почему виводит ошибку cannot find symbol на єтой строке System.out.println(stack.pop());

import java.util.*; public class mria{ private static Scanner scn = new Scanner(System.in); private static boolean isN(String string) { try { Double.parseDouble(string); return true; } catch (NumberFormatException ex) {return false;} } public static void main (String[] args) { System.out.println(stack.pop()); System.out.println("rr"); String str = scn.nextLine(); String[] strings = str.split(" "); Stack<Double> stack = new Stack<Double>(); for (int i = 0; i < strings.length; i++) { if(isN(strings[i])) { // stack.push(Double.parseDouble(strings[i])); } else { double tmp1 = stack.pop(); double tmp2 = stack.pop(); switch (strings[i]) { case "+": stack.push(tmp1 + tmp2); break; case "-": stack.push(tmp1 - tmp2); break; case "*": stack.push(tmp1 * tmp2); break; case "/": stack.push(tmp1 / tmp2); break; } // } } } }