识别底片的表达式评估器

Question

我有一个表达式评估程序，但问题是它没有识别出-12 / -4的否定，我的IDE认为它除以零。我测试了我已经拥有的类似代码，似乎它以某种方式将12改为0，我不知道在哪里。任何人都可以提供的帮助将非常感激！

import java.util.Scanner;
class Expressions {
    String e;

    void setExpressions(String ee) {
        e = ee;
    }

    String getE() {
        return e;
    }

    //evaluate arithemetic expressions e
    //assuming e has only *, /, and operands.
    //12*345/3/72*890
    int evalMD(String e) {
        int r = 1, n = 0;
        char op = '*';
        for (int i = 0; i < e.length(); i++)
            if (e.charAt(i) == '*' || e.charAt(i) == '/') {
                if (op == '*') 
                    r *= n;
                else 
                    r /= n; 
                n = 0;
                op = e.charAt(i);
            }
            else //digits
                n = 10*n+(e.charAt(i)-'0');

        if (op == '*') //last number
            r *= n;
        else 
            r /= n; 

        return r;
    }

    //evaluate arithemetic expressions e
    //assuming e has +, -, *, /, and operands.
    //12*3-34/5*56+3-72
    int evalE() {
        int r = 0;
        String n = "";
        char op = '+';
        for (int i = 0; i < e.length(); i++)
            if (e.charAt(i) == '+' || e.charAt(i) == '-') {
                if (op == '+') 
                    r += evalMD(n);
                else 
                    r -= evalMD(n); 
                n = "";
                op = e.charAt(i);
            }
            else //digits
                n += e.charAt(i);

        if (op == '+') //last number
            r += evalMD(n);
        else 
            r -= evalMD(n); 

        return r;
    }
}

    public class Lab13 {
    public static void main(String[] args) {
        Expressions myE = new Expressions();
        Scanner in = new Scanner(System.in);

        System.out.print("Enter expression(Type q to stop): ");
        String e = in.nextLine();
        while (e.charAt(0) != 'q' && e.charAt(0) != 'Q') {
            myE.setExpressions(e);
            System.out.println(myE.evalE());

            System.out.print("Enter expression(Type q to stop): ");
            e = in.nextLine();
        }
        System.out.println("Thank you for using my program!!!");
    }
}