Multiply Strings

Given two numbers represented as strings, return multiplication of the numbers as a string.
Note: The numbers can be arbitrarily large and are non-negative.

public class MultiplyStrings {

	static String add(String s1, String s2) {
		StringBuffer sb = new StringBuffer();

		int carrier = 0;
		int p1 = s1.length() - 1;
		int p2 = s2.length() - 1;

		while (p1 >= 0 || p2 >= 0) {
			int a = 0;
			int b = 0;

			if (p1 >= 0) {
				a = s1.charAt(p1) - '0';
				p1--;
			}

			if (p2 >= 0) {
				b = s2.charAt(p2) - '0';
				p2--;
			}
			sb.append((a + b + carrier) % 10);
			carrier = (a + b + carrier) / 10;
		}

		return sb.reverse().toString();
	}

	static String mul(String a, int c, int b) {
		StringBuffer sb = new StringBuffer();
		int carrier = 0;
		for (int i = a.length() - 1; i >= 0; i--) {
			int t = (a.charAt(i) - '0') * c;
			sb.append((t + carrier) % 10);
			carrier = (t + carrier) / 10;
		}

		sb.reverse();
		for (int i = 0; i < b; i++)
			sb.append(0);

		return sb.toString();
	}

	public static String multiply(String num1, String num2) {
		String result = "";
		for (int i = 0; i < num2.length(); i++) {
			result = add(result,
					mul(num1, num2.charAt(num2.length() - 1 - i) - '0', i));
		}

		return result;
	}

	public static void main(String[] args) {
		String str1 = "123456789";
		String str2 = "987654321";
		System.out.println("result is"+multiply(str1, str2));
	}
}