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

• Improve the code with Generics
• Apply Generics
• Refactor the code with Generics

Reference

For detailed guidance, examples, and constraints, see references/128-java-generics.md.