Intro to TDD with Kotlin and JUnit

Introduction to Practice

Welcome to your first lesson in this course dedicated to practicing Test-Driven Development (TDD) using Kotlin and JUnit . Test-Driven Development is an effective approach that emphasizes writing tests before coding. This methodology helps you design your application with testing as a core element, ensuring each component behaves as expected. In this lesson, you'll learn the fundamentals of TDD along with the Red-Green-Refactor cycle, understanding their roles in creating robust and maintainable code. This course involves hands-on practice, where you'll receive requirements through tests incrementally. Your task is to implement code that makes each test pass, mirroring a real-world TDD environment. Consider this course guide as your pair programmer, providing you with test-driven prompts to enhance your skills as you advance.

Understanding the Red-Green-Refactor Cycle

As a reminder, the Red-Green-Refactor cycle is central to TDD, guiding your development process: Red : Start by writing a failing test to define the next step. Green : Implement just enough code to pass the test, focusing on functionality. Refactor : Optimize the code for clarity and efficiency without changing its behavior.

Requirements for `calculateDiscount` Function

Below, you’ll find the detailed requirements and corresponding test cases for the calculateDiscount function. Each requirement focuses on a specific behavior the function must fulfill, guiding you through the TDD process step by step.

1. Correct Discount Application

Description : The function must correctly apply a percentage-based discount to an original price. Test Case : Kotlin import kotlin.test.Test import kotlin.test.assertEquals class DiscountCalculatorTest {@Test fun testCalculateDiscountAppliesCorrectDiscount() {// Arrange val discountCalculator = DiscountCalculator() val originalPrice = 100.0 val discountPercentage = 20.0 val expectedDiscountedPrice = 80.0 // Act val result = discountCalculator.calculateDiscount(originalPrice, discountPercentage) // Assert assertEquals(expectedDiscountedPrice, result, 0.01)}} The test ensures that the function applies the discount correctly and tolerates minor floating-point deviations.

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Kotlin

import kotlin.test.Test
import kotlin.test.assertEquals

class DiscountCalculatorTest {

    @Test
    fun testCalculateDiscountReturnsOriginalPriceWhenDiscountIsZero() {
        // Arrange
        val discountCalculator = DiscountCalculator()
        val originalPrice = 50.0
        val discountPercentage = 0.0

        // Act
        val result = discountCalculator.calculateDiscount(originalPrice, discountPercentage)

        // Assert
        assertEquals(originalPrice, result, 0.01)
    }
}

import kotlin.test.Test
import kotlin.test.assertEquals

class DiscountCalculatorTest {

    @Test
    fun testCalculateDiscountReturnsOriginalPriceWhenDiscountIsZero() {
        // Arrange
        val discountCalculator = DiscountCalculator()
        val originalPrice = 50.0
        val discountPercentage = 0.0

        // Act
        val result = discountCalculator.calculateDiscount(originalPrice, discountPercentage)

        // Assert
        assertEquals(originalPrice, result, 0.01)
    }
}

Kotlin

import kotlin.test.Test
import kotlin.test.assertEquals

class DiscountCalculatorTest {

    @Test
    fun testCalculateDiscountHandlesDecimalDiscountsCorrectly() {
        // Arrange
        val discountCalculator = DiscountCalculator()
        val originalPrice = 100.0
        val discountPercentage = 33.333
        val expectedDiscountedPrice = 66.67

        // Act
        val result = discountCalculator.calculateDiscount(originalPrice, discountPercentage)

        // Assert
        assertEquals(expectedDiscountedPrice, result, 0.01)
    }
}

import kotlin.test.Test
import kotlin.test.assertEquals

class DiscountCalculatorTest {

    @Test
    fun testCalculateDiscountHandlesDecimalDiscountsCorrectly() {
        // Arrange
        val discountCalculator = DiscountCalculator()
        val originalPrice = 100.0
        val discountPercentage = 33.333
        val expectedDiscountedPrice = 66.67

        // Act
        val result = discountCalculator.calculateDiscount(originalPrice, discountPercentage)

        // Assert
        assertEquals(expectedDiscountedPrice, result, 0.01)
    }
}

Kotlin

import kotlin.test.Test
import kotlin.test.assertFailsWith

class DiscountCalculatorTest {

    @Test
    fun testCalculateDiscountShouldThrowErrorForNegativePrices() {
        // Arrange
        val discountCalculator = DiscountCalculator()
        val originalPrice = -50.0
        val discountPercentage = 10.0

        // Act & Assert
        assertFailsWith<IllegalArgumentException> {
            discountCalculator.calculateDiscount(originalPrice, discountPercentage)
        }
    }
}

import kotlin.test.Test
import kotlin.test.assertFailsWith

class DiscountCalculatorTest {

    @Test
    fun testCalculateDiscountShouldThrowErrorForNegativePrices() {
        // Arrange
        val discountCalculator = DiscountCalculator()
        val originalPrice = -50.0
        val discountPercentage = 10.0

        // Act & Assert
        assertFailsWith<IllegalArgumentException> {
            discountCalculator.calculateDiscount(originalPrice, discountPercentage)
        }
    }
}

Kotlin

import kotlin.test.Test
import kotlin.test.assertFailsWith

class DiscountCalculatorTest {

    @Test
    fun testCalculateDiscountThrowsErrorForDiscountsGreaterThan100Percent() {
        // Arrange
        val discountCalculator = DiscountCalculator()
        val originalPrice = 100.0
        val discountPercentage = 110.0

        // Act & Assert
        assertFailsWith<IllegalArgumentException> {
            discountCalculator.calculateDiscount(originalPrice, discountPercentage)
        }
    }
}

import kotlin.test.Test
import kotlin.test.assertFailsWith

class DiscountCalculatorTest {

    @Test
    fun testCalculateDiscountThrowsErrorForDiscountsGreaterThan100Percent() {
        // Arrange
        val discountCalculator = DiscountCalculator()
        val originalPrice = 100.0
        val discountPercentage = 110.0

        // Act & Assert
        assertFailsWith<IllegalArgumentException> {
            discountCalculator.calculateDiscount(originalPrice, discountPercentage)
        }
    }
}

Kotlin

import kotlin.test.Test
import kotlin.test.assertEquals

class DiscountCalculatorTest {

    @Test
    fun testCalculateDiscountReturnsOriginalPriceWhenDiscountIsZero() {
        // Arrange
        val discountCalculator = DiscountCalculator()
        val originalPrice = 50.0
        val discountPercentage = 0.0

        // Act
        val result = discountCalculator.calculateDiscount(originalPrice, discountPercentage)

        // Assert
        assertEquals(originalPrice, result, 0.01)
    }
}

Kotlin

import kotlin.test.Test
import kotlin.test.assertEquals

class DiscountCalculatorTest {

    @Test
    fun testCalculateDiscountHandlesDecimalDiscountsCorrectly() {
        // Arrange
        val discountCalculator = DiscountCalculator()
        val originalPrice = 100.0
        val discountPercentage = 33.333
        val expectedDiscountedPrice = 66.67

        // Act
        val result = discountCalculator.calculateDiscount(originalPrice, discountPercentage)

        // Assert
        assertEquals(expectedDiscountedPrice, result, 0.01)
    }
}

Kotlin

import kotlin.test.Test
import kotlin.test.assertFailsWith

class DiscountCalculatorTest {

    @Test
    fun testCalculateDiscountShouldThrowErrorForNegativePrices() {
        // Arrange
        val discountCalculator = DiscountCalculator()
        val originalPrice = -50.0
        val discountPercentage = 10.0

        // Act & Assert
        assertFailsWith<IllegalArgumentException> {
            discountCalculator.calculateDiscount(originalPrice, discountPercentage)
        }
    }
}

Kotlin

import kotlin.test.Test
import kotlin.test.assertFailsWith

class DiscountCalculatorTest {

    @Test
    fun testCalculateDiscountThrowsErrorForDiscountsGreaterThan100Percent() {
        // Arrange
        val discountCalculator = DiscountCalculator()
        val originalPrice = 100.0
        val discountPercentage = 110.0

        // Act & Assert
        assertFailsWith<IllegalArgumentException> {
            discountCalculator.calculateDiscount(originalPrice, discountPercentage)
        }
    }
}

Introduction to Practice

Understanding the Red-Green-Refactor Cycle

Requirements for `calculateDiscount` Function

1. Correct Discount Application

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2. Zero Discount Handling

3. Decimal Discount Precision

4. Negative Price Handling

5. Discount Percentage Greater Than 100%

Summary and Preparation for Practice