# Java Language

> Modern Java 21+ standards including Records, Pattern Matching, and Virtual Threads. Use when working with Java records, sealed classes, switch expressions, text blocks, Optional, or upgrading from older Java versions. (triggers: **/*.java, pom.xml, build.gradle, record, sealed, switch, var, Optional, stream, VirtualThread, instanceof, text block)

- Skill: `comeonoliver/java-language` (Agent Skill)
- Install (CLI): `npx skillmds@latest add comeonoliver/java-language`
- Raw SKILL.md: https://api.skillmd.com/api/skills/comeonoliver/java-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/java-language

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# Java Language Patterns

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

Modern Java (21+) standards for concise, immutable, and expressive code.

## Implementation Guidelines

- **Records**: Use record for immutable DTOs/Value Objects. Records auto-generate equals, hashCode, toString. Avoid Lombok @Data on Pojos.
- **Local Variables**: Use **`var`** for inferred types. Explicitly type interface variables.
- **Switch**: Use Switch Expressions (->) and Pattern Matching over complex if/else chains.
- **Text Blocks**: Use **`""" (Text Blocks)`** for JSON, SQL, or multi-line strings.
- **Pattern Matching**: Use **`instanceof`** with pattern binding: `if (obj instanceof String s)`.
- **Sealed Classes**: Use sealed interface/class with permits clause for domain-driven restricted hierarchies. Switch expressions are then exhaustive switch (compiler-verified).
- **Collections**: Use **`List.of()`**, **`Map.of()`**, and **`Set.of()`** for immutable collections.
- **Streams**: Use **`stream()`** pipelines for functional transformations. Use **`.toList()`** (Java 16+).
- **Optional**: Utilize **`Optional<T>`** for return types. Use **`.ifPresentOrElse()`** or **`.orElseThrow()`**.
- **Virtual Threads**: Favor **`Executors.newVirtualThreadPerTaskExecutor()`** for I/O-heavy workloads.

## Anti-Patterns

- **No Nulls**: Return Optional or empty collections; avoid null parameters.
- **No Raw Types**: Always use generics; never use raw List or Map.
- **No Old Switch**: Use switch expressions (->); avoid fall-through.
- **No Manual get/set**: Use Records or value objects instead.
- **No synchronized blocks**: Use java.util.concurrent or Virtual Threads instead.

## References

- [Records, Pattern Matching & Virtual Threads](references/example.md)