Unit Testing vs Integration Testing: Key Differences

package io.thecodeforge.payment;

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

// UNIT TEST — no database, no HTTP, no external dependencies
class PaymentAmountCalculatorTest {

    @Test
    void calculate_appliesDiscountForGoldTier() {
        // Arrange — mock only the external dependency (discount lookup)
        DiscountService discountService = mock(DiscountService.class);
        when(discountService.getDiscountRate("GOLD")).thenReturn(0.15);

        PaymentAmountCalculator calculator =
            new PaymentAmountCalculator(discountService);

        // Act
        long result = calculator.calculate(100_00, "GOLD"); // £100.00

        // Assert
        assertEquals(85_00, result); // 15% off = £85.00
        verify(discountService, times(1)).getDiscountRate("GOLD");
    }

    @Test
    void calculate_noDiscountForUnknownTier() {
        DiscountService discountService = mock(DiscountService.class);
        when(discountService.getDiscountRate("UNKNOWN")).thenReturn(0.0);

        PaymentAmountCalculator calculator =
            new PaymentAmountCalculator(discountService);

        assertEquals(100_00, calculator.calculate(100_00, "UNKNOWN"));
    }
}
// Runs in: ~50ms
// Tests: pure business logic, isolated from real DiscountService implementation

package io.thecodeforge.payment;

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.orm.jpa.DataJpaTest;
import static org.junit.jupiter.api.Assertions.*;

// INTEGRATION TEST — uses real database (H2 in-memory for @DataJpaTest)
// Tests that the JPA mapping, queries, and constraints work correctly
@DataJpaTest
class PaymentRepositoryIntegrationTest {

    @Autowired
    private PaymentRepository paymentRepository;

    @Test
    void save_andFind_roundTrip() {
        // Arrange
        Payment payment = Payment.builder()
            .customerId("customer-42")
            .amountInPence(100_00)
            .currency("GBP")
            .status(PaymentStatus.PENDING)
            .build();

        // Act — actually hits the database
        Payment saved = paymentRepository.save(payment);
        Payment found = paymentRepository.findById(saved.getId()).orElseThrow();

        // Assert — verifies JPA mapping, column types, NOT NULL constraints
        assertEquals("customer-42", found.getCustomerId());
        assertEquals(100_00, found.getAmountInPence());
        assertEquals(PaymentStatus.PENDING, found.getStatus());
        assertNotNull(found.getCreatedAt()); // Verify @CreatedDate audit field
    }

    @Test
    void findByCustomerId_returnsAllPaymentsForCustomer() {
        paymentRepository.save(buildPayment("customer-42", 100_00));
        paymentRepository.save(buildPayment("customer-42", 200_00));
        paymentRepository.save(buildPayment("customer-99", 50_00));

        // Tests the actual JPQL query — unit test with a mock wouldn't catch typos
        var results = paymentRepository.findByCustomerId("customer-42");
        assertEquals(2, results.size());
    }
}
// Runs in: ~2-8 seconds (database startup)
// Tests: JPA mapping, query correctness, constraint enforcement