Здравствуйте! Нужна помощь с заданием. 2 и 3 пункт

public class Calulator {
    public static String ExpressionToRPN(String Expr) {
        String current = "";
        Stack<Character> stack = new Stack<>();
        int priority;
        for (int i = 0; i < Expr.length(); i++) {
            priority = getP(Expr.charAt(i));
            if (priority == 0) current += Expr.charAt(i);
            if (priority == 1) stack.push(Expr.charAt(i));
            if (priority > 1) {
                current += ' ';
                while (!stack.empty()) {
                    if (getP(stack.peek()) >= priority) current += stack.pop();
                    else break;
                }
                stack.push(Expr.charAt(i));
            }
            if (priority == -1) {
                current += ' ';
                while (getP(stack.peek()) != 1) current += stack.pop();
                stack.pop();
            }
        }
        while (!stack.empty()) current += stack.pop();

        return current;
    }

    public static double RPNtoAnswer(String rpn) {
        String operand = new String();
        Stack<Double> stack = new Stack<>();
        for (int i = 0; i < rpn.length(); i++) {
            if (rpn.charAt(i) == ' ') continue;
            if (getP(rpn.charAt(i)) == 0) {
                while (rpn.charAt(i) != ' ' && getP(rpn.charAt(i)) == 0) {
                    operand += rpn.charAt(i++);
                    if (i == rpn.length())
                        break;
                    stack.push(Double.parseDouble(operand));
                    operand = new String();
                }
            }
            if (getP(rpn.charAt(i)) > 1) {
                double a = stack.pop(), b = stack.pop();
                if (rpn.charAt(i) == '+') stack.push(b + a);
                if (rpn.charAt(i) == '-') stack.push(b - a);
                if (rpn.charAt(i) == '*') stack.push(b * a);
            }
        }


        return stack.pop();
    }

    private static int getP(char token) {
        if (token == '*' || token == '/') return 3;
        else if (token == '+' || token == '-') return 2;
        else if (token == '(') return 1;
        else if (token == ')') return -1;
        else return 0;
    }



}