122-java-type-design

# 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 ## 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 ## Reference For detailed guidance, examples, and constraints, see [references/122-java-type-design.md](references/122-java-type-design.md).