Kotlin
Java

Practice with Collections
Kotlin

Created By: Geoffrey Challen
/ Updated: 2022-09-26

Let's pause before moving on to get more practice with Kotlin's collections—the lists, maps, and sets that are so useful for solving problems. We'll also learn how we can combine these collections together to build more interesting data structures. Let's get started!

Nested Collections

In the past lessons we've seen how to create and use several standard Kotlin collections: Lists, Maps, and Sets:

These collections are quite useful on their own! However, they can also be combined to great effect. Let's see an example.

Show how to use a mutableMapOf<String, MutableList<String>> to create a list of tasks for individual people. Demonstrate how to handle the case where the initial value is not present.

Lists of Maps of Sets

You can combine Lists, Maps, and Sets in many interesting ways to build data structures to solve problems. You can create Lists of Maps:

Or Sets of Lists:

But generally, it's more common for the top-level data structure to be a Map: Maps of Maps, Maps of Lists, and Maps of Sets. We'll get some practice working with these on this lesson's practice and homework problems.

Kotlin Maps and Nullability

One thing you may have noticed is that Kotlin's null checking doesn't work so well in some cases on maps. For example:

Let's briefly discuss why this is, and what we can do about it.

Discuss how null checking and Kotlin Maps collide, and make the case for the null assertion !! operator here.

Lists to Map Warm-Up

We'll spend the rest of the lesson working on some problems that test our understanding of how to nest collections. First, we're asked to parse a List<String> into a Map<Set<String>>. Let's do an example of that together, which you can use as a starting point for the practice problem that follows.

Set up the next homework problem and provide an example of how to insert a Set into a Map.

Show how to complete the homework problem above. Feel free to cover multiple approaches!

Homework Problem Warm-Up

Today's homework problem is a challenge! Your goal is to complete the implementation of the Hawaiian Word translator you began earlier this semester. Since this problem is more difficult than other homework problems, we're giving you an extra day to complete it. And, at the end of the day, please remember that this is just one problem, and you have homework drops.

We'll also get you started with a walkthrough to help you think about how to approach this problem.

Note that the problem asks you to throw an exception in certain cases, something that we have not yet covered. The walkthrough describes how to do that.

Help students get their loop set up properly for the Hawaiian Words problem.

Show how to complete the homework problem above. Feel free to cover multiple approaches!

More Practice

Need more practice? Head over to the practice page.