# Kotlin Language

> Idiomatic Kotlin 1.9+ standards including null safety, sealed classes, and expression syntax. Use when working with Kotlin null safety, data classes, sealed interfaces, extension functions, or migrating Java code to Kotlin. (triggers: **/*.kt, **/*.kts, val, var, ?., ?:, !!, data class, sealed, when, extension, lazy, lateinit, object)

- Skill: `comeonoliver/kotlin-language` (Agent Skill)
- Install (CLI): `npx skillmds@latest add comeonoliver/kotlin-language`
- Raw SKILL.md: https://api.skillmd.com/api/skills/comeonoliver/kotlin-language/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: ComeOnOliver (https://skillmd.com/u/comeonoliver)
- Updated: 2026-09-08
- Page: https://skillmd.com/skills/comeonoliver/kotlin-language

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# Kotlin Language Patterns

## **Priority: P0 (CRITICAL)**

Modern, idiomatic Kotlin standards for safety and conciseness.

## Implementation Guidelines

- **Immutability**: Use `val` by default. Only use `var` if mutation is required locally.
- **Null Safety**: Use `?` for nullable types. Use safe call `?.` and Elvis `?:` over `!!`.
- **Expressions**: Prefer expression bodies `fun foo() = ...` for one-liners. Use `if`/`try` as expressions.
- **Classes**: Use `data class` for DTOs. Use `sealed interface/class` for state hierarchies (e.g., `Success`, `Error`, `Loading`). Access members as a computed property rather than a function.
- **Extension Functions**: Prefer over utility classes (`StringUtil`). Keep private/internal if module-specific.
- **Named Arguments**: Use for clarity, especially with booleans or multiple same-type params.
- **String Templates**: Use `"$var"` over concatenation. Use `"""` for multiline strings (SQL/JSON).

## Anti-Patterns

- **No !! Operator**: Never use in production; prefer safe calls or requireNotNull.
- **No Java-isms**: Use properties not get/set; prefer top-level functions over companion object statics.
- **No Lateinit Abuse**: Prefer nullable types or lazy delegates instead.
- **No Silenced Errors**: Never swallow exceptions without logging or handling.

## References

- [Sealed Class, When Expression & Extension Examples](references/example.md)