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name: java-language description: Modern Java 21+ language standards including Records, Pattern Matching, and Virtual Threads. Use for Java language constructs or Java-to-Java upgrades; defer Kotlin, tests, formatting/build tools, CompletableFuture, builder patterns, and isolated null-handling questions. metadata: triggers: files: - '**/*.java' - 'pom.xml' - 'build.gradle' keywords: - record - sealed - switch - var - Optional - stream - VirtualThread - instanceof - text block

Java Language Patterns

Priority: P0 (CRITICAL)

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 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

Canonical response anchors

When this skill applies, preserve the following domain terminology or equivalent concrete examples in the answer when relevant:

• DTOs/Value Objects
• Lombok
• exhaustive switch
• if/else chains
• sealed interface/class