// Code for head, copy, and tail are taken from the book Functional Programming in Java
public static <T> T head(List<T> list) {
  if (list.size() == 0) {
    throw new IllegalStateException("head of empty list");
  }
  return list.get(0);
}

private static <T> List<T> copy(List<T> ts) {
  return new ArrayList<>(ts);
}

public static <T> List<T> tail(List<T> list) {
  if (list.size() == 0) {
    throw new IllegalStateException("tail of empty list");
  }
  List<T> workList = copy(list);
  workList.remove(0);
  return Collections.unmodifiableList(workList);
}

public static int recusive_sum(List<Integer> x) {
  return sum_helper(x, 0);
}

private static int sum_helper(List<Integer> x, int accumulator) {
  // base case
  if (x.isEmpty()) {
    return accumulator;
  }
  // processing logic leading to base case
  return sum_helper(tail(x), accumulator + head(x));
}