unit-test-utility-methods

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

import org.junit.jupiter.api.Test;
import static org.assertj.core.api.Assertions.*;

class StringUtilsTest {

    @Test
    void shouldCapitalizeFirstLetter() {
        assertThat(StringUtils.capitalize("hello")).isEqualTo("Hello");
    }

    @Test
    void shouldReturnNullForNullInput() {
        assertThat(StringUtils.capitalize(null)).isNull();
    }

    @Test
    void shouldHandleEmptyString() {
        assertThat(StringUtils.capitalize("")).isEmpty();
    }

    @Test
    void shouldHandleSingleCharacter() {
        assertThat(StringUtils.capitalize("a")).isEqualTo("A");
    }
}