4.x: Java Migration; records, newer API, newer syntax

[akarnokd]

• Turn classes into records where possible.
• Use List.sort where possible.
• Use switch expression where possible.
• Use record extractor pattern where possible.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.613 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

src/main/java/io/reactivex/rxjava4/core/Completable.java: 4.835
src/main/java/io/reactivex/rxjava4/core/Flowable.java: 4.779
src/main/java/io/reactivex/rxjava4/core/Maybe.java: 4.843
src/main/java/io/reactivex/rxjava4/core/MaybeObserver.java: 4.811
src/main/java/io/reactivex/rxjava4/core/Observable.java: 4.758
src/main/java/io/reactivex/rxjava4/core/Single.java: 4.832
src/main/java/io/reactivex/rxjava4/disposables/Disposable.java: 4.677
src/main/java/io/reactivex/rxjava4/exceptions/OnErrorNotImplementedException.java: 4.876
src/main/java/io/reactivex/rxjava4/exceptions/UndeliverableException.java: 4.960
src/main/java/io/reactivex/rxjava4/internal/disposables/DisposableHelper.java: 4.412
src/main/java/io/reactivex/rxjava4/internal/fuseable/CancellableQueueFuseable.java: 4.883
src/main/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableGroupBy.java: 4.043
src/main/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableWindowBoundarySelector.java: 4.039
src/main/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableWithLatestFromMany.java: 4.466
src/main/java/io/reactivex/rxjava4/internal/operators/maybe/MaybeCount.java: 4.696
src/main/java/io/reactivex/rxjava4/internal/operators/observable/ObservableWindowBoundarySelector.java: 4.091
src/main/java/io/reactivex/rxjava4/internal/operators/single/SingleTakeUntil.java: 4.601
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromPublisher.java: 4.463
src/main/java/io/reactivex/rxjava4/internal/subscribers/SinglePostCompleteSubscriber.java: 4.637
src/main/java/io/reactivex/rxjava4/internal/subscriptions/EmptySubscription.java: 4.691

✅ No secrets or suspicious high-entropy strings found.

Mid-4 beauty heuristic in action — powered by our entropy chats! 😊

[codecov]

Codecov Report

❌ Patch coverage is 99.16898% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.33%. Comparing base (557a600) to head (0964943).
⚠️ Report is 1 commits behind head on 4.x.

Files with missing lines	Patch %	Lines
.../main/java/io/reactivex/rxjava4/core/Streamer.java	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8187      +/-   ##
============================================
- Coverage     98.36%   98.33%   -0.03%     
+ Complexity     6929     6927       -2     
============================================
  Files           785      785              
  Lines         48981    48766     -215     
  Branches       6595     6594       -1     
============================================
- Hits          48181    47955     -226     
- Misses          610      615       +5     
- Partials        190      196       +6     

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