Обратная польская нотация

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Stack;

public class Hello {

    public static void mymain() {
        int ans = calculate("-2+2*(9+8)");
        System.out.println(ans);
    }
    static int calculate(String mathString) {

        List<String> operations = new ArrayList<>(Arrays.asList("+", "-", "*", "/", "(", ")"));

        int res;
        Stack<String> stack = new Stack<>();
        List<String> arr = new ArrayList<>();
        StringBuilder currentNum = new StringBuilder();

        for (int i = 0; i < mathString.length(); i++) {
            String c = String.valueOf(mathString.charAt(i));
            if (operations.contains(c)) {
                if (currentNum.length() != 0) {
                    arr.add(currentNum.toString());
                    currentNum.setLength(0);
                }

                if (c.equals("-") && (i == 0 || String.valueOf(mathString.charAt(i - 1)).equals("("))) {
                    arr.add(c);
                }
                else if (c.equals(")")) {
                    while (!stack.peek().equals("(")) {
                        arr.add(String.valueOf(stack.pop()));
                    }
                    stack.pop();
                }
                else {
                    if (!stack.isEmpty() && (c.equals("+") || c.equals("-")) && (!stack.peek().equals(")") && !stack.peek().equals("("))) {
                        String peekStack = stack.pop();
                        arr.add(String.valueOf(peekStack));
                    }
                    stack.push(c);
                }
            }
            else {
                currentNum.append(c);
            }
        }
        if (currentNum.length() != 0) {
            arr.add(currentNum.toString());
            currentNum.setLength(0);
        }
        while (!stack.isEmpty())
            arr.add(String.valueOf(stack.pop()));
        System.out.println(Arrays.toString(arr.toArray()));

        int a, b, c;
        boolean negative = false;
        for (String s : arr) {
            switch (s) {
                case "+":
                    a = Integer.parseInt(stack.pop());
                    b = Integer.parseInt(stack.pop());
                    c = b + a;
                    stack.push(String.valueOf(c));
                    break;
                case "-":
                    if (stack.isEmpty() || stack.size() % 2 != 0) {
                        negative = true;
                    }
                    else {
                        a = Integer.parseInt(stack.pop());
                        b = Integer.parseInt(stack.pop());
                        c = b - a;
                        stack.push(String.valueOf(c));
                    }
                    break;
                case "/":
                    a = Integer.parseInt(stack.pop());
                    b = Integer.parseInt(stack.pop());
                    c = b / a;
                    stack.push(String.valueOf(c));
                    break;
                case "*":
                    a = Integer.parseInt(stack.pop());
                    b = Integer.parseInt(stack.pop());
                    c = b * a;
                    stack.push(String.valueOf(c));
                    break;
                default:
                    if (negative) {
                        s = String.valueOf(Integer.parseInt(s) * -1);
                        negative = false;
                    }
                    stack.push(s);
            }
        }

        return Integer.parseInt(stack.pop());
    }
}