Provides patterns for testing utility classes, static methods, and helper functions. Validates pure functions, null handling, edge cases, and boundary conditions. Generates AssertJ assertions and @ParameterizedTest for string utils, math utils, validators, and collection helpers. Use when testing ut

Unit Testing Utility Classes and Static Methods Overview This skill generates tests for utility classes with static helper methods and pure functions. It provides patterns for testing null handling, edge cases, boundary conditions, and common utilities like string manipulation, calculations, data validation, and collections. Pure functions require no mocking. When to Use Use this skill when: Writing tests for utility/helper classes with static methods Testing pure functions with no state or side effects Testing string manipulation, formatting, or transformation utilities Testing calculation, conversion, or math helper functions Testing data validation and formatter utilities Verifying null/empty input handling in utility code Testing collections or array helper methods Instructions 1. Create test class : Name it after the utility (e.g., StringUtilsTest ) 2. Test happy path : Valid inputs with expected outputs 3. Test edge cases : null, empty, whitespace, single elements 4. Test boundary conditions : max/min values, large numbers, precision 5. Use descriptive names : shouldCapitalizeFirstLetter instead of test1 6. Use AssertJ : For readable, chainable assertions 7. Use @ParameterizedTest : For multiple similar inputs (see references/parameterized tests.md ) 8. Avoid mocking : Pure utilities need no mocks Examples Basic Static Utility Test Comprehensive Example: isEmpty Implementation Null Safe Utility Math/Calculation Utility Collection Utility Data Validation Utility Utility with Clock Dependency (Rare) Best Practices Test pure functions exclusively no side effects or state dependency Cover happy path and edge cases null, empty, whitespace, extreme values Use descriptive test names shouldReturnNullWhenInputIsNull Use @ParameterizedTest for multiple similar inputs (see references/parameterized tests.md ) Test boundary conditions min/max values, overflow, precision Avoid mocking pure functions only mock external dependencies like Clock Keep tests independent no order dependency between tests Constraints and Warnings No mocking static methods : Use reflection utilities only when absolutely necessary Pure function requirement : Stateful utilities are harder to test; prefer immutability Floating point precision : Never use exact equality; use isCloseTo(delta) Null handling consistency : Decide whether utility returns null or throws; test accordingly Thread safety : Static utilities must be thread safe; verify concurrent behavior separately Immutable inputs : Document whether utilities modify input parameters Edge cases reference : See references/edge cases.md for boundary testing patterns

unit-test-utility-methods