ArrayList field not correctly gathered - kotlin

Question

I'm actually new at Kotlin, and I encounter the following problematic:
I have a class holding an ArrayList of EnsembleVerifier class.
This other class is instantiated with an ArrayList of Square.
When I tried to get this ArrayList, I discovered that this one has no element inside.
Is there any absurdity/misconception in my code? Is it something else? Thank you in advance! :)
GridVerifiers.kt
class GridVerifiers(private val grid: Grid) {
private var verifiers: ArrayList<EnsembleVerifier> = ArrayList()
init {
generateVerifiers()
}
private fun generateVerifiers() {
generateLineVerifiers()
generateColumnVerifiers()
generateSubGridVerifiers()
}
private fun generateLineVerifiers() {
val line: ArrayList<Square> = ArrayList()
for (lineIndex in grid.gridState.indices) {
for (columnIndex in grid.gridState.indices)
line.add(grid.gridState[lineIndex][columnIndex])
println(line.size) // returns 9
verifiers.add(EnsembleVerifier(line))
line.clear()
}
}
...
EnsembleVerifier.kt
class EnsembleVerifier(private val squares: ArrayList<Square>) {
...
fun isValid(): Boolean {
val buffer: ArrayList<Int> = ArrayList()
println(squares.size) // returns 0!
for (square in squares) {
if (square.value in buffer) return false
buffer.add(square.value)
}
return true
}

Answer 1

In java most of the time you are working with references to objects. This means in your case, that your are always working with the same array line. Therefore, when you call line.clear you are cleaning the array that that reference is pointing at, and that's causing your issue with empty arrays.
You need to generate new objects every time instead of cleaning the list.
private fun generateLineVerifiers() {
for (lineIndex in grid.gridState.indices) {
val line: ArrayList<Square> = ArrayList()
for (columnIndex in grid.gridState.indices)
line.add(grid.gridState[lineIndex][columnIndex])
println(line.size) // returns 9
verifiers.add(EnsembleVerifier(line))
}
}