Adding and subtracting string numbers in Java

Question

I'm supposed to create a program where a user enters two numbers that could be negative or positive and could contain decimal places or not. Theoretically, when you add say "256.78 + 78.6783" it is supposed to carry the one like a normal addition problem and finish the operation.

I have figured out how to add numbers of any length only when they are positive which took me FOREVER, but when I add negative numbers or even subtract the number I don't get the correct result. This is supposed to work with any set of two numbers that a user enters.

Here is my code so far, any suggestions?
P.S. I'm NOT allowed to convert these numbers to int's or double's before the operation so Parsing them is out of the question.

public class Number {
    static Scanner kbd = new Scanner (System.in);
    private String sign; 
    private String whole;
    private String decimal;
    private String fraction;
    private static double firstNumber;
    private static double secondNumber;

    public static void main(String[] args) {

        System.out.println("Please enter the first number: ");
        firstNumber = kbd.nextDouble();
        System.out.println("Next, enter the second number: ");
        secondNumber = kbd.nextDouble();

        Number x = new Number (firstNumber);
        Number y = new Number (secondNumber);
        Number sum = x.add(y);
        System.out.println("x = " + x);
        System.out.println("y = " + y);
        System.out.println("x + y = " + sum);
        Number subtract = x.subtract(y);
        System.out.println("x - y = " + subtract);
    }
    public Number() 
    {
        whole = "0";
        decimal = "0";
        sign = "+";
    }
    public String toString() 
    {
        return sign + whole + "." + decimal;
    }
    public Number (double n) 
    {
        whole = "0";
        decimal = "0";
        sign = "+";

        String numString = new Double(n).toString();
        if (numString.charAt(0) == '-') {
            sign ="-";
            numString = numString.substring(1);
        }
        int position = numString.indexOf(".");
        if (position == -1)
            whole = numString;
        else
        {
            whole = numString.substring(0,position);
            decimal = numString.substring(position+1);
            fraction = "";
        }
    }

    public Number add (Number RHS) {
        this.fixWhole (RHS);
        this.fixDecimal(RHS);
        return this.addNum (RHS);
    }
    public Number subtract (Number RHS) {
        this.fixWhole(RHS);
        this.fixDecimal(RHS);
        return this.subtractNum (RHS);
    }
    private void fixWhole (Number RHS) {
        int firstWholeNum = this.whole.length();
        int secondWholeNum = RHS.whole.length();
        int difference = firstWholeNum - secondWholeNum;
        if (difference > 0) {
            for (int i = 1; i <= difference; i++) 
                RHS.whole = "0" + RHS.whole;
        }
        else if (difference < 0 ) {
            difference = Math.abs(difference);
            for (int i = 1; i <= difference; i++)
                this.whole = "0" + this.whole;
        }
    }

    private void fixDecimal  (Number RHS ) {
        int firstDecimalNum = this.decimal.length();
        int secondDecimalNum = RHS.decimal.length();
        int difference = firstDecimalNum - secondDecimalNum;

        if (difference > 0) {
            for (int i = 1; i <=  difference; i++)
                RHS.decimal = RHS.decimal + "0";
        }
        else if (difference < 0 ) 
        {
            difference = Math.abs(difference);
            for (int i = 1; i <= difference; i ++)
                this.decimal = this.decimal + "0";
        }
    }

    private Number addNum (Number RHS ) {
        Number sum = new Number();
        sum.decimal = "";
        int carry = 0;
        int decimalNum = this.decimal.length();
        for (int i = decimalNum - 1; i >= 0; i --) {
            char c1 = this.decimal.charAt(i); 
            char c2 = RHS.decimal.charAt(i); 
            int tempSum= (c1 - 48) + (c2 - 48) + carry;
            carry =  tempSum/ 10;
            int sumDigit = tempSum % 10;
            sum.decimal = (char) (sumDigit + 48) + sum.decimal;
        }
        sum.whole = "";
        int wholeNum = this.whole.length();
        for (int i = wholeNum - 1; i >= 0; i --) {
            char c1 = this.whole.charAt(i);
            char c2 = RHS.whole.charAt(i);
            int tempSum = (c1 - 48) + (c2 - 48 ) + carry;
            carry = tempSum / 10;
            int sumDigit = tempSum % 10;
            sum.whole = (char) (sumDigit + 48) + sum.whole;
        }
        if (carry != 0) 
            sum.whole = "1" + sum.whole;
        return sum;
    }

    private Number subtractNum (Number RHS ) {
        Number sum = new Number();
        sum.decimal = "";
        int carry = 0;
        int decimalNum = this.decimal.length();
        for (int i = decimalNum - 1; i >= 0; i --) {
            char c1 = this.decimal.charAt(i); 
            char c2 = RHS.decimal.charAt(i); 
            int tempSum= (c1 - 48) - (c2 - 48) - carry; 
            carry =  tempSum/ 10;
            int sumDigit = tempSum % 10;
            sum.decimal = (char) (sumDigit - 48) + sum.decimal;
        }
        sum.whole = "";
        int wholeNum = this.whole.length();
        for (int i = wholeNum - 1; i >= 0; i --) {
            char c1 = this.whole.charAt(i);
            char c2 = RHS.whole.charAt(i);
            int tempSum = (c1 - 48) - (c2 - 48 ) + carry;
            carry = tempSum / 10;
            int sumDigit = tempSum % 10;
            sum.whole = (char) (sumDigit + 48) + sum.whole;
        }
        if (carry != 0) 
            sum.whole = "1" + sum.whole;
        return sum;
    }
}

Answer 1

take both the numbers as strings and store the signs into the sign string into the corresponding Numbers objects and call your method like

System.out.println("Please enter the first number: ");
        firstNumber = kbd.nextString();
        System.out.println("Next, enter the second number: ");
        secondNumber = kbd.nextString();

        Number x = new Number (firstNumber.substring(1),firstNumber.charAt(0));
        Number y = new Number (secondNumber.substring(1),secondNumber.charAt(0));

/*convert the firstNumber.substring(1) and secondNumber.substring(1) to doubles using  Double.parseDouble()*/

public String doTheOperation(Number other){
 if(this.sign.equals(otherNumber.sign)){
  /*simply the double values and put the sign*/ in front of it and return it
 }

 else{
  do the simple double subtraction and by looking at your code i believe you can find out the bigger double among them
 }
}