122-java-type-design

name	122-java-type-design
description	Use when you need to review, improve, or refactor Java code for type design quality — including establishing clear type hierarchies, applying consistent naming conventions, eliminating primitive obsession with domain-specific value objects, leveraging generic type parameters, creating type-safe wrappers, designing fluent interfaces, ensuring precision-appropriate numeric types (BigDecimal for financial calculations), and improving type contrast through interfaces and method signature alignment. Part of the skills-for-java project
license	Apache-2.0
metadata	{"author":"Juan Antonio Breña Moral","version":"0.13.0-SNAPSHOT"}

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.