Optional<Stream>.filter(Optional::isPresent).map(Optional::get) to Java 11 .flatMap(Optional::stream)

[aslaski2]

Fixes #313

Problem

Java 8 Optional class didn't provide ::stream method. As a result of this unfortunate mistake a popular idiom was created to flatten a Stream of Optionals: .filter(Optional::isPresent).map(Optional::get).
This was corrected in Java 9 and the idiom may be replaced with .flatMap(Optional::stream)

Anything in particular you'd like reviewers to focus on?

This is my first adventure with OpenRewrite, please check everything carefully.

Checklist

• I've added unit tests to cover both positive and negative cases
• I've added the license header to any new files through ./gradlew licenseFormat
• I've used the IntelliJ IDEA auto-formatter on affected files except for the flatMap chain

[aslaski2]

Hi @timtebeek! What happens now? Will someone come and review?

[timtebeek]

Impressed with the work you've done here! Really like the attention to detail in handling edge cases, and the suite of tests to show what's correctly handled. I've added some comments that I think would improve this PR still, and bring it closer to a merge.

Let me know if you'd be open to making those changes still!

[timtebeek]

Thanks a lot for this addition @aslaski2 ! Let us know how this works for you; you can try it out through app.moderne.io soon after the merge.