Kotlin Collections Overview

Collection types

                  val numbers = mutableListOf("one", "two", "three", "four")
numbers.add("five")   // this is OK    
//numbers = mutableListOf("six", "seven")      // compilation error
                

                    let numbers = mutableListOf("one", "two", "three", "four")
numbers.add("five")   // this is OK    
//numbers = mutableListOf("six", "seven")      // compilation error
                  

Collection

                  fun printAll(strings: Collection<String>) {
        for(s in strings) print("$s ")
        println()
    }
    
fun main() {
    val stringList = listOf("one", "two", "one")
    printAll(stringList)
    
    val stringSet = setOf("one", "two", "three")
    printAll(stringSet)
}
                

                    func printAll(strings: Collection<String>) {
        for(s in strings) print("$s ")
        print()
    }
    
func main() {
    let stringList = listOf("one", "two", "one")
    printAll(stringList)
    
    let stringSet = setOf("one", "two", "three")
    printAll(stringSet)
}
                  

                  fun List<String>.getShortWordsTo(shortWords: MutableList<String>, maxLength: Int) {
    this.filterTo(shortWords) { it.length <= maxLength }
    // throwing away the articles
    val articles = setOf("a", "A", "an", "An", "the", "The")
    shortWords -= articles
}
​
fun main() {
    val words = "A long time ago in a galaxy far far away".split(" ")
    val shortWords = mutableListOf<String>()
    words.getShortWordsTo(shortWords, 3)
    println(shortWords)
}
                

                    func List<String>.getShortWordsTo(shortWords: MutableList<String>, maxLength: Int) {
    this.filterTo(shortWords) { it.length <= maxLength }
    // throwing away the articles
    let articles = setOf("a", "A", "an", "An", "the", "The")
    shortWords -= articles
}
​
func main() {
    let words = "A long time ago in a galaxy far far away".split(" ")
    let shortWords = mutableListOf<String>()
    words.getShortWordsTo(shortWords, 3)
    print(shortWords)
}
                  

List

                  val numbers = listOf("one", "two", "three", "four")
println("Number of elements: ${numbers.size}")
println("Third element: ${numbers.get(2)}")
println("Fourth element: ${numbers[3]}")
println("Index of element \"two\" ${numbers.indexOf("two")}")
                

                    let numbers = listOf("one", "two", "three", "four")
print("Number of elements: ${numbers.size}")
print("Third element: ${numbers.get(2)}")
print("Fourth element: ${numbers[3]}")
print("Index of element \"two\" ${numbers.indexOf("two")}")
                  

                  val bob = Person("Bob", 31)
val people = listOf(Person("Adam", 20), bob, bob)
val people2 = listOf(Person("Adam", 20), Person("Bob", 31), bob)
println(people == people2)
bob.age = 32
println(people == people2)
                

                    let bob = Person("Bob", 31)
let people = listOf(Person("Adam", 20), bob, bob)
let people2 = listOf(Person("Adam", 20), Person("Bob", 31), bob)
print(people == people2)
bob.age = 32
print(people == people2)
                  

                  val numbers = mutableListOf(1, 2, 3, 4)
numbers.add(5)
numbers.removeAt(1)
numbers[0] = 0
numbers.shuffle()
println(numbers)
                

                    let numbers = mutable[1, 2, 3, 4]
numbers.add(5)
numbers.removeAt(1)
numbers[0] = 0
numbers.shuffle()
print(numbers)
                  

Set

                  val numbers = setOf(1, 2, 3, 4)
println("Number of elements: ${numbers.size}")
if (numbers.contains(1)) println("1 is in the set")
​
val numbersBackwards = setOf(4, 3, 2, 1)
println("The sets are equal: ${numbers == numbersBackwards}")
                

                    let numbers = setOf(1, 2, 3, 4)
print("Number of elements: ${numbers.size}")
if (numbers.contains(1)) print("1 is in the set")
​
let numbersBackwards = setOf(4, 3, 2, 1)
print("The sets are equal: ${numbers == numbersBackwards}")
                  

                  val numbers = setOf(1, 2, 3, 4)  // LinkedHashSet is the default implementation
val numbersBackwards = setOf(4, 3, 2, 1)
​
println(numbers.first() == numbersBackwards.first())
println(numbers.first() == numbersBackwards.last())
                

                    let numbers = setOf(1, 2, 3, 4)  // LinkedHashSet is the default implementation
let numbersBackwards = setOf(4, 3, 2, 1)
​
print(numbers.first() == numbersBackwards.first())
print(numbers.first() == numbersBackwards.last())
                  

Map

                  val numbersMap = mapOf("key1" to 1, "key2" to 2, "key3" to 3, "key4" to 1)
​
println("All keys: ${numbersMap.keys}")
println("All values: ${numbersMap.values}")
if ("key2" in numbersMap) println("Value by key \"key2\": ${numbersMap["key2"]}")    
if (1 in numbersMap.values) println("The value 1 is in the map")
if (numbersMap.containsValue(1)) println("The value 1 is in the map") // same as previous
                

                    let numbersMap = mapOf("key1" to 1, "key2" to 2, "key3" to 3, "key4" to 1)
​
print("All keys: ${numbersMap.keys}")
print("All values: ${numbersMap.values}")
if ("key2" in numbersMap) print("Value by key \"key2\": ${numbersMap["key2"]}")    
if (1 in numbersMap.values) print("The value 1 is in the map")
if (numbersMap.containsValue(1)) print("The value 1 is in the map") // same as previous
                  

                  val numbersMap = mapOf("key1" to 1, "key2" to 2, "key3" to 3, "key4" to 1)    
val anotherMap = mapOf("key2" to 2, "key1" to 1, "key4" to 1, "key3" to 3)
​
println("The maps are equal: ${numbersMap == anotherMap}")
                

                    let numbersMap = mapOf("key1" to 1, "key2" to 2, "key3" to 3, "key4" to 1)    
let anotherMap = mapOf("key2" to 2, "key1" to 1, "key4" to 1, "key3" to 3)
​
print("The maps are equal: ${numbersMap == anotherMap}")
                  

                  val numbersMap = mutableMapOf("one" to 1, "two" to 2)
numbersMap.put("three", 3)
numbersMap["one"] = 11
​
println(numbersMap)
                

                    let numbersMap = mutableMapOf("one" to 1, "two" to 2)
numbersMap.put("three", 3)
numbersMap["one"] = 11
​
print(numbersMap)