at least start with adding @Descriptions here and there.

[werli]

I can try to come up with something later today. 🤞

[Stephan202]

We could IMO at least start with adding @Descriptions here and there.

I would defer that to a separate PR, as I'm still unsure about that feature. (Perhaps we can generate the @Description annotation at compile time, based on the Javadoc: that would make for a much nicer developer experience.)

[rickie]

Let's go with Javadoc then, deal?

[Stephan202]

Rebased and added a commit; PTAL.

[rickie]

LGTM!

[werli]

Yeah, would indeed feel like a different concern. Would have been weird starting this now in this PR. 🤔

[Stephan202]

Unable to resolve action actions/[email protected], unable to find version v3.7.0

Build fails due to actions/setup-java#422...

[github-actions]

Looks good. No mutations were possible for these changes.
Mutation testing report by Pitest. Review any surviving mutants by inspecting the line comments under Files changed.

[rickie]

Swapped the order of the names to make 'em sorted 😉.