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writing-java-kotlin

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Idiomatic modern Java and Kotlin JVM development. Use when writing `.java`, `.kt`, or `.kts` code; changing Gradle or Maven builds; or working on Spring, Micronaut, Quarkus, Ktor, Android JVM modules, JUnit, Mockito, Kotest, ktlint, detekt, or JVM CLI/services. Emphasizes JDK toolchains, null-safety, fast focused Gradle/Maven feedback, deterministic formatting, and minimal dependencies. NOT for JavaScript/TypeScript, C#/.NET, Python, shell scripts, or infra-only work.

src/skills/writing-java-kotlin/SKILL.md

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Last checked
Nov 11, 2026

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38
Repository last updated
Oct 25, 2026
License
MIT

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# Java and Kotlin Development

## Build baseline

- Read the wrapper properties, `settings.gradle*`/`build.gradle*`/`gradle.properties` or `pom.xml`, and CI before relying on version-specific behavior.
- Use `./gradlew`/`./mvnw` when the repo has them; scope tests to the changed module (see testing.md).
- The compile target is the declared Java toolchain and Kotlin `jvmToolchain`, not the shell's `JAVA_HOME`; without one, the running JDK compiles.
- Toolchain, plugin, and dependency versions live in convention plugins, version catalogs, BOMs, or parent POMs, set once there. Raising the Java or Kotlin toolchain is a project-wide decision: ask, and change it in shared build config, not one module.
- A request to use a preview feature is not approval to enable it: ask first, name a non-preview fallback, and, once approved, add `--enable-preview` to the build's compile and run/test tasks, not a local flag. Previews can be withdrawn between JDKs: string templates were withdrawn in JDK 23.
- Wire coverage, mutation, or other heavy analysis as its own task or under `check`; never make `test` depend on or be finalized by the report, which would still run it on every plain `test`. Use a separate command or CI step, even when adding it is the task.

## Defaults

- Kotlin: structured concurrency only — no `GlobalScope`; pass a `coroutineScope` or an injected scope through the call chain. Test coroutine code with `kotlinx-coroutines-test`'s `runTest`, not `runBlocking` or real delays. Keep blocking calls off event-loop and coroutine dispatcher threads unless the framework allows it.
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