在Java中添加和减去字符串

Question

我需要一些我应该创建的程序的帮助。我应该创建一个程序，读取用户输入的任意长度的两个字符串，并将它们减去或加在一起。我不允许在操作之前将这些字符串转换为数字。这就是我到目前为止所做的。我的老师提到了将字符串转换为单一代码以添加和减去它们的内容，但我不知道如何做到这一点，因为我们甚至没有学过单代码。这是我的代码：

import java.util.Scanner;
public class Number {
    private char Sign;
    private String Whole;
    private String Fraction;

    public static void main(String[] args) {
        Scanner Keyboard = new Scanner (System.in);
        System.out.println("This program adds or subtracts numbers of any lengths, please add two numbers: ");
        String num1 = Keyboard.nextLine();
        System.out.println("Enter the second number: ");
        String num2 = Keyboard.nextLine();

        String sum = " ";
        int length = num1.length();
        int carry = 0;

        public Number Add(Number RHS) {

            for (int i = length -1 ; i >= 0; i--) {
                char c1 = num1.charAt(i);
                char c2 = num2.charAt(i);

                int tempSum = (c1 - 48) + (c2 - 48) + carry;
                carry = tempSum / 10;

                int sumDigit = tempSum % 10;
                sum = (char) (sumDigit + 48) + sum;

                if (carry == 1) {
                    sum = "1" + sum;
                }
            }
        }
    }
    public Number (double n) {
        Whole = " ";
        Fraction = " ";
        if (n >= 0) {
            Sign = '+';
        }
        else 
        {
            Sign = '-';
            n = Math.abs(n);
            String numString = new Double(n).toString();
            int position = numString.indexOf(".");
        }

    }
}