Unit Testing Utility Classes and Static Methods

Overview

This skill provides comprehensive patterns for testing utility classes and static methods using JUnit 5. It covers testing pure functions without side effects, edge case handling, boundary conditions, and common utility patterns like string manipulation, calculations, collections, and data validation.

When to Use

Use this skill when:

• Testing utility classes with static helper methods
• Testing pure functions with no state or side effects
• Testing string manipulation and formatting utilities
• Testing calculation and conversion utilities
• Testing collections and array utilities
• Want simple, fast tests without mocking complexity
• Testing data transformation and validation helpers

Instructions

Follow these steps to test utility classes and static methods:

1. Create Test Class

Create a JUnit 5 test class named after the utility class being tested (e.g., StringUtilsTest).

2. Test Happy Path

Write tests for typical use cases with valid inputs to verify correct behavior.

3. Test Edge Cases

Test null inputs, empty strings, zero values, and boundary conditions.

4. Test Error Cases

Verify proper exception throwing for invalid inputs when applicable.

5. Use Descriptive Test Names

Name tests to clearly indicate what scenario is being tested (e.g., shouldCapitalizeFirstLetter).

6. Use AssertJ Assertions

Leverage AssertJ’s readable assertion methods for clear test code.

7. Consider Parameterized Tests

Use @ParameterizedTest for testing multiple similar scenarios with different inputs.

Examples

Basic Pattern: Static Utility Testing

Simple String Utility

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

class StringUtilsTest {

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

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

  @Test
  void shouldHandleNullInput() {
    String result = StringUtils.capitalize(null);
    assertThat(result).isNull();
  }

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

  @Test
  void shouldNotChangePascalCase() {
    String result = StringUtils.capitalize("Hello");
    assertThat(result).isEqualTo("Hello");
  }
}

Testing Null Handling

Null-Safe Utility Methods

class NullSafeUtilsTest {

  @Test
  void shouldReturnDefaultValueWhenNull() {
    Object result = NullSafeUtils.getOrDefault(null, "default");
    assertThat(result).isEqualTo("default");
  }

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

  @Test
  void shouldReturnFalseWhenStringIsNull() {
    boolean result = NullSafeUtils.isNotBlank(null);
    assertThat(result).isFalse();
  }

  @Test
  void shouldReturnTrueWhenStringHasContent() {
    boolean result = NullSafeUtils.isNotBlank("   text   ");
    assertThat(result).isTrue();
  }
}

Testing Calculations and Conversions

Math Utilities

class MathUtilsTest {

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

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

  @Test
  void shouldRoundToTwoDecimalPlaces() {
    double result = MathUtils.round(3.14159, 2);
    assertThat(result).isEqualTo(3.14);
  }

  @Test
  void shouldHandleNegativeNumbers() {
    int result = MathUtils.absoluteValue(-42);
    assertThat(result).isEqualTo(42);
  }
}

Testing Collection Utilities

List/Set/Map Operations

class CollectionUtilsTest {

  @Test
  void shouldFilterList() {
    List<Integer> numbers = List.of(1, 2, 3, 4, 5);
    List<Integer> evenNumbers = CollectionUtils.filter(numbers, n -> n % 2 == 0);
    assertThat(evenNumbers).containsExactly(2, 4);
  }

  @Test
  void shouldReturnEmptyListWhenNoMatches() {
    List<Integer> numbers = List.of(1, 3, 5);
    List<Integer> evenNumbers = CollectionUtils.filter(numbers, n -> n % 2 == 0);
    assertThat(evenNumbers).isEmpty();
  }

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

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

  @Test
  void shouldHandleEmptyList() {
    String result = CollectionUtils.join(List.of(), "-");
    assertThat(result).isEmpty();
  }

  @Test
  void shouldDeduplicateList() {
    List<String> input = List.of("apple", "banana", "apple", "cherry", "banana");
    Set<String> unique = CollectionUtils.deduplicate(input);
    assertThat(unique).containsExactlyInAnyOrder("apple", "banana", "cherry");
  }
}

Testing String Transformations

Format and Parse Utilities

class FormatUtilsTest {

  @Test
  void shouldFormatCurrencyWithSymbol() {
    String result = FormatUtils.formatCurrency(1234.56);
    assertThat(result).isEqualTo("$1,234.56");
  }

  @Test
  void shouldHandleNegativeCurrency() {
    String result = FormatUtils.formatCurrency(-100.00);
    assertThat(result).isEqualTo("-$100.00");
  }

  @Test
  void shouldParsePhoneNumber() {
    String result = FormatUtils.parsePhoneNumber("5551234567");
    assertThat(result).isEqualTo("(555) 123-4567");
  }

  @Test
  void shouldFormatDate() {
    LocalDate date = LocalDate.of(2024, 1, 15);
    String result = FormatUtils.formatDate(date, "yyyy-MM-dd");
    assertThat(result).isEqualTo("2024-01-15");
  }

  @Test
  void shouldSluggifyString() {
    String result = FormatUtils.sluggify("Hello World! 123");
    assertThat(result).isEqualTo("hello-world-123");
  }
}

Testing Data Validation

Validator Utilities

class ValidatorUtilsTest {

  @Test
  void shouldValidateEmailFormat() {
    boolean valid = ValidatorUtils.isValidEmail("user@example.com");
    assertThat(valid).isTrue();

    boolean invalid = ValidatorUtils.isValidEmail("invalid-email");
    assertThat(invalid).isFalse();
  }

  @Test
  void shouldValidatePhoneNumber() {
    boolean valid = ValidatorUtils.isValidPhone("555-123-4567");
    assertThat(valid).isTrue();

    boolean invalid = ValidatorUtils.isValidPhone("12345");
    assertThat(invalid).isFalse();
  }

  @Test
  void shouldValidateUrlFormat() {
    boolean valid = ValidatorUtils.isValidUrl("https://example.com");
    assertThat(valid).isTrue();

    boolean invalid = ValidatorUtils.isValidUrl("not a url");
    assertThat(invalid).isFalse();
  }

  @Test
  void shouldValidateCreditCardNumber() {
    boolean valid = ValidatorUtils.isValidCreditCard("4532015112830366");
    assertThat(valid).isTrue();

    boolean invalid = ValidatorUtils.isValidCreditCard("1234567890123456");
    assertThat(invalid).isFalse();
  }
}

Testing Parameterized Scenarios

Multiple Test Cases with @ParameterizedTest

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.ValueSource;
import org.junit.jupiter.params.provider.CsvSource;

class StringUtilsParametrizedTest {

  @ParameterizedTest
  @ValueSource(strings = {"", " ", "null", "undefined"})
  void shouldConsiderFalsyValuesAsEmpty(String input) {
    boolean result = StringUtils.isEmpty(input);
    assertThat(result).isTrue();
  }

  @ParameterizedTest
  @CsvSource({
    "hello,HELLO",
    "world,WORLD",
    "javaScript,JAVASCRIPT",
    "123ABC,123ABC"
  })
  void shouldConvertToUpperCase(String input, String expected) {
    String result = StringUtils.toUpperCase(input);
    assertThat(result).isEqualTo(expected);
  }
}

Testing with Mockito for External Dependencies

Utility with Dependency (Rare Case)

import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;
import static org.mockito.Mockito.when;

@ExtendWith(MockitoExtension.class)
class DateUtilsTest {

  @Mock
  private Clock clock;

  @Test
  void shouldGetCurrentDateFromClock() {
    Instant fixedTime = Instant.parse("2024-01-15T10:30:00Z");
    when(clock.instant()).thenReturn(fixedTime);

    LocalDate result = DateUtils.today(clock);
    
    assertThat(result).isEqualTo(LocalDate.of(2024, 1, 15));
  }
}

Edge Cases and Boundary Testing

class MathUtilsEdgeCaseTest {

  @Test
  void shouldHandleMaxIntegerValue() {
    int result = MathUtils.increment(Integer.MAX_VALUE);
    assertThat(result).isEqualTo(Integer.MAX_VALUE);
  }

  @Test
  void shouldHandleMinIntegerValue() {
    int result = MathUtils.decrement(Integer.MIN_VALUE);
    assertThat(result).isEqualTo(Integer.MIN_VALUE);
  }

  @Test
  void shouldHandleVeryLargeNumbers() {
    BigDecimal result = MathUtils.add(
      new BigDecimal("999999999999.99"),
      new BigDecimal("0.01")
    );
    assertThat(result).isEqualTo(new BigDecimal("1000000000000.00"));
  }

  @Test
  void shouldHandleFloatingPointPrecision() {
    double result = MathUtils.multiply(0.1, 0.2);
    assertThat(result).isCloseTo(0.02, within(0.0001));
  }
}

Best Practices

• Test pure functions exclusively – no side effects or state
• Cover happy path and edge cases – null, empty, extreme values
• Use descriptive test names – clearly state what’s being tested
• Keep tests simple and short – utility tests should be quick to understand
• Use @ParameterizedTest for testing multiple similar scenarios
• Avoid mocking when not needed – only mock external dependencies
• Test boundary conditions – min/max values, empty collections, null inputs

Common Pitfalls

• Testing framework behavior instead of utility logic
• Over-mocking when pure functions need no mocks
• Not testing null/empty edge cases
• Not testing negative numbers and extreme values
• Test methods too large – split complex scenarios

Constraints and Warnings

• Static methods cannot be mocked: Use reflection-based utilities like PowerMock only when absolutely necessary
• Pure function requirement: Utility methods should be pure functions; testing stateful utilities is difficult
• Floating point precision: Never use exact equality for floating point comparisons; use tolerance
• Null handling consistency: Decide whether utilities return null or throw exceptions for invalid input
• Immutable inputs: Document clearly whether utility methods modify input parameters
• Thread safety: Static utilities must be thread-safe; verify under concurrent access
• External dependencies: Minimize external dependencies in utility classes for easier testing

Examples

Input: Utility Method Without Tests

public class StringUtils {
    public static boolean isEmpty(String str) {
        return str == null || str.trim().isEmpty();
    }
}

Output: Complete Test Coverage

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();
    }
}

Input: Calculation Without Edge Case Testing

public static double percentage(int value, int total) {
    return (value / total) * 100.0;
}

Output: Tests Covering Edge Cases

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

    @Test
    void shouldHandleZeroDivisor() {
        assertThat(MathUtils.percentage(50, 0)).isEqualTo(0.0);
    }

    @Test
    void shouldHandleZeroValue() {
        assertThat(MathUtils.percentage(0, 100)).isEqualTo(0.0);
    }
}

Constraints and Warnings

Floating point precision issues: Use isCloseTo() with delta instead of exact equality.

Null handling inconsistency: Decide whether utility returns null or throws exception, then test consistently.

Complex utility logic belongs elsewhere: Consider refactoring into testable units.

References

• JUnit 5 Parameterized Tests
• AssertJ Assertions
• Testing Edge Cases and Boundaries