How to Sort List in Kotlin: Simple Syntax and Examples

In Kotlin, you can sort a list using the sorted() function to get a new sorted list or sort() to sort a mutable list in place. You can also use sortedBy or sortedWith for custom sorting rules.

Syntax

Kotlin provides several ways to sort lists:

• list.sorted(): Returns a new list sorted in ascending order.
• mutableList.sort(): Sorts the mutable list in place.
• list.sortedBy { selector }: Sorts by a specific property or value.
• list.sortedWith(comparator): Sorts using a custom comparator.

kotlin

val list = listOf(3, 1, 4, 2)
val sortedList = list.sorted()  // returns new sorted list

val mutableList = mutableListOf(3, 1, 4, 2)
mutableList.sort()  // sorts in place

val sortedByLength = listOf("apple", "kiwi", "banana").sortedBy { it.length }

val sortedWithComparator = listOf("apple", "kiwi", "banana").sortedWith(compareBy { it.length })

Example

This example shows how to sort a list of numbers and a list of strings by length.

kotlin

fun main() {
    val numbers = listOf(5, 3, 8, 1)
    val sortedNumbers = numbers.sorted()
    println("Sorted numbers: $sortedNumbers")

    val fruits = listOf("pear", "apple", "banana", "kiwi")
    val sortedFruitsByLength = fruits.sortedBy { it.length }
    println("Fruits sorted by length: $sortedFruitsByLength")

    val mutableNumbers = mutableListOf(10, 2, 7, 4)
    mutableNumbers.sort()
    println("Mutable numbers sorted in place: $mutableNumbers")
}

Output

Sorted numbers: [1, 3, 5, 8]
Fruits sorted by length: [pear, kiwi, apple, banana]
Mutable numbers sorted in place: [2, 4, 7, 10]

Common Pitfalls

One common mistake is trying to use sort() on an immutable list, which causes a compile error because sort() modifies the list in place and only works on mutable lists. Another is expecting sorted() to change the original list, but it returns a new sorted list instead.

kotlin

fun main() {
    val immutableList = listOf(3, 2, 1)
    // immutableList.sort() // Error: Unresolved reference: sort

    val sortedList = immutableList.sorted() // Correct: returns new sorted list
    println("Original list: $immutableList")
    println("Sorted list: $sortedList")
}

Output

Original list: [3, 2, 1]
Sorted list: [1, 2, 3]

Quick Reference

Here is a quick summary of Kotlin list sorting functions:

Function	Description
list.sorted()	Returns a new list sorted in ascending order.
mutableList.sort()	Sorts the mutable list in place.
list.sortedBy { selector }	Sorts by a property or value.
list.sortedWith(comparator)	Sorts using a custom comparator.

Key Takeaways

Use sorted() to get a new sorted list without changing the original.

Use sort() only on mutable lists to sort them in place.

Use sortedBy or sortedWith for custom sorting rules.

Immutable lists cannot be sorted in place; always use sorted() for them.

Remember that sorted() returns a new list and does not modify the original.