// Java 17–25 policy and pitfalls. Use when writing, reviewing, or refactoring Java code — enforces idioms around records, sealed types, switch exhaustiveness, Optional, virtual threads, and null-safety that LLMs frequently get wrong.
| name | java-engineer |
| description | Java 17–25 policy and pitfalls. Use when writing, reviewing, or refactoring Java code — enforces idioms around records, sealed types, switch exhaustiveness, Optional, virtual threads, and null-safety that LLMs frequently get wrong. |
Baseline knowledge of the Java language (records, sealed, pattern matching, text blocks, streams, var, generics, Optional API) is assumed — this skill does not teach the language. It encodes the project policy and the traps that keep appearing in code review.
Before writing non-trivial code, verify:
java --version, and target version in pom.xml (<maven.compiler.release>) or build.gradle(.kts) (java { toolchain { languageVersion = JavaLanguageVersion.of(21) } }). Adjust feature use if < 17. Current LTS: Java 25 (released Sept 2025); Java 21 is the previous LTS. Prefer LTS releases for new projects../mvnw / ./gradlew, never a globally installed mvn / gradle.lombok in dependencies. If present, prefer @RequiredArgsConstructor / @Slf4j / @Builder; if absent, do not introduce Lombok unless asked.error-prone, checkstyle, spotbugs, pmd. Respect existing rules; do not add new violations.--enable-preview, some features behave differently. Check flag before relying on preview syntax.Optional is a return type only. Never a parameter, field, or element of a collection.record. DTOs, events, results, config snapshots. Not JPA entities (records are final and have no no-arg constructor).sealed + exhaustive switch. Compiler catches new cases.switch expression over if / else if chains. Pattern-matching switch over instanceof chains.final unless you have a reason to reassign.Objects.requireNonNull in public-API constructors; validate invariants in record compact constructors.try-with-resources for anything AutoCloseable — including ExecutorService (Java 19+).Executors.newVirtualThreadPerTaskExecutor()). Not for CPU-bound work.try (var scope = new StructuredTaskScope.ShutdownOnFailure()) { ... }.log.info("user_updated userId={} status={}", id, status).Optional.get() without prior isPresent() — use .map(...).orElseThrow(...) or .orElse(default).List instead of List<Foo>), no @SuppressWarnings("rawtypes") to silence warnings.null from a collection-returning method. Return List.of() / Collections.emptyList().Stream / lambda. Wrap into an unchecked domain exception at the boundary.Exception / Throwable to "make the compiler happy". Handle a specific type or propagate.List.copyOf(input).synchronized around a blocking call on a virtual thread — it pins the carrier thread. Use ReentrantLock.Thread.sleep / blocking I/O on parallelStream() — shared common pool, starves other work.Stream.toList() result — it's unmodifiable (JDK 16+).| Load when | File |
|---|---|
| Debugging subtle language / API behavior (Optional, Stream, switch, records, generics) | references/pitfalls.md |
| Records, sealed hierarchies, pattern-matching switch, preview features (pitfalls & traps, not syntax tutorials) | references/modern-java.md |
Writing concurrent code (virtual threads, structured concurrency, ExecutorService lifecycle, ThreadLocal traps) | references/concurrency.md |
| File / stream / socket I/O, charsets, partial reads, NIO buffers | references/io.md |
When producing code:
When reviewing code: call out MUST-DO / MUST-NOT violations explicitly and suggest the minimal fix.
Senior Android engineer workflows — Kotlin-first (Java legacy supported), Jetpack Compose + View system, MVVM, Coroutines/Flow, Room, Retrofit, Hilt, plus device orchestration via plugin tools (device management, logcat, crash reports, app run) and mobile-mcp for UI interaction (element tree, taps, swipes). Use when implementing features, debugging crashes, fixing builds, writing tests, reviewing Android code, or running QA on an emulator.
Use when working on Laravel projects. Covers architecture decisions (Services vs Actions), thin controllers, Form Requests, API Resources, and API versioning. Enforces Larastan verification after code generation.
Up-to-date library and framework documentation via Context7 MCP. Use when setup, API, or version-specific questions require current documentation.
Modern PHP 8.x policy and pitfalls. Use when writing, reviewing, or refactoring PHP code — enforces strict types, enum/readonly/match/DNF policy, PHPStan discipline, and catches the subtle traps (type coercion, mixed abuse, readonly mutation through references, enum serialization, fiber lifecycle, PDO emulation) that LLMs get wrong by default.
Redis policy & pitfalls — key naming, atomicity, TTL strategy, distributed locks, cache patterns, production traps. Use when designing Redis-backed caches, locks, queues, rate limiters, or sessions. Covers the traps LLMs get wrong by default: SETNX + EX (deprecated), KEYS on production, Pub/Sub no-persistence, HGETALL on big hashes, maxmemory-policy defaults, cluster hash tags, RDB fork memory.
SQL policy & pitfalls — query correctness, indexing strategy, safe migrations. Use when writing SQL, diagnosing slow queries, designing schemas, or reviewing Flyway/Liquibase migrations. Covers the traps LLMs miss by default: NOT IN with NULLs, function-on-column breaking indexes, OFFSET on large tables, NOT NULL column lock, CREATE INDEX blocking writes, immutable migrations.
