131-java-testing-unit-testing

# Java Unit testing guidelines Review and improve Java unit tests using modern JUnit 5, AssertJ, and Mockito best practices. **What is covered in this Skill?** - JUnit 5 annotations: `@Test`, `@BeforeEach`, `@AfterEach`, `@DisplayName`, `@Nested`, `@ParameterizedTest` - AssertJ fluent assertions: `assertThat`, `assertThatThrownBy` - Given-When-Then test structure, descriptive test naming, single-responsibility tests - Test independence and isolated state - Parameterized tests: `@ValueSource`/`@CsvSource`/`@MethodSource` - Mockito dependency mocking: `@Mock`, `@InjectMocks`, `MockitoExtension` - Code coverage guidance (JaCoCo), package-private test visibility - Testing anti-patterns: reflection, shared state, hard-coded values, testing implementation details - Error handling: `assertThatThrownBy`, exception messages - JSpecify null-safety: `@NullMarked`, `@Nullable` - RIGHT-BICEP coverage principles, A-TRIP test quality, CORRECT boundary condition verification **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 unit test 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 — compilati...