128-java-generics

# Java Generics Best Practices Review and improve Java code using comprehensive generics best practices that enforce compile-time type safety and enable flexible, reusable APIs. **What is covered in this Skill?** - Type safety: avoiding raw types, eliminating unsafe casts - Code reusability: generic methods and types for multiple type contexts - PECS wildcards: `? extends` for producers, `? super` for consumers - Diamond operator for type inference - Type erasure awareness: type tokens, factory patterns, array creation - Generic inheritance and variance: invariance, covariance, contravariance - `@SafeVarargs` for heap pollution prevention - Wildcard capture helpers, self-bounded generics (CRTP) for fluent builders - Proper wildcard API design: `Comparator<? super T>`, `Function<? super T, ? extends R>` - Arrays-vs-generics covariance pitfalls, serialization with `TypeReference`/`TypeToken` - Generic naming conventions (`T`, `E`, `K/V`, `?`), typesafe heterogeneous containers - Integration with Records, sealed types, and pattern matching **Scope:** The reference is organized by examples (good/bad code patterns) for each core area. Apply recommendations based on applicable examples. ## Constraints Before applying any generics changes, ensure the project compiles. If compilation fails, stop immediately — do not proceed until resolved. After applying improvements, run full verification. - **MANDATORY**: Run `./mvnw compile` or `mvn compile` before applying any change - **SAFETY**: If compilation fails, stop immediately and do not proceed — compilation failure is a blocking condition - **VERIFY**: Run `./mvnw clean verify` or `mvn clean verify` after applying improvements - **BEFORE APPLYING**: Read the reference for detailed examples, good/bad patterns, and constraints - **EDGE CASE**: If request scope is ambiguous, stop and ask a clarifying question before applying changes - **EDGE CASE**: If required inputs, files, or tooling are missing, report what is missing and ask whether to proceed with setup guidance ## When to use this skill - Improve the code with Generics - Apply Generics - Refactor the code with Generics ## Workflow 1. **Compile project before generics changes** Run `./mvnw compile` or `mvn compile` and stop immediately if compilation fails. 2. **Read generics reference and assess type safety** Read `references/128-java-generics.md` and identify raw types, unsafe casts, wildcard misuse, and API variance opportunities. 3. **Apply generics refactorings** Implement selected generic type and API improvements while preserving behavior. 4. **Verify with full build** Run `./mvnw clean verify` or `mvn clean verify` after applying improvements. ## Reference For detailed guidance, examples, and constraints, see [references/128-java-generics.md](references/128-java-generics.md).