List or array is always empty in Kotlin - kotlin

Question

I'm trying to write simple Stack on Kotlin, but all data containers are always throws ArrayIndexOutOfBoundsException. I can't find out what can cause this problem:
class StackX(size: Int) {
private var maxSize: Int = size
private var stackArray: Array<Long> = arrayOf(maxSize.toLong())
private var top = -1
fun push(data: Long) {
stackArray[++top] = data
}
fun pop() : Long {
return stackArray[top--]
}
fun peek() : Long {
return stackArray[top]
}
fun isEmpty() : Boolean {
return (top == -1)
}
fun isFull() : Boolean {
return (top == maxSize -1)
}
}
Could you please explain me the right patter of arrays declaration in this case? I want just this:
int a[] = new int[10];
P.S. It's test code, I even doesn't call pop. It throws on push. I'm just trying to understand what's wrong with declaration.

Answer 1

This line is the problem
private var stackArray: ArrayList<Long> = arrayListOf().
It creates an array which length is 0.
Perhaps, you want something like this
val stackArray: LongArray = LongArray(size).

Answer 2

The problem is in your push() method.
private var stackArray: ArrayList<Long> = arrayListOf() // size = 0
private var top = -1
fun push(data: Long) {
stackArray[++top] = data // top = 0
}
What you're trying to do is to get the 0th element of an empty list.
Possible fix:
fun push(data: Long) {
stackArray.add(data)
++top
}
Updated.
Creating an int array of the specified size:
int[] a = new int[10]; // Java
val a = IntArray(10) // Kotlin
All elements are initialized to zero.
Creating a DataItem array:
DataItem[] b = new DataItem[10]; // Java
val b = arrayOfNulls<DataItem>(10) // Kotlin
All elements are initialized to null.