JUnit

Ludovic Deneuville

Unit tests with JUnit

https://junit.org/junit5/

Reminder: Unit Testing

Testing individual units of code
Verifying that each unit behaves as expected
Essential for detecting bugs
Avoid regressions (Prevents new changes from breaking existing code)

Test classes

One-to-One Relationship
Each class you write should have a corresponding test class
Book.java ➡️ BookTest.java
- placed in the same package as the class being tested
- but in the src/test/java directory

Example

Class Fraction

Fraction.java

public class Fraction {
    @Getter private int numerator;
    @Getter private int denominator;

    public Fraction(int numerator, int denominator) {
        if (denominator == 0)
            throw new IllegalArgumentException("Denominator cannot be zero.");
        this.numerator = numerator;
        this.denominator = denominator;
    }

    public Fraction invert() {
        if (numerator == 0)
            throw new ArithmeticException("Cannot invert a fraction with a zero numerator.");
        return new Fraction(denominator, numerator);
    }
}

Test class

FractionTest.java

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

public class FractionTest {

    @Test
    void testInvertValid() {
        // GIVEN
        Fraction fraction = new Fraction(2, 3);

        // WHEN
        Fraction inverted = fraction.invert();

        // THEN
        assertEquals(3, inverted.getNumerator());
        assertEquals(2, inverted.getDenominator());
    }

    @Test
    void testInvertZeroNumerator() {
        Fraction fraction = new Fraction(0, 5);
        assertThrows(ArithmeticException.class, fraction::invert);
    }
}

Assertions

assertEquals(expected, actual)
assertTrue(condition) / assertFalse(condition)
assertNull(object) / assertNotNull(object)
assertThrows(expectedType, executable)

Annotations

@Test: Marks a method as a test case
@BeforeEach: Executes before each test method
@AfterEach: Executes after each test method
@BeforeAll: Executes once before all test methods
@AfterAll: Executes once after all test methods

Mocking

Replacing real dependencies with controlled substitutes during unit testing
Allows you to isolate the unit under test from external factors
See Mockito frameworks

Mutation Testing

Evaluate the effectiveness of your tests
Introduces small changes (mutations) to your code
Checks if your tests can detect these changes

https://pitest.org/