Solution: Implementing the Product class

class Product(
    val name: String,
    val price: Double,
    initialQuantity: Int
) {
    var quantity: Int = initialQuantity
        set(value) {
            field = if (value >= 0) value else 0
        }

    fun calculateTotalValue(): Double {
        return price * quantity
    }

    fun restock(additionalQuantity: Int) {
        if (additionalQuantity > 0) {
            quantity += additionalQuantity
        }
    }
}

Example solution in playground

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

class Product(
    val name: String,
    val price: Double,
    initialQuantity: Int
) {
    var quantity: Int = initialQuantity
        set(value) {
            field = if (value >= 0) value else 0
        }

    fun calculateTotalValue(): Double {
        return price * quantity
    }

    fun restock(additionalQuantity: Int) {
        if (additionalQuantity > 0) {
            quantity += additionalQuantity
        }
    }
}

fun main() {
    val laptop = Product("Laptop", 999.99, 5)
    
    println(laptop.name) // Laptop
    println(laptop.quantity) // 5
    println(laptop.calculateTotalValue()) // 4999.95
    
    laptop.restock(3)
    
    println(laptop.quantity) // 8
    println(laptop.calculateTotalValue()) // 7999.92
    
    laptop.quantity = -2
    
    println(laptop.quantity) // 0
    println(laptop.calculateTotalValue()) // 0.0
    
    laptop.quantity = 10
    
    println(laptop.quantity) // 10
    println(laptop.calculateTotalValue()) // 9999.9
}

class ProductTest {
   @Test
   fun `should keep name, price and quantity`() {
       val product = Product("Apple", 1.0, 10)
       assertEquals("Apple", product.name)
       assertEquals(1.0, product.price)
       assertEquals(10, product.quantity)
   }
 
   @Test
   fun `should calculate total value`() {
       assertEquals(10.0, Product("Apple", 1.0, 10).calculateTotalValue())
       assertEquals(0.0, Product("Apple", 1.0, 0).calculateTotalValue())
       assertEquals(0.0, Product("Apple", 0.0, 10).calculateTotalValue())
       assertEquals(4.8, Product("Apple", 1.2, 4).calculateTotalValue())
   }
 
   @Test
   fun `should restock`() {
       val product = Product("Apple", 1.0, 10)
       product.restock(5)
       assertEquals(15, product.quantity)
       product.restock(-5)
       assertEquals(15, product.quantity)
       product.restock(0)
       assertEquals(15, product.quantity)
   }
 
   @Test
   fun `should not allow negative quantity`() {
       val product = Product("Apple", 1.0, 10)
       product.quantity = -5
       assertEquals(0, product.quantity)
       product.quantity = 8
       assertEquals(8, product.quantity)
       product.quantity = 0
       assertEquals(0, product.quantity)
   }
}