How to use the spread operator with ArrayList in kotlin? - kotlin

Question

I've read the documentation and I run into this issue of using the spread operator with an ArrayList Collection, and I want to know how to solve the mismatch type or implement a way to use it with ArrayList
I'll attach an image of the code along with the code.
fun howSum(targetSum: Int, numbers: ArrayList<Int>): ArrayList<Int>? {
if (targetSum == 0) return arrayListOf();
if (targetSum < 0) return null;
for (number: Int in numbers){
val remainder = targetSum - number;
val remainderResult = howSum(remainder, numbers);
if (remainderResult != null){
return arrayListOf(*remainderResult, number)
}
}
return null
}
Any comment could be helpful...

Answer 1

I think you need to give us more information about what you are trying to do for a better answer.
The spread operator is for passing an array in place of a varargs argument, but you can't add additional arguments to the array at the same time.
If you want a new ArrayList that contains the contents of another ArrayList with an extra element added, you can do something like this:
fun main() {
val foo = arrayListOf(1, 2, 3)
val bar = arrayListOf<Int>().apply {
addAll(foo)
add(4)
}
println(foo)
println(bar)
}
Output:
[1, 2, 3]
[1, 2, 3, 4]
But it's not efficient, because it copies all the items of foo into bar.

Answer 2

Spread operator is not applicable to Lists, it's intended only for arrays:
fun howSum(targetSum: Int, numbers: ArrayList<Int>): IntArray? {
if (targetSum == 0) return intArrayOf()
if (targetSum < 0) return null
for (number: Int in numbers) {
val remainder = targetSum - number;
val remainderResult = howSum(remainder, numbers);
if (remainderResult != null) {
return intArrayOf(*remainderResult, number)
}
}
return null
}
If you want to create new List of the other one with addition of some element, you can use + operator:
fun howSum(targetSum: Int, numbers: ArrayList<Int>): List<Int>? {
if (targetSum == 0) return arrayListOf();
if (targetSum < 0) return null;
for (number: Int in numbers){
val remainder = targetSum - number;
val remainderResult = howSum(remainder, numbers);
if (remainderResult != null){
return remainderResult + number
}
}
return null
}