Android: List.of() (API 30+) used in core since 0.24.0 contradicts the README's "minimum API level is 19"

まず、commonmark モジュールにある List.of、Set.of、Map.of の記載された使用箇所と、.github/workflows/ci.yml の Android テスト設定を確認します。core library desugaring なしで API 24～29 の失敗を再現し、そのうえで、選択した解決策によって README、実行時の動作、Android サポートに関する想定の整合性が保たれることを確認します。

Summary

Since 0.24.0 the core commonmark module uses java.util.List.of(...) (and a few Set.of/Map.of). On Android these methods exist only from API 30 (Android 11). The README still says:

It works on Android too, but that is on a best-effort basis, please report problems. For Android the minimum API level is 19

An app with minSdk < 30 that does not enable core library desugaring crashes on the first parse on API < 30 devices:

java.lang.NoSuchMethodError: No static method of(Ljava/lang/Object;)Ljava/util/List;
    at org.commonmark.internal.InlineParserImpl.parse(InlineParserImpl.java)

This is the same class of problem as #366 / #373 (Objects.requireNonNullElseGet, fixed by #369). The List.of usages were introduced deliberately in #322 as a Java 11 clean-up, so this is a report that the clean-up and the Android support statement now disagree — not a claim that the change was wrong.

Steps to reproduce

Android app, minSdk 24, no coreLibraryDesugaring, org.commonmark:commonmark:0.30.0, run on an API 24–29 emulator:

Parser.builder().build().parse("*a*")   // NoSuchMethodError: List.of

With isCoreLibraryDesugaringEnabled = true + com.android.tools:desugar_jdk_libs:2.1.5 the same code works on API 24 (verified on an API 24 emulator).

Where the API 30+ calls live (0.30.0, commonmark module)

List.of / Set.of / Map.of occurrences by file:

0.22.0 had none; 0.24.0 introduced 8 in InlineParserImpl.

Why CI did not catch it

commonmark-android-test is only run through ./gradlew :app:lint in .github/workflows/ci.yml (minSdk 19, no emulator run), so a runtime NoSuchMethodError never surfaces there.

Possible resolutions

Either would be fine from a consumer's point of view — the important part is that the README and the code agree:

1. Keep API 19 support — replace the factory calls with Java 8 equivalents (Collections.emptyList() / singletonList(x) / unmodifiableList(Arrays.asList(a, b)), same for Set/Map). Mechanical, ~26 sites, keeps immutability. I can send a PR if you want this.
2. Document the new requirement — state in the README that Android needs API 30+ or core library desugaring, and ideally have commonmark-android-test run on an emulator (or fail lint on NewApi) so future regressions are visible.

Context: I hit this while building an Android Markdown renderer on top of commonmark-java (https://github.com/Jimmy-Jung/RichMarkdown-Android). We chose minSdk 30 for now, so this is not blocking us — reporting because the README statement is likely to mislead other Android users.