Explore Kotlin.Collections API

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Kirill Suslov Explore Kotlin.Collections API

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- friend “Lol! This is an intern interview question in JavaScript ”

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Less Code is (usually) Better*

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No content

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1. Helps making the code much more concise 2. Part of kotlin-stdlib 3. Built on top of Java Collections API Kotlin.Collections highlights

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Collection Mutable Read-only Immutable

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Collection Mutable Read-only • MutableList • MutableSet • MutableMap • List • Set • Map

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Neat stuﬀ

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data class Movie( val title: String, val releaseDate: Date, val genre: String, val averageRating: Double) Model

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val madMax = Movie("Mad Max", "2015-05-13".toDate(), "action", 7.3) val dunkirk = Movie("Dunkirk", "2017-07-19".toDate(), "action", 7.6) val birdman = Movie("Birdman", "2014-10-17".toDate(), "drama", 7.4) val titanic = Movie("Titanik", "1997-12-19".toDate(), "romance", 7.6) val lamb = Movie("The Silence of the Lambs", "1991-01-30".toDate(), "thriller", 8.1) val nemo = Movie("Finding Nemo", "2003-05-30".toDate(), "animation", 7.7) Some movies

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fun String.toDate(): Date { val formatter = SimpleDateFormat("yyyy-mm-dd") return formatter.parse(this) } Extension function

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Creation functions Neat stuﬀ. Part 1

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List and Mutable List val movieList = listOf(madMax, dunkirk, birdman, titanic, lamb, nemo) val movieMutableList = mutableListOf(madMax, dunkirk, birdman, titanic, lamb, nemo)

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Set and Mutable Set val movieSet = setOf(madMax, dunkirk, birdman, titanic, lamb, nemo) val movieMutableSet = mutableSetOf(madMax, dunkirk, birdman, titanic, lamb, nemo) Map and Mutable Map val movieMap = mapOf(madMax.title to madMax, dunkirk.title to dunkirk) val movieMutableMap = mutableMapOf(madMax.title to madMax, dunkirk.title to dunkirk) // PROTIP madMax.title to madMax == Pair(madMax.title, madMax)

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Aggregate functions Neat stuﬀ. Part 2

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Count() movieList.count() // Count: 6 // movieList.count() == movieList.size

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MaxBy() movieList.maxBy { it.averageRating } // Max rating: The Silence of the Lambs, 8.1 Note: max() if element is a number

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MinBy() movieList.minBy { it.averageRating } // Min rating: Mad Max, 7.3 Note: min() if element is a number

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Average() arrayOf(7.3, 7.6, 7.4, 7.6, 8.1, 7.7).average() // Average: 7.616666666666667

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Sum() arrayOf(7.3, 7.6, 7.4, 7.6, 8.1, 7.7).sum() // Sum: 45.7 movieList.sumByDouble { it.averageRating }

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Mode() and Median() aren’t there

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Search functions Neat stuﬀ. Part 3

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First() movieList.first { it.averageRating < 7.5 } // First: Mad Max movieList.first { it.averageRating < 3.0 } // NoSuchElementException

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Find() movieList.find { it.averageRating < 7.5 } // First: Mad Max movieList.find { it.averageRating < 3.0 } // First: null // movieList.find() == movieList.firstOrNull()

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Last() movieList.last { it.averageRating < 7.5 } // First: Birdman movieList.last { it.averageRating < 3.0 } // NoSuchElementException

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FindLast() movieList.findLast { it.averageRating < 7.5 } // First: Birdman movieList.findLast { it.averageRating < 3.0 } // First: null // movieList.findLast() == movieList.lastOrNull()

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Single() movieList.single { it.averageRating == 7.7 } // Single: Finding Nemo movieList.single { it.averageRating == 7.6 } // IllegalArgumentException

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SingleOrNull() movieList.singleOrNull { it.averageRating == 7.7 } // Single: Finding Nemo movieList.singleOrNull { it.averageRating == 7.6 } // Single: null

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Filter functions Neat stuﬀ. Part 4

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Filter() movieList.filter { it.averageRating > 7.5 } // Better than 7.5: // [Dunkirk, Titanik, The Silence of the Lambs, Finding Nemo]

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FilterNot(), FilterNotNull() movieList.filterNot { it.averageRating > 7.5 } // Not better than 7.5: [Mad Max, Birdman] movieList.filterNotNull() // Kinda useless here

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FilterTo(), filterNotTo() val listToFilterInto = mutableListOf() movieList.filterTo(listToFilterInto, { it.averageRating > 7.5 }) // Better than 7.5: [Dunkirk, Titanik, The Silence of the Lambs, Finding Nemo] movieList.filterNotTo(listToFilterInto, { it.averageRating > 7.5 }) // Not better than 7.5: [Mad Max, Birdman] movieList.filterNotNullTo(listToFilterInto) // Kinda useless here

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FilterIndexed() movieList.filterIndexed { index, movie -> index.even() } // Even Indexed: [Mad Max, Birdman, The Silence of the Lambs]

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Transform functions Neat stuﬀ. Part 5

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Map() movieList.map { it.title } // [Mad Max, Dunkirk, Birdman, Titanik, // The Silence of the Lambs, Finding Nemo] movieList.mapIndexed { index, movie -> index to movie.title } // [(0, Mad Max), (1, Dunkirk), (2, Birdman), // (3, Titanik), (4, The Silence of the Lambs), (5, Finding Nemo)]

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val madMax = Movie("Mad Max", "2015-05-13".toDate(), listOf("action"), 7.3) val dunkirk = Movie("Dunkirk", "2017-07-19".toDate(), listOf("action", "drama"), 7.6) val birdman = Movie("Birdman", "2014-10-17".toDate(), listOf("drama"), 7.4) val titanic = Movie("Titanik", "1997-12-19".toDate(), listOf("romance"), 7.6) val lamb = Movie("The Silence of the Lambs", "1991-01-30".toDate(), listOf("thriller"), 8.1) val nemo = Movie("Finding Nemo", "2003-05-30".toDate(), listOf("animation"), 7.7) Add some changes

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FlatMap() movieList.map { it.genre } // [[action], [action, drama], [drama], [romance], [thriller], [animation]] movieList.flatMap { it.genre } // [action, action, drama, drama, romance, thriller, animation]

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Distinct() // [action, action, drama, drama, romance, thriller, animation] genreList.distinct() // [action, drama, romance, thriller, animation]

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Associate(), AssociateBy() movieList.associate { it.title to it.averageRating } movieList.associateBy({ it.title }, { it.averageRating }) // {Mad Max=7.3, Dunkirk=7.6, Birdman=7.4, Titanik=7.6, // The Silence of the Lambs=8.1, Finding Nemo=7.7}

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GroupBy() movieList.groupBy({ it.genre }, { it.title }) // {action=[Mad Max, Dunkirk], drama=[Birdman], romance=[Titanik], // thriller=[The Silence of the Lambs], animation=[Finding Nemo]}

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Neat stuﬀ. Final Part

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val average = movieList.filter { it.genre == "action" } .map { it.averageRating } .average() // 7.449999999999999 var count = 0 var total = 0.0 for (movie in movieList) { if (movie.genre == "action") { count++ total += movie.averageRating } } val average = total / count // 7.449999999999999 Collection API Old school

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9 lines of old fashioned code 3 lines of neatness Comparison Slide

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Chart Title Lines 0 2 5 7 9 Java style Collection API

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val averagePerCategory = movieList .groupBy { it.genre } .mapValues { it.value .map { it.averageRating } .average() } /* {action=7.449999999999999, drama=7.4, romance=7.6, thriller=8.1, animation=7.7} */ More of Collection API