--- name: jabrena/121-java-object-oriented-design source: https://app.decimal.ai/s/jabrena-121-java-object-oriented-design@1/SKILL.md source_sha256: ed7f2532605c --- # 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 a blocking condition - **VERIFY**: Run `./mvnw clean verify` or `mvn clean verify` after applying improvements - **PROGRESSIVE DISCLOSURE**: Classify the applicable OOD concerns before reading references, then read only the references mapped to those concerns - **MULTI-CONCERN CHANGES**: Read multiple focused references only when the request or diagnosed code problems cross concern boundaries - **PRESERVE BEHAVIOR**: Apply focused, incremental refactorings without changing observable business behavior - **INCREMENTAL SAFETY**: Compile after each significant refactoring and keep changes easy to revert if validation reveals a regression - **DEPENDENCY SAFETY**: Confirm refactoring does not break imports, dependencies, class relationships, or established contracts - **ADJACENT SKILLS**: Use skill 122 for dedicated type-design work and skill 126 for dedicated exception-handling work; use the overlapping skill 121 references only when those topics are part of a broader OOD review - **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. 2. **Assess code and classify OOD concerns** Analyze the request and affected Java code before reading implementation guidance. Classify each relevant problem into one or more concerns: - Fundamental principles: SOLID, DRY, or YAGNI violations. - Core OOP concepts: encapsulation, inheritance, or polymorphism misuse. - Code smells: God Class, Feature Envy, Inappropriate Intimacy, Refused Bequest, Shotgun Surgery, or Data Clumps. - Object creation: factories, builders, singletons, dependency injection, or unnecessary object creation. - Classes and interfaces: accessibility, public fields, mutability, composition, or inheritance design. - Enums and annotations: int constants, ordinals, bit fields, ordinal indexing, or missing `@Override`. - Methods: parameter validation, defensive copies, signatures, null collections, or `Optional`. - Exceptions: exception contracts that are part of the broader OOD review. If the request is dedicated type-design work, use skill 122. If it is dedicated exception-handling work, use skill 126. 3. **Read only applicable OOD references** Map the classified concerns to focused references: - For SOLID, DRY, or YAGNI, read `references/121-java-object-oriented-design-principles.md`. - For encapsulation, inheritance, or polymorphism, read `references/121-java-object-oriented-design-oop-concepts.md`. - For object-oriented code smells, read `references/121-java-object-oriented-design-code-smells.md`. - For factories, builders, singletons, dependency injection, or unnecessary objects, read `references/121-java-object-oriented-design-object-creation.md`. - For class and interface boundaries, accessibility, mutability, composition, or inheritance design, read `references/121-java-object-oriented-design-classes-interfaces.md`. - For enums, ordinals, bit fields, enum lookup, or `@Override`, read `references/121-java-object-oriented-design-enums-annotations.md`. - For method contracts, validation, defensive copies, signatures, collection returns, or `Optional`, read `references/121-java-object-oriented-design-methods.md`. - For exception contracts within a broader OOD review, read `references/121-java-object-oriented-design-exceptions.md`. Read every reference required by a cross-concern refactoring, but do not read unrelated references. 4. **Apply focused refactorings** Prioritize findings by impact: CRITICAL, MAINTAINABILITY, FLEXIBILITY, or CODE_QUALITY. Apply the smallest suitable refactoring for each diagnosed concern while preserving observable behavior: - Extract classes or methods to restore focused responsibilities. - Introduce or refine abstractions for extension, interface segregation, and dependency inversion. - Correct inheritance hierarchies to preserve substitutability, or replace inheritance with composition. - Move behavior to eliminate Feature Envy and improve encapsulation. - Introduce cohesive value or parameter objects for Data Clumps. - Hide exposed state and reduce unnecessary coupling. Apply changes incrementally and compile after each significant refactoring. 5. **Verify with full build** Run `./mvnw clean verify` or `mvn clean verify` after applying improvements. 6. **Report applied OOD improvements** Report findings by impact and concern, focused references used, refactorings applied, maintainability/flexibility/testability benefits, behavior-preservation evidence, and the final compilation and test result. ## Reference For detailed guidance, examples, and constraints, see: - [references/121-java-object-oriented-design-principles.md](references/121-java-object-oriented-design-principles.md) - [references/121-java-object-oriented-design-oop-concepts.md](references/121-java-object-oriented-design-oop-concepts.md) - [references/121-java-object-oriented-design-code-smells.md](references/121-java-object-oriented-design-code-smells.md) - [references/121-java-object-oriented-design-object-creation.md](references/121-java-object-oriented-design-object-creation.md) - [references/121-java-object-oriented-design-classes-interfaces.md](references/121-java-object-oriented-design-classes-interfaces.md) - [references/121-java-object-oriented-design-enums-annotations.md](references/121-java-object-oriented-design-enums-annotations.md) - [references/121-java-object-oriented-design-methods.md](references/121-java-object-oriented-design-methods.md) - [references/121-java-object-oriented-design-exceptions.md](references/121-java-object-oriented-design-exceptions.md)