Unit Testing Boundary Conditions and Edge Cases Skill

Unit Testing Boundary Conditions and Edge Cases

Overview

Systematic patterns for testing boundary conditions, corner cases, and limit values in Java using JUnit 5. Covers numeric boundaries, string edge cases, collection states, floating-point precision, date/time limits, and off-by-one scenarios.

When to Use

Numeric min/max limits, null/empty/whitespace inputs
Overflow/underflow validation, collection boundaries
Off-by-one errors, floating-point precision

Instructions

Identify boundaries: List numeric limits (MIN_VALUE, MAX_VALUE, zero), string states (null, empty, whitespace), collection sizes (0, 1, many)
Apply parameterized tests: Use @ParameterizedTest with @ValueSource or @CsvSource for multiple boundary values
Test both sides of boundaries: Cover values just below, at, and just above each boundary
Run tests after adding each boundary category to catch issues early
Verify floating-point precision: Use isCloseTo(expected, within(tolerance)) with AssertJ
Test collection states: Explicitly test empty (0), single (1), and many (>1) element scenarios
Handle overflow/underflow: Use Math.addExact() and Math.subtractExact() to detect arithmetic overflow
Test date/time edges: Verify leap years, month boundaries, timezone transitions
Iterate based on failures: When a boundary test fails, analyze the error to discover additional untested boundaries; add test cases for the newly discovered edge conditions

Examples

Requires: junit-jupiter, junit-jupiter-params, assertj-core.

Integer Boundary Testing

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.ValueSource;
import static org.assertj.core.api.Assertions.*;

class IntegerBoundaryTest {

  @ParameterizedTest
  @ValueSource(ints = {Integer.MIN_VALUE, Integer.MIN_VALUE + 1, 0, Integer.MAX_VALUE - 1, Integer.MAX_VALUE})
  void shouldHandleIntegerBoundaries(int value) {
    assertThat(value).isNotNull();
  }

  @Test
  void shouldDetectIntegerOverflow() {
    assertThatThrownBy(() -> Math.addExact(Integer.MAX_VALUE, 1))
      .isInstanceOf(ArithmeticException.class);
  }

  @Test
  void shouldDetectIntegerUnderflow() {
    assertThatThrownBy(() -> Math.subtractExact(Integer.MIN_VALUE, 1))
      .isInstanceOf(ArithmeticException.class);
  }

  @Test
  void shouldHandleZeroEdge() {
    int result = MathUtils.divide(0, 5);
    assertThat(result).isZero();

    assertThatThrownBy(() -> MathUtils.divide(5, 0))
      .isInstanceOf(ArithmeticException.class);
  }
}

String Boundary Testing

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

class StringBoundaryTest {

  @ParameterizedTest
  @ValueSource(strings = {"", " ", "  ", "\t", "\n"})
  void shouldRejectEmptyAndWhitespace(String input) {
    boolean result = StringUtils.isNotBlank(input);
    assertThat(result).isFalse();
  }

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

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

  @Test
  void shouldHandleVeryLongString() {
    String longString = "x".repeat(1000000);

    assertThat(longString.length()).isEqualTo(1000000);
    assertThat(StringUtils.isNotBlank(longString)).isTrue();
  }
}

Collection Boundary Testing

class CollectionBoundaryTest {

  @Test
  void shouldHandleEmptyList() {
    List<String> empty = List.of();

    assertThat(empty).isEmpty();
    assertThat(CollectionUtils.first(empty)).isNull();
    assertThat(CollectionUtils.count(empty)).isZero();
  }

  @Test
  void shouldHandleSingleElementList() {
    List<String> single = List.of("only");

    assertThat(single).hasSize(1);
    assertThat(CollectionUtils.first(single)).isEqualTo("only");
    assertThat(CollectionUtils.last(single)).isEqualTo("only");
  }

  @Test
  void shouldHandleLargeList() {
    List<Integer> large = new ArrayList<>();
    for (int i = 0; i < 100000; i++) {
      large.add(i);
    }

    assertThat(large).hasSize(100000);
    assertThat(CollectionUtils.first(large)).isZero();
    assertThat(CollectionUtils.last(large)).isEqualTo(99999);
  }

  @Test
  void shouldHandleNullInCollection() {
    List<String> withNull = new ArrayList<>(List.of("a", null, "c"));

    assertThat(withNull).contains(null);
    assertThat(CollectionUtils.filterNonNull(withNull)).hasSize(2);
  }
}

Floating-Point Boundary Testing

class FloatingPointBoundaryTest {

  @Test
  void shouldHandleFloatingPointPrecision() {
    double result = 0.1 + 0.2;
    assertThat(result).isCloseTo(0.3, within(0.0001));
  }

  @Test
  void shouldHandleSpecialFloatingPointValues() {
    assertThat(Double.POSITIVE_INFINITY).isGreaterThan(Double.MAX_VALUE);
    assertThat(Double.NEGATIVE_INFINITY).isLessThan(Double.MIN_VALUE);
    assertThat(Double.NaN).isNotEqualTo(Double.NaN);
  }

  @Test
  void shouldHandleZeroInDivision() {
    assertThat(1.0 / 0.0).isEqualTo(Double.POSITIVE_INFINITY);
    assertThat(-1.0 / 0.0).isEqualTo(Double.NEGATIVE_INFINITY);
    assertThat(0.0 / 0.0).isNaN();
  }
}

Date/Time Boundary Testing

class DateTimeBoundaryTest {

  @Test
  void shouldHandleMinAndMaxDates() {
    LocalDate min = LocalDate.MIN;
    LocalDate max = LocalDate.MAX;

    assertThat(min).isBefore(max);
    assertThat(DateUtils.isValid(min)).isTrue();
    assertThat(DateUtils.isValid(max)).isTrue();
  }

  @Test
  void shouldHandleLeapYearBoundary() {
    LocalDate leapYearEnd = LocalDate.of(2024, 2, 29);
    assertThat(leapYearEnd).isNotNull();
  }

  @Test
  void shouldRejectInvalidDateInNonLeapYear() {
    assertThatThrownBy(() -> LocalDate.of(2023, 2, 29))
      .isInstanceOf(DateTimeException.class);
  }
}

Array Index Boundary Testing

class ArrayBoundaryTest {

  @Test
  void shouldHandleFirstElementAccess() {
    int[] array = {1, 2, 3, 4, 5};
    assertThat(array[0]).isEqualTo(1);
  }

  @Test
  void shouldHandleLastElementAccess() {
    int[] array = {1, 2, 3, 4, 5};
    assertThat(array[array.length - 1]).isEqualTo(5);
  }

  @Test
  void shouldThrowOnNegativeIndex() {
    int[] array = {1, 2, 3};
    assertThatThrownBy(() -> array[-1])
      .isInstanceOf(ArrayIndexOutOfBoundsException.class);
  }

  @Test
  void shouldThrowOnOutOfBoundsIndex() {
    int[] array = {1, 2, 3};
    assertThatThrownBy(() -> array[10])
      .isInstanceOf(ArrayIndexOutOfBoundsException.class);
  }

  @Test
  void shouldHandleEmptyArray() {
    int[] empty = {};
    assertThat(empty.length).isZero();
    assertThatThrownBy(() -> empty[0])
      .isInstanceOf(ArrayIndexOutOfBoundsException.class);
  }
}

Best Practices

Test at boundaries explicitly: don't rely on random testing
Test null and empty separately from valid inputs
Use parameterized tests for multiple boundary cases
Test both sides of boundaries (just below, at, just above)
Verify error messages for invalid boundary inputs
Document why specific boundaries matter for your domain
Test overflow/underflow for all numeric operations

Constraints and Warnings

Integer overflow: Use Math.addExact() to detect silent overflow
Floating-point precision: Never use exact equality; always use tolerance-based assertions
NaN behavior: NaN != NaN; use Float.isNaN() or Double.isNaN()
Collection size limits: Be mindful of memory with large test collections
String encoding: Test with Unicode characters for internationalization
Date/time boundaries: Account for timezone transitions and daylight saving
Array indexing: Always test index 0, length-1, and out-of-bounds

References

Integer.MIN_VALUE/MAX_VALUE
Double.MIN_VALUE/MAX_VALUE
AssertJ Floating Point
Boundary Value Analysis
references/concurrent-testing.md - Thread safety patterns
references/parameterized-patterns.md - Off-by-one and parameterized examples

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