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
When to use this skill
- Review Java code for generics
- Apply best practices for generics in Java code
Reference
For detailed guidance, examples, and constraints, see references/128-java-generics.md.
1---2name: "128-java-generics-2"3description: Use when you need to review, improve, or refactor Java code for generics quality — including avoiding raw types, applying the PECS (Producer Extends Consumer Super) principle for wildcards, using bounded type parameters, designing effective generic methods, leveraging the diamond operator, understanding type erasure implications, handling generic inheritance correctly, preventing heap pollution with @SafeVarargs, and integrating generics with modern Java features like Records, sealed types, and pattern matching. Part of the skills-for-java project4license: Apache-2.05---6# Java Generics Best Practices78Review and improve Java code using comprehensive generics best practices that enforce compile-time type safety and enable flexible, reusable APIs.910**What is covered in this Skill?**1112- Type safety: avoiding raw types, eliminating unsafe casts13- Code reusability: generic methods and types for multiple type contexts14- PECS wildcards: `? extends` for producers, `? super` for consumers15- Diamond operator for type inference16- Type erasure awareness: type tokens, factory patterns, array creation17- Generic inheritance and variance: invariance, covariance, contravariance18- `@SafeVarargs` for heap pollution prevention19- Wildcard capture helpers, self-bounded generics (CRTP) for fluent builders20- Proper wildcard API design: `Comparator<? super T>`, `Function<? super T, ? extends R>`21- Arrays-vs-generics covariance pitfalls, serialization with `TypeReference`/`TypeToken`22- Generic naming conventions (`T`, `E`, `K/V`, `?`), typesafe heterogeneous containers23- Integration with Records, sealed types, and pattern matching2425**Scope:** The reference is organized by examples (good/bad code patterns) for each core area. Apply recommendations based on applicable examples.2627## Constraints2829Before applying any generics changes, ensure the project compiles. If compilation fails, stop immediately — do not proceed until resolved. After applying improvements, run full verification.3031- **MANDATORY**: Run `./mvnw compile` or `mvn compile` before applying any change32- **SAFETY**: If compilation fails, stop immediately and do not proceed — compilation failure is a blocking condition33- **VERIFY**: Run `./mvnw clean verify` or `mvn clean verify` after applying improvements34- **BEFORE APPLYING**: Read the reference for detailed examples, good/bad patterns, and constraints3536## When to use this skill3738- Review Java code for generics39- Apply best practices for generics in Java code4041## Reference4243For detailed guidance, examples, and constraints, see [references/128-java-generics.md](references/128-java-generics.md).
jabrena