Kotlin collections methods: examples and syntax

By Alvin Alexander. Last updated: February 6, 2019

As a quick note today, if you ever need some examples of how the Kotlin collections methods work, I hope these examples are helpful.

Sample data

First, here’s some sample data:

val a = listOf(10, 20, 30, 40, 10)
val names = listOf("joel", "ed", "chris", "maurice")

Kotlin collection methods examples

Next, here are examples of many Kotlin methods, especially the most common Kotlin methods and their syntax:

a.any{it > 20}              //true
a.contains(10)              //true
a.count()                   //5
a.count{it > 10}            //3
a.distinct()                //[10, 20, 30, 40]
a.distinctBy()

a.drop(1)                   //[20, 30, 40, 10]
a.drop(2)                   //[30, 40, 10]
a.dropLast(1)               //[10, 20, 30, 40]
a.dropLast(2)               //[10, 20, 30]
a.dropWhile{it < 30}        //[30, 40, 10]
a.dropLastWhile{it != 30}   //[10, 20, 30]

a.filter{it != 10}          //[20, 30, 40]
a.find{it != 10}            //20
a.first()                   //10
a.first{}
a.firstOrNull()             //TODO
a.fold(0){acc, x -> acc+x}  //110 (sum function)
a.forEach{println(it)}      //prints out the list values

a.getOrElse(0){0}           //10
a.getOrElse(1){0}           //20
a.getOrElse(11){0}          //0
TODO: better groupBy
a.groupBy({it}, {it+1})     //{10=[11, 11], 20=[21], 30=[31], 40=[41]}
a.indexOf(10)               //0
a.indexOf(30)               //2
a.indexOfFirst()
a.indexOfLast()

a.intersect()
a.isEmpty()                 //false
a.isNotEmpty()              //true
a.last()                    //10
names.last{it.length < 4}   //ed
a.lastOrNull()              //10

a.map{it + 1}               //[11, 21, 31, 41, 11]
a.map{it * 2}               //[20, 40, 60, 80, 20]
a.max()                     //40
a.maxBy{it + 3}             //40
maxWith                     //TODO
a.min()                     //10
a.minBy{it + 3}             //10
minWith                     //TODO
a.onEach{println(it)}       //prints each element and returns 
                            //a copy of the list
a.partition{it >10}         //([20, 30, 40], [10, 10])
a.reduce{acc, x -> acc+x}   //110 (sum function)
a.slice(0..2)               //[10, 20, 30]
a.slice(1..2)               //[20, 30]
a.sorted()                  //[10, 10, 20, 30, 40]
a.sortedBy{it}              //[10, 10, 20, 30, 40]
names.sortedBy{it.length}   //[ed, joel, chris, maurice]
a.sortedWith()              //TODO

a.sum()                     //110
a.sumBy{it + 1}             //115

a.take(1)                   //[10]
a.take(2)                   //[10, 20]
a.takeLast(1)               //[10]
a.takeLast(2)               //[40, 10]
a.takeLastWhile{it < 40}    //[10]
a.takeWhile{it < 40}        //[10, 20, 30]
a.union(names)              //[10, 20, 30, 40, joel, ed, chris, maurice]
a.zip(names)                //[(10, joel), (20, ed), (30, chris), (40, maurice)]
names.zip(a)                //[(joel, 10), (ed, 20), (chris, 30), (maurice, 40)]

Again, if you needed to see examples of the most common Kotlin methods and their syntax, I hope these examples are helpful.

kotlin

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