Initialize a list in Kotlin

This article explores different ways to initialize a list in Kotlin in a single line.

1. Using `listOf()` function

The listOf() function returns an immutable list of the given elements, which does not permit the addition or removal of elements. This is similar to Arrays.asList() function in Java.

fun main() {

val immutableList: List<Int> = listOf(4, 2, 5, 4, 1)

println(immutableList)

}

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To get an immutable list with all null elements excluded, you can use the listOfNotNull() function.

fun main() {

val list: List<Int> = listOfNotNull(4, 2, 5, 4, null, 1)

println(list)

}

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2. Using `mutableListOf()` function

If we need a resizable list that can expand or shrink, you can use the mutableListOf() function.

fun main() {

val mutableList: MutableList<Int> = mutableListOf(4, 2, 5, 4, 1)

println(mutableList)

}

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You can also use arrayListOf() function, which returns an ArrayList implementation.

fun main() {

val mutableList: MutableList<Int> = arrayListOf(4, 2, 5, 4, 1)

println(mutableList)

}

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3. Using `emptyList()` function

If you need an empty read-only list, you can use the emptyList() function.

fun main() {

val emptyList: List<Int> = emptyList()

println(emptyList)

}

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4. Using Builder functions

Another plausible way is to use builder functions for collections. To get a read-only list, as a result, you can use the buildList, which takes a lambda with a receiver as an argument.

fun main() {

val initial = listOf(1, 3, 5, 7)

val ints = buildList {

add(0)

initial.mapTo(this) { it + 1 }

}

println(ints) // [0, 2, 4, 6, 8]

}

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5. Using Double Brace Initialization

Finally, you can use Double Brace Initialization, which creates an anonymous inner class with just an instance initializer in it.

fun main() {

val mutableList: MutableList<Int> = object : ArrayList<Int>()

{

init {

add(1)

add(2)

add(3)

}

println(mutableList)

}

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That’s all about initializing a list in Kotlin.

Initialize a list in Kotlin

1. Using listOf() function

2. Using mutableListOf() function

3. Using emptyList() function

4. Using Builder functions

5. Using Double Brace Initialization

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1. Using `listOf()` function

2. Using `mutableListOf()` function

3. Using `emptyList()` function