Java Big Num Processing

package com.DTT;

public class BigNumOperator {

    public static String AddPositiveNum(String a, String b) {
        // normalize two string
        if (a.length() < b.length()) {
            String tmp = a; a = b; b = tmp;
        }
        // make same length
        while(a.length() != b.length()) b = "0" + b;

        int remain = 0;
        StringBuilder res = new StringBuilder();
        for (int i = a.length() - 1; i >= 0; i--) {
            int add = (a.charAt(i) - 48) + (b.charAt(i) - 48) + remain;
            res.append(add % 10);
            remain = add / 10;
        }
        if (remain != 0) res.append(remain);
        return res.reverse().toString();
    }

    public static String MultiplyPositive(String num1, char num2) {
        int remain = 0;
        int num = num2 - 48;
        StringBuilder res = new StringBuilder();
        for (int i = num1.length() - 1; i >= 0; i--) {
            int mul = num * (num1.charAt(i)-48) + remain;
            res.append(mul % 10);
            remain = mul / 10;
        }
        if (remain != 0) res.append(remain);
        return res.reverse().toString();
    }

    public static String MultiplyPositive(String num1, String num2) {
        String res = "";
        for (int i = num2.length() - 1; i >= 0; i--) {
            String mul = MultiplyPositive(num1, num2.charAt(i));
            for (int j = 0; j < num2.length() - 1 - i; j++) mul += "0";
            res = AddPositiveNum(res, mul);
        }
        return res;
    }

    public static String Multiply(String num1, String num2) {
        int op = 1;
        if (num1.charAt(0) == '-') { num1 = num1.substring(1, num1.length()); op *= -1; }
        if (num2.charAt(0) == '-') { num2 = num2.substring(1, num2.length()); op *= -1; }
        String res = MultiplyPositive(num1, num2);
        if (op == -1) res = '-' + res;
        return Normalize(res);
    }

    public static String Normalize(String num) {
        if (num.length() > 1 && num.charAt(0) == '0') return Normalize(num.substring(1, num.length()));
        else return num;
    }

    public static void main(String[] args) {
        String a = "999";
        char b = '9';
        String res = MultiplyPositive(a, b);
        System.out.println("Mul: " + res);

        String add1 = "3";
        String add2 = "333";
        res = AddPositiveNum(add1, add2);
        System.out.println("Add: " + res);

        String big1 = "999";
        String big2 = "012";
        res = MultiplyPositive(big1, big2);
        System.out.println("Mul Big Num: " + res);

        String gen1 = "999";
        String gen2 = "-012";
        res = Multiply(gen1, gen2);
        System.out.println("Mul General: " + res);

        System.out.println("Normalize: " + Normalize("0000"));
    }

}