WELCOME KOTLIN / Everywhere Nairobi Study Jam 3 / 6

Expressive & Efficient Collection Processing in Kotlin HARUN WANGEREKA ANDROID DEVELOPER, APPS:LAB

Agenda ● Basics ● Collections Playground ● Summary

Heavily inspired by Scala

Built on top of Java Collections API

Defines collections in the kotlin.collections package

public interface Iterable { public operator fun iterator(): Iterator } Basis for all collections

Iterator object knows how to iterate over a collection public interface Iterator { public operator fun next(): T public operator fun hasNext(): Boolean }

Next on hierarchy is Collection public interface Collection : Iterable { public val size: Int public fun isEmpty(): Boolean public operator fun contains(element: @UnsafeVariance E): Boolean override fun iterator(): Iterator public fun containsAll(elements: Collection<@UnsafeVariance E>): Boolean }

Next hierarchy is Collection ● List ● Set ● Map

Collection Mutable Read-Only ● MutableList ● MutableSet ● MutableMap ● List ● Set ● Map

Collections Playground

Inspired by the “collector”

When the speaker is ready, the collector will appear

No content

Summary ● Create - We have mutable and read-only ● Operators - Functional operations plus,minus, plus assign and minus assign ● Transformers - build new collections based on conditions provided ● Aggregators - operations that return single value based on collection content ● Searching - getting particular elements ● Filtering - filter-in with some predicate/condition ● Actions - [action] on a particular iterable

● Retrieving Collections Parts - Retrieves parts of collections - ● Retrieving Single Elements ● Collection Ordering ● Collections write operations - add(),remove() ● List/Set/Map Specific Operations

Neat Stuff. The cool way

..the old way var count = 0 var total = 0.0 for (album in albumList){ if (album.genre == "HipHop"){ count ++ total += album.averageRating } } val average = total/ count average.print()

The collections.way val avg = albumList.filter { it.genre == "HipHop" } .map { it.averageRating } .average() .print()

..the old way val speakersList :List = getSpeakersInfo() val sessionList : List = snapshots.toObjects() sessionList.map { sessionModel -> val sessionSpeakerIdList = sessionModel.speaker_id sessionSpeakerIdList.forEach { speakerId -> speakersList.forEach {speakerModel -> if (sessionSpeakerIdList.contains(speakerModel.id)){ sessionModel.speakerList.add(speakerModel) } } } }

The collections.way val speakerById : Map = getSpeakerInfo().associateBy { it.id} val sessionList : List = snapshots.toObjects() sessionList.onEach { sessionsModel -> sessionsModel.speaker_id.mapNotNull { speakerById[it]?.let { speakerModel -> sessionsModel.speakerList.add(speakerModel) } } }

articlesList.filter { it == "Kotlin Collections" }

Resources ● http://jussi.hallila.com/Kollections/ ● https://antonioleiva.com/collection-operations-kotlin/ ● https://kotlinlang.org/docs/reference/collections-overview.html ● https://github.com/wangerekaharun/CollectionsPlayGround