Test-Driven Development

The TDD Cycle

• Red: Write a failing test

• Green: Write minimal code to pass

• Refactor: Improve code while keeping tests green

Workflows

• Write Test: Write a test that describes desired behavior

• Run Test: Verify it fails (Red)

• Implement: Write minimal code to pass

• Run Test: Verify it passes (Green)

• Refactor: Clean up while tests stay green

• Repeat: Next test case

TDD Example (Java + JUnit 5)

Step 1: Red - Write Failing Test

class CalculatorTest { @Test void add_TwoNumbers_ReturnsSum() { // Arrange Calculator calc = new Calculator();

    // Act
    int result = calc.add(2, 3);

    // Assert
    assertThat(result).isEqualTo(5);
}

}

// Run: mvn test // FAIL - Cannot resolve symbol 'Calculator'

Step 2: Green - Minimal Implementation

public class Calculator { public int add(int a, int b) { return a + b; } }

// Run: mvn test // PASS ✓

Step 3: Refactor (if needed)

Code is already clean, move to next test.

Step 4: Next Test

@Test void subtract_TwoNumbers_ReturnsDifference() { // Arrange Calculator calc = new Calculator();

// Act
int result = calc.subtract(5, 3);

// Assert
assertThat(result).isEqualTo(2);

}

// Run: mvn test // FAIL - Cannot resolve method 'subtract' in 'Calculator'

Step 5: Implement Subtract

public class Calculator { public int add(int a, int b) { return a + b; }

public int subtract(int a, int b) {
    return a - b;
}

}

// Run: mvn test // PASS ✓ (2 tests passing)

TDD Benefits

• Design Feedback: Tests reveal design issues early

• Documentation: Tests document expected behavior

• Confidence: Refactor fearlessly with test safety net

• Focus: One behavior at a time

TDD Tips

• Start Simple: Begin with the simplest test case

• One Assert: Each test should verify one behavior

• Descriptive Names: Test names are documentation

• No Logic in Tests: Tests should be obvious

• Fast Feedback: Tests should run in milliseconds

When to Use TDD

• New features with clear requirements

• Bug fixes (write failing test first)

• Complex business logic

• API contract development

When TDD is Less Useful

• Exploratory/prototype code

• UI layout changes

• Simple CRUD operations