Java语言怎么用字符串的方式表示一个表达式，并且计算表达式的值是什么

import java.util.*;

public class ExpressionCalculator {
    public static int calculateExpression(String expression) {
        List<String> rpn = convertToRPN(expression);
        return calculateRPN(rpn);
    }

    private static List<String> convertToRPN(String expression) {
        List<String> rpn = new ArrayList<>();
        Stack<String> stack = new Stack<>();

        for (int i = 0; i < expression.length(); i++) {
            char c = expression.charAt(i);

            if (Character.isDigit(c)) {
                StringBuilder num = new StringBuilder();
                num.append(c);
                while (i + 1 < expression.length() && Character.isDigit(expression.charAt(i + 1))) {
                    num.append(expression.charAt(i + 1));
                    i++;
                }
                rpn.add(num.toString());
            } else if (c == '+' || c == '-' || c == '*' || c == '/') {
                while (!stack.isEmpty() && isHigherPriority(stack.peek(), Character.toString(c))) {
                    rpn.add(stack.pop());
                }
                stack.push(Character.toString(c));
            } else if (c == '(') {
                stack.push(Character.toString(c));
            } else if (c == ')') {
                while (!stack.isEmpty() && !stack.peek().equals("(")) {
                    rpn.add(stack.pop());
                }
                stack.pop();
            }
        }

        while (!stack.isEmpty()) {
            rpn.add(stack.pop());
        }

        return rpn;
    }

    private static boolean isHigherPriority(String operator1, String operator2) {
        if (operator1.equals("*") || operator1.equals("/")) {
            return true;
        } else if (operator1.equals("+") || operator1.equals("-")) {
            return !operator2.equals("*") && !operator2.equals("/");
        }
        return false;
    }

    private static int calculateRPN(List<String> rpn) {
        Stack<Integer> stack = new Stack<>();

        for (String token : rpn) {
            if (Character.isDigit(token.charAt(0))) {
                stack.push(Integer.parseInt(token));
            } else {
                int operand2 = stack.pop();
                int operand1 = stack.pop();
                int result = 0;

                switch (token) {
                    case "+":
                        result = operand1 + operand2;
                        break;
                    case "-":
                        result = operand1 - operand2;
                        break;
                    case "*":
                        result = operand1 * operand2;
                        break;
                    case "/":
                        result = operand1 / operand2;
                        break;
                }

                stack.push(result);
            }
        }

        return stack.pop();
    }

    public static void main(String[] args) {
        String expression = "2 + 3 * 5 - 4";
        int result = calculateExpression(expression);
        System.out.println(result);
    }
}