Make an ArrayList whose attributes have ArrayList too - for-loop

Question

I have a problem to make data ArrayList whose attributes have ArrayList too. The model data looks like this code.
data class DataArrayInArray02(
val no: Int? = null,
val dataArray: ArrayList<Int>
)
I want to get a data DataArrayInArray02 like this.
This is my code
fun main() {
val dataArrayInArray = ArrayList<DataArrayInArray02>()
val dataChildrenArray = ArrayList<Int>()
for (i in 0..3) {
val data = (0..10).random()
for (j in 0..data) {
val d = (1..1000).random()
dataChildrenArray.add(d)
}
dataArrayInArray.add(DataArrayInArray02(i+1, dataChildrenArray))
println("ID : ${dataArrayInArray[i].no}, Data : ${dataArrayInArray[i].dataArray}")
dataChildrenArray.clear()
}
}
When I run this code, I get the result like the picture above.
I call dataArrayInArray using looping "for" looks like this.
for (j in 0 until dataArrayInArray.size) {
println("ID : ${dataArrayInArray[j].no}, Data : ${dataArrayInArray[j].dataArray}")
}
But, I get the result like this.
So, which code is incorrect? Is it because of using dataChildrenArray.clear()? If I delete that code. The result will get like this:
ID : 1, Data : [915, 565, 591, 254, 67]
ID : 2, Data : [915, 565, 591, 254, 67, 258, 57, 767, 866, 986, 558, 187, 976]
where ID 2 should only display data [258, 57, 767, 866, 986, 558, 187, 976].
How to solve that problem?

Answer 1

You're passing the same instance of an ArrayList<Int> to each instance of DataArrayInArray02 in the top level list. To fix your code, move the line
val dataChildrenArray = ArrayList<Int>()
inside your for loop so a new one is created for each child.
If you're OK with using List instead of ArrayList, you could simplify this:
val dataArrayInArray = (1..4).map { i ->
val innerListSize = (0..10).random()
val innerList = (0..innerListSize).map { (1..1000).random() }
DataArrayInArray02(i, innerList)
}