Kotlin sum() and sumBy() method for List, Map of Objects example

#kotlin #sum #sumby

This Kotlin tutorial shows you example that uses sum() and sumBy() method for List, Map of Objects.

I. Technology

Java 1.8
Kotlin 1.1.2
II. Overview
The goal is to calculate total quantity of all Product(name, quantity) objects in List/Map using:
sum() method: Firstly, map() will create a List of quantity first, then we invoke:
```
List.sum()
```
sumBy() method: It can apply on List of Product, but we must provide selector for indicating quantity:
```
List.sumBy(selector: product -> quantity)
```
III. Practice

0. Product Class
```
package com.javasampleapproach.fold
```