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

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:

sumBy() method:
It can apply on List of Product, but we must provide selector for indicating quantity:

III. Practice

0. Product Class

1. List


2. Map


By JavaSampleApproach | August 29, 2017.

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