Join two arrays in Kotlin

This article explores different ways to join two arrays in Kotlin.

The solution should generate a new array containing all elements of the first array, followed by all the second array elements while preserving the original order of elements in both arrays.

1. Using Plus Operator

A simple and fairly efficient solution to combine two arrays in Kotlin is with the plus operator. This is demonstrated below:

fun <T> join(first: Array<T>, second: Array<T>): Array<T> {

return first + second

}

fun main() {

val first = arrayOf("A", "B")

val second = arrayOf("C", "D", "E")

val result = join(first, second)

println(result.contentToString()) // [A, B, C, D, E]

}

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An equivalent way is to use the plus() function.

fun <T> join(first: Array<T>, second: Array<T>): Array<T> {

return first.plus(second)

}

fun main() {

val first = arrayOf("A", "B")

val second = arrayOf("C", "D", "E")

val result = join(first, second)

println(result.contentToString()) // [A, B, C, D, E]

}

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2. Using Spread Operator

This can also be done using the Spread operator (prefix the array with *). You can pass the contents of both arrays to the Array constructor using the Spread operator.

inline fun <reified T> join(first: Array<T>, second: Array<T>): Array<T> {

return arrayOf(*first, *second)

}

fun main() {

val first = arrayOf("A", "B")

val second = arrayOf("C", "D", "E")

val result = join(first, second)

println(result.contentToString()) // [A, B, C, D, E]

}

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3. Using `MutableList`

You can also create a MutableList object and add all elements of the first array, followed by all elements of the second array. Finally, return the list as an array.

inline fun <reified T> join(first: Array<T>, second: Array<T>): Array<T> {

val list : MutableList<T> = ArrayList();

list.addAll(first);

list.addAll(second);

return list.toTypedArray()

}

fun main() {

val first = arrayOf("A", "B")

val second = arrayOf("C", "D", "E")

val result = join(first, second)

println(result.contentToString()) // [A, B, C, D, E]

}

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This can be shortened to:

inline fun <reified T> join(first: Array<T>, second: Array<T>): Array<T> {

val list = first.toMutableList();

list.addAll(second);

return list.toTypedArray()

}

4. Using `System.arraycopy()` function

The idea is to allocate memory to the new array to accommodate all the elements present in both arrays and use System.arraycopy() to copy given arrays into the new array, as shown below:

inline fun <reified T> join(first: Array<T>, second: Array<T>): Array<T?> {

val arr = arrayOfNulls<T>(first.size + second.size)

System.arraycopy(first, 0, arr, 0, first.size)

System.arraycopy(second, 0, arr, first.size, second.size)

return arr

}

fun main() {

val first = arrayOf("A", "B")

val second = arrayOf("C", "D", "E")

val result = join(first, second)

println(result.contentToString()) // [A, B, C, D, E]

}

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You can replace the first System.arraycopy() call to the copyOf() function:

fun <T> join(first: Array<T>, second: Array<T>): Array<T?> {

val result = first.copyOf(first.size + second.size)

System.arraycopy(second, 0, result, first.size, second.size)

return result

}

5. Using Java 8 Stream

We can use Stream in Kotlin to merge two arrays. This can be easily done using the Stream.concat() function:

import java.util.*

import java.util.stream.Stream

fun <T> join(first: Array<T>, second: Array<T>): Array<Any> {

return Stream.concat(Arrays.stream(first), Arrays.stream(second))

.toArray()

}

fun main() {

val first = arrayOf("A", "B")

val second = arrayOf("C", "D", "E")

val result = join(first, second)

println(result.contentToString()) // [A, B, C, D, E]

}

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That’s all about joining two arrays in Kotlin.

1. Using Plus Operator

2. Using Spread Operator

3. Using MutableList

4. Using System.arraycopy() function

5. Using Java 8 Stream

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3. Using `MutableList`

4. Using `System.arraycopy()` function