Welcome back to our course on Test-Driven Development (TDD) using Kotlin. In our previous lesson, we introduced the fundamentals of TDD along with the Red-Green-Refactor workflow. Now, we will elevate our TDD skills by focusing on generalizing solutions and enhancing the complexity of our testing scenarios.

As a brief reminder, TDD involves a repetitive cycle known as Red-Green-Refactor:

• Red: Write a failing test to clarify the new functionality you aim to implement.
• Green: Develop the smallest amount of code needed to make that test pass.
• Refactor: Clean up the code, enhancing its quality while maintaining its functionality and ensuring all tests remain passing.

In this lesson, we will expand upon the sum function, demonstrating how to generalize it while following these TDD principles.

Before we dive into coding, let's review our current setup. You are already familiar with the sum function, which is implemented in Calculator.kt and its corresponding test in CalculatorTest.kt:

import org.junit.jupiter.api.Test
import kotlin.test.assertEquals

class CalculatorTest {

    @Test
    fun sumShouldAddTwoNumbersCorrectly() {
        val calculator = Calculator()
        val result = calculator.sum(2, 3)
        assertEquals(5, result)
    }
}

class Calculator {

    fun sum(a: Int, b: Int): Int {
        return 5
    }
}

This existing setup serves as a foundation. Now, we'll focus on expanding your understanding by generalizing the approach using TDD principles. Understanding where you've come from will help ensure future changes enhance our function without straying too far from the core logic.

To embrace the Red phase, let's introduce a new test case that will fail.

Update CalculatorTest.kt to include more input scenarios:

class CalculatorTest {

    @Test
    fun sumShouldAddTwoNumbersCorrectly() {
        val calculator = Calculator()
        val result = calculator.sum(2, 3)
        assertEquals(5, result)
    }
    
    @Test
    fun sumShouldAddTwoMoreNumbersCorrectly() {
        val calculator = Calculator()
        val result = calculator.sum(5, 6)
        assertEquals(11, result)
    }
}

By including a new scenario with new inputs, this step is intentionally set to fail to define our target goal clearly. Running this test will confirm that additional functionality needs addressing.