unit-test-utility-methods

SKILL.md

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("user@example.com")).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