Imported from jabrena/plinth (skills/122-java-type-design/SKILL.md). Install upstream with npx skills add jabrena/plinth --skill 122-java-type-design. Copyright stays with the author (Apache-2.0).

Type Design Thinking in Java

Review and improve Java code using comprehensive type design principles that apply typography concepts to code structure and organization for maximum clarity and maintainability.

What is covered in this Skill?

• Clear type hierarchies: nested static classes, logical structure
• Consistent naming conventions: domain-driven patterns, uniform interface/implementation naming
• Strategic whitespace for readability
• Type-safe wrappers: value objects replacing primitive obsession (EmailAddress, Money)
• Generic type parameters: flexible reusable types, bounded parameters
• Domain-specific fluent interfaces: builder pattern, method chaining
• Type weights: conceptual importance — core domain vs supporting vs utility
• Type contrast through interfaces: contract vs implementation separation
• Aligned method signatures: consistent parameter and return types across related classes
• Self-documenting code: clear descriptive names
• BigDecimal for precision-sensitive calculations (financial/monetary operations)
• Strategic type selection: Optional, Set vs List, interfaces over concrete types

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 type design 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

• Review Java code for type design
• Improve type design in Java code
• Fix primitive obsession in Java code
• Create value objects in Java code
• Create type hierarchies in Java code
• Create fluent interfaces in Java code

Workflow

1. Compile project before type-design changes

Run ./mvnw compile or mvn compile and stop immediately if compilation fails.

2. Read type-design reference and inspect code

Read references/122-java-type-design.md and identify type hierarchy, naming, and value-object improvements.

3. Apply type-design refactorings

Implement selected type-safety and readability improvements based on applicable patterns.

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/122-java-type-design.md.