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");
    }
}

Comprehensive Example: isEmpty Implementation

// Input: public static boolean isEmpty(String str)
//   { return str == null || str.trim().isEmpty(); }

class StringUtilsTest {

    @Test
    void shouldReturnTrueForNullString() {
        assertThat(StringUtils.isEmpty(null)).isTrue();
    }

    @Test
    void shouldReturnTrueForEmptyString() {
        assertThat(StringUtils.isEmpty("")).isTrue();
    }

    @Test
    void shouldReturnTrueForWhitespaceOnly() {
        assertThat(StringUtils.isEmpty("   ")).isTrue();
    }

    @Test
    void shouldReturnFalseForNonEmptyString() {
        assertThat(StringUtils.isEmpty("hello")).isFalse();
    }
}

Null-Safe Utility

class NullSafeUtilsTest {

    @Test
    void shouldReturnDefaultWhenNull() {
        assertThat(NullSafeUtils.getOrDefault(null, "default")).isEqualTo("default");
    }

    @Test
    void shouldReturnValueWhenNotNull() {
        assertThat(NullSafeUtils.getOrDefault("value", "default")).isEqualTo("value");
    }

    @Test
    void shouldReturnFalseWhenBlank() {
        assertThat(NullSafeUtils.isNotBlank(null)).isFalse();
        assertThat(NullSafeUtils.isNotBlank("   ")).isFalse();
    }
}

Math/Calculation Utility

class MathUtilsTest {

    @Test
    void shouldCalculatePercentage() {
        assertThat(MathUtils.percentage(25, 100)).isEqualTo(25.0);
    }

    @Test
    void shouldHandleZeroDivisor() {
        assertThat(MathUtils.percentage(50, 0)).isZero();
    }

    @Test
    void shouldRoundToDecimalPlaces() {
        assertThat(MathUtils.round(3.14159, 2)).isEqualTo(3.14);
    }

    @Test
    void shouldHandleFloatingPointWithTolerance() {
        assertThat(MathUtils.multiply(0.1, 0.2))
            .isCloseTo(0.02, within(0.0001));
    }
}

Collection Utility

class CollectionUtilsTest {

    @Test
    void shouldFilterList() {
        List<Integer> result = CollectionUtils.filter(List.of(1, 2, 3, 4), n -> n % 2 
how to use unit-test-utility-methods
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How to use unit-test-utility-methods on Cursor
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Prerequisites
Before installing skills in Cursor, ensure your development environment meets these requirements:
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›Node.js version 16.0+ with npm package manager (verify with node --version)
›Active project directory or workspace where you want to add unit-test-utility-methods
2
Execute installation command
Execute the skills CLI command in your project's root directory to begin installation:
$npx skills add https://github.com/giuseppe-trisciuoglio/developer-kit --skill unit-test-utility-methodscopy
The skills CLI fetches unit-test-utility-methods from GitHub repository giuseppe-trisciuoglio/developer-kit and configures it for Cursor.
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Confirm successful installation by checking the skill directory location:
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