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
- Improve generic type safety in Java APIs
- Fix raw types and unchecked casts in Java code
Workflow
- Compile project before generics changes
Run ./mvnw compile or mvn compile and stop immediately if compilation fails.
- 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.
- Apply generics refactorings
Implement selected generic type and API improvements while preserving behavior.
- 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.
1---
2name: 128-java-generics
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. This should trigger for requests such as Improve the code with Generics; Apply Generics; Refactor the code with Generics; Improve generic type safety in Java APIs; Fix raw types and unchecked casts in Java code. Part of Plinth Toolkit
4license: Apache-2.0
5metadata:
6 author: Juan Antonio Breña Moral
7 version: 0.18.0
8---
9
10# Java Generics Best Practices
11
12Review and improve Java code using comprehensive generics best practices that enforce compile-time type safety and enable flexible, reusable APIs.
13
14**What is covered in this Skill?**
15
16- Type safety: avoiding raw types, eliminating unsafe casts
17- Code reusability: generic methods and types for multiple type contexts
18- PECS wildcards: `? extends` for producers, `? super` for consumers
19- Diamond operator for type inference
20- Type erasure awareness: type tokens, factory patterns, array creation
21- Generic inheritance and variance: invariance, covariance, contravariance
22- `@SafeVarargs` for heap pollution prevention
23- Wildcard capture helpers, self-bounded generics (CRTP) for fluent builders
24- Proper wildcard API design: `Comparator<? super T>`, `Function<? super T, ? extends R>`
25- Arrays-vs-generics covariance pitfalls, serialization with `TypeReference`/`TypeToken`
26- Generic naming conventions (`T`, `E`, `K/V`, `?`), typesafe heterogeneous containers
27- Integration with Records, sealed types, and pattern matching
28
29**Scope:** The reference is organized by examples (good/bad code patterns) for each core area. Apply recommendations based on applicable examples.
30
31## Constraints
32
33Before applying any generics changes, ensure the project compiles. If compilation fails, stop immediately — do not proceed until resolved. After applying improvements, run full verification.
34
35- **MANDATORY**: Run `./mvnw compile` or `mvn compile` before applying any change
36- **SAFETY**: If compilation fails, stop immediately and do not proceed — compilation failure is a blocking condition
37- **VERIFY**: Run `./mvnw clean verify` or `mvn clean verify` after applying improvements
38- **BEFORE APPLYING**: Read the reference for detailed examples, good/bad patterns, and constraints
39- **EDGE CASE**: If request scope is ambiguous, stop and ask a clarifying question before applying changes
40- **EDGE CASE**: If required inputs, files, or tooling are missing, report what is missing and ask whether to proceed with setup guidance
41
42## When to use this skill
43
44- Improve the code with Generics
45- Apply Generics
46- Refactor the code with Generics
47- Improve generic type safety in Java APIs
48- Fix raw types and unchecked casts in Java code
49
50## Workflow
51
521. **Compile project before generics changes**
53
54Run `./mvnw compile` or `mvn compile` and stop immediately if compilation fails.
55
562. **Read generics reference and assess type safety**
57
58Read `references/128-java-generics.md` and identify raw types, unsafe casts, wildcard misuse, and API variance opportunities.
59
603. **Apply generics refactorings**
61
62Implement selected generic type and API improvements while preserving behavior.
63
644. **Verify with full build**
65
66Run `./mvnw clean verify` or `mvn clean verify` after applying improvements.
67
68## Reference
69
70For detailed guidance, examples, and constraints, see [references/128-java-generics.md](references/128-java-generics.md).
jabrena