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
• 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 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

Workflow

1. Compile project before OOD changes

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

1. Read OOD reference and assess code

Read references/121-java-object-oriented-design.md and identify applicable SOLID/OOP/code-smell improvements.

1. Apply focused refactorings

Implement the selected object-oriented design improvements while preserving behavior.

1. Verify with full build

Run ./mvnw clean verify or mvn clean verify after applying improvements.

Reference

For detailed guidance, examples, and constraints, see [references/121-java-object-oriented-design.md](references/121-java-object-oriented-design.md).