Question

So far I have

def toDoubleArray(list: ArrayList<Double>): Array[Double] {
  ...
}

Which is not compiling and is underlined but giving a very cryptic error message

Answer 1

import scala.collection.JavaConversions._

def toDoubleArray(list: ArrayList[Double]): Array[Double] = list.toArray

import will allow automatic conversion of Java ArrayList to scala ListBuffer ArrayBuffer

More generic solution:

def [T] toDoubleArray(list: ArrayList[T]): Array[T] = list.toArray

But in reality you don’t even need a function to do that.