Adding And Subtracting Positive Or Negative Two Numbers Of Any Length With Strings?

You are to design a Java application to carry out additions and subtractions for numbers of any length. A number is represented as an object which includes a sign and two strings for the whole and decimal parts of the number. And, the operations must be done by adding or subtracting characters directly. You are not allowed to convert these strings to numbers before the operation.

The program must use a "Number" class which includes at least the following methods:

Number ( );
Number (double n);
Number add (Number RHS);
Number subtract (Number RHS);
String toString ( );

This is what i have but it only adds positive numbers and it doesn't subtract problems like 7.05-8.96. Also some of it was what our teacher gave us like alignwhole method

import java.util.Scanner; 
public class Number{
private String whole;
private String decimal;
private String sign;
  public static void main (String[] args){
 System.out.println("Enter two numbers");

[code]....