Kotlin Koans—Part 19

This section of the Kotlin Koans tutorial continued onward with Kotlin’s collection API enhancement. The challenge in this section use to total the price of all products a customer purchased. Here is the code

fun Customer.getTotalOrderPrice(): Double {
    // Return the sum of prices of all products that a customer has ordered.
    // Note: a customer may order the same product for several times.
    return orders.sumByDouble { it.products.sumByDouble { it.price } }
}

The collection API in Kotlin has a sumByDouble method, which takes a lambda expression. The lambda let’s developers chain function calls. In this case, each Customer had a collection of Products in each Order. To get the price of all Products ordered, I needed the sum of the price of all products in a order. This was easy enough to do because I just made a nested call to sumByDouble on it.products and then told it to sum on it.price.

Here is Java code that solves the same problem.

public static double getTotalOrderPrice(Customer customer){
    return customer
            .getOrders()
            .stream()
            .mapToDouble(
                    order -> order.getProducts()
                                    .stream()
                                    .mapToDouble(Product::getPrice)
                            .sum())
            .sum();
}

You can click here to see Part 18