Guava has a Doubles class which has a toArray
that takes a Collection<? extends Number>
and will thereby take a List<Double>
and convert to an array. So...
List<List<Double>> myList;
double[][] myArray = new double[myList.size()][];
for (int i=0, n<myList.size(); i<n; i++){
myArray[i] = Doubles.toArray(myList.get(i));
}