Test-Driven Development (TDD)

What is TDD?

Test-Driven Development is a software development approach where you write tests before writing the actual code. The TDD cycle follows these steps:

1. Red: Write a failing test
2. Green: Write the minimum code to make the test pass
3. Refactor: Improve the code while keeping tests green

TDD Example

Step 1: Write a Failing Test

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

public class StringCalculatorTest {
    @Test
    void add_EmptyString_ReturnsZero() {
        StringCalculator calculator = new StringCalculator();
        assertEquals(0, calculator.add(""));
    }
}

Step 2: Write Minimum Code to Pass

public class StringCalculator {
    public int add(String numbers) {
        return 0;
    }
}

Step 3: Add More Tests

@Test
void add_SingleNumber_ReturnsThatNumber() {
    StringCalculator calculator = new StringCalculator();
    assertEquals(1, calculator.add("1"));
}

@Test
void add_TwoNumbers_ReturnsSum() {
    StringCalculator calculator = new StringCalculator();
    assertEquals(3, calculator.add("1,2"));
}

Step 4: Implement the Feature

public class StringCalculator {
    public int add(String numbers) {
        if (numbers.isEmpty()) {
            return 0;
        }
        String[] nums = numbers.split(",");
        int sum = 0;
        for (String num : nums) {
            sum += Integer.parseInt(num.trim());
        }
        return sum;
    }
}

TDD Benefits

• Better Design: Tests drive the design of the code
• Documentation: Tests serve as living documentation
• Confidence: High test coverage provides confidence in changes
• Maintainability: Easier to maintain and refactor code
• Bug Prevention: Catches issues early in development

TDD Best Practices

Test First

• Write tests before implementing features
• Start with the simplest test case
• Focus on behavior, not implementation
• Use descriptive test names

Test Independence

• Each test should be independent
• Tests should not depend on each other
• Clean up after each test
• Use appropriate test fixtures

Refactoring

• Refactor only when tests are green
• Keep refactoring small and focused
• Run tests after each refactoring
• Maintain test readability

Common TDD Patterns

Test Doubles

import org.mockito.Mock;
import org.mockito.Mockito;
import static org.mockito.Mockito.*;

public class OrderServiceTest {
    @Mock
    private PaymentGateway paymentGateway;

    @Test
    void processOrder_ValidPayment_ReturnsSuccess() {
        when(paymentGateway.processPayment(anyDouble()))
            .thenReturn(true);

        OrderService service = new OrderService(paymentGateway);
        assertTrue(service.processOrder(new Order(100.0)));
    }
}

Behavior-Driven Development (BDD)

@DisplayName("User Registration")
class UserRegistrationTest {
    @Test
    @DisplayName("Should create new user with valid data")
    void shouldCreateNewUserWithValidData() {
        // Given
        UserRegistration registration = new UserRegistration();
        UserData userData = new UserData("john@example.com", "password123");

        // When
        User user = registration.register(userData);

        // Then
        assertNotNull(user);
        assertEquals("john@example.com", user.getEmail());
    }
}

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