Unit Test Utility Methods Agent Skill

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 utils, test helpers, helper functions, static methods, or verifying utility code correctness.

3k tokens

context cost

the whole folder, loaded on every use

files

instructions only

copies elsewhere

how many repositories repackaged it

316

stars on the repo

on the repository, not the skill itself

Install

one command, takes just this skill from the repository

npx skills add https://github.com/giuseppe-trisciuoglio/developer-kit --skill unit-test-utility-methods

The instruction itself

14 sections, as written by the author

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

Create test class: Name it after the utility (e.g., StringUtilsTest)
Test happy path: Valid inputs with expected outputs
Test edge cases: null, empty, whitespace, single elements
Test boundary conditions: max/min values, large numbers, precision
Use descriptive names: shouldCapitalizeFirstLetter instead of test1
Use AssertJ: For readable, chainable assertions
Use @ParameterizedTest: For multiple similar inputs (see references/parameterized-tests.md)
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 == 0);
        assertThat(result).containsExactly(2, 4);
    }

    @Test
    void shouldHandleNullList() {
        assertThat(CollectionUtils.filter(null, n -> true)).isEmpty();
    }

    @Test
    void shouldJoinWithSeparator() {
        assertThat(CollectionUtils.join(List.of("a", "b", "c"), "-")).isEqualTo("a-b-c");
    }

    @Test
    void shouldDeduplicateList() {
        assertThat(CollectionUtils.deduplicate(List.of("a", "b", "a")))
            .containsExactlyInAnyOrder("a", "b");
    }
}

Data Validation Utility

class ValidatorUtilsTest {

    @Test
    void shouldValidateEmailFormat() {
        assertThat(ValidatorUtils.isValidEmail("[email protected]")).isTrue();
        assertThat(ValidatorUtils.isValidEmail("invalid")).isFalse();
    }

    @Test
    void shouldValidateUrlFormat() {
        assertThat(ValidatorUtils.isValidUrl("https://example.com")).isTrue();
        assertThat(ValidatorUtils.isValidUrl("not a url")).isFalse();
    }

    @Test
    void shouldValidateCreditCard() {
        assertThat(ValidatorUtils.isValidCreditCard("4532015112830366")).isTrue();
        assertThat(ValidatorUtils.isValidCreditCard("1234567890123456")).isFalse();
    }
}

Utility with Clock Dependency (Rare)

@ExtendWith(MockitoExtension.class)
class DateUtilsTest {

    @Mock
    private Clock clock;

    @Test
    void shouldGetDateFromClock() {
        when(clock.instant()).thenReturn(Instant.parse("2024-01-15T10:00:00Z"));
        assertThat(DateUtils.today(clock)).isEqualTo(LocalDate.of(2024, 1, 15));
    }
}

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

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Take giuseppe-trisciuoglio/unit-test-utility-methods from the repository into ~/.claude/skills for personal use, or into .claude/skills inside a project.

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