121-java-object-oriented-design

# Java Object-Oriented Design Guidelines Review and improve Java code using focused object-oriented design references selected after assessing the request and affected code. **What is covered in this Skill?** - Fundamental design principles (SOLID, DRY, YAGNI) - Class and interface design: composition over inheritance, immutability, accessibility minimization, accessor methods - Core OOP concepts: encapsulation, inheritance, polymorphism - Object creation patterns: static factory methods, Builder, Singleton, dependency injection, avoiding unnecessary objects - OOD code smells: God Class, Feature Envy, Inappropriate Intimacy, Refused Bequest, Shotgun Surgery, Data Clumps - Method design: parameter validation, defensive copies, careful signatures, empty collections over nulls, Optional usage - Exception handling: checked vs. runtime exceptions, standard exceptions, failure-capture messages, no silent ignoring **Scope:** Classify the applicable OOD concerns first, then load only the focused references needed for those concerns. Load multiple references when a refactoring crosses concern boundaries. ## Constraints Before applying any OOD 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 ...