JUnit

Slides

Unit tests with JUnit

https://junit.org/junit5/

A popular open-source testing framework for Java

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);
    }
}

• @Getter : using Lombok
• Create method getAttribute()

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/

• Mutations are automatically seeded into your code,
• then your tests are run.
• If your tests fail then the mutation is killed
• if your tests pass then the mutation lived
• The quality of your tests can be gauged from the percentage of mutations killed

Replace :

• + with -
• > with >=
• …