Unit Testing in Java: JUnit 5 and Mockito for Beginners

public class MiniTest {
    static int divide(int a, int b) {
        if (b == 0) throw new IllegalArgumentException("divisor must not be zero");
        return a / b;
    }

    public static void main(String[] args) {
        int result = divide(10, 2);                                  // Arrange and Act
        System.out.println(result == 5);                             // true   (Assert)

        try {
            divide(1, 0);
            System.out.println("no exception was thrown");
        } catch (IllegalArgumentException e) {
            System.out.println(e.getMessage());                      // divisor must not be zero
        }
    }
}

// Illustrative: needs JUnit 5 (org.junit.jupiter:junit-jupiter) on the classpath.
import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.*;
import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.CsvSource;

class CalculatorTest {
    private Calculator calculator;

    @BeforeEach
    void setUp() {
        calculator = new Calculator();                // a fresh object for every test
    }

    @Test
    @DisplayName("adds two numbers")
    void add() {
        assertEquals(5, calculator.add(2, 3));
    }

    @Test
    void divideByZeroIsRejected() {
        IllegalArgumentException e = assertThrows(IllegalArgumentException.class, () -> calculator.divide(1, 0));
        assertEquals("divisor must not be zero", e.getMessage());
    }

    @ParameterizedTest
    @CsvSource({"10, 2, 5", "9, 3, 3", "-8, 2, -4"})
    void divide(int a, int b, int expected) {
        assertEquals(expected, calculator.divide(a, b));
    }
}

// Illustrative: needs JUnit 5 and Mockito (org.mockito:mockito-junit-jupiter) on the classpath.
import static org.junit.jupiter.api.Assertions.*;
import static org.mockito.Mockito.*;
import java.util.Optional;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;

@ExtendWith(MockitoExtension.class)
class EmployeeServiceTest {
    @Mock
    EmployeeRepository repository;                 // the database is replaced by a mock

    @InjectMocks
    EmployeeService service;                       // the mock is injected through the constructor

    @Test
    void findReturnsTheStoredEmployee() {
        when(repository.findById(1L)).thenReturn(Optional.of(new Employee("Asha", "IT")));

        Optional<Employee> found = service.find(1L);

        assertTrue(found.isPresent());
        assertEquals("Asha", found.get().name());
        verify(repository).findById(1L);           // the repository was asked exactly once
    }

    @Test
    void findReturnsEmptyForUnknownId() {
        when(repository.findById(99L)).thenReturn(Optional.empty());

        assertTrue(service.find(99L).isEmpty());
        verify(repository, never()).save(any());   // nothing was saved
    }
}