121-java-object-oriented-design

# Java Object-Oriented Design Guidelines Review and improve Java code using comprehensive object-oriented design guidelines and refactoring practices. **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:** The reference is organized by examples (good/bad code patterns) for each core area. Apply recommendations based on applicable examples. ## 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 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 object-oriented design - Refactor Java code for object-oriented design - Improve Java code for object-oriented design - Fix OOP concept misuse in Java code - Identify and resolve code smells in Java code - Improve object creation patterns in Java code - Improve method design in Java code - Improve exception handling in Java code ## Reference For detailed guidance, examples, and constraints, see [references/121-java-object-oriented-design.md](references/121-java-object-oriented-design.md).