public static void main(String[] args) {

MutiplyString ms = new MutiplyString();

String rst = ms.multiply("0", "456");

System.out.println(rst);

}

public String multiply(String num1, String num2) {

if (num1 == null || num2 == null) {

return null;

}

if (num1.equals("0") || num2.equals("0")) {

return "0";

}

StringBuilder rst = new StringBuilder();

int carry = 0;

for(int i = 0; i < num1.length(); i++) {

for (int j = 0; j < num2.length(); j++) {

int product = Character.getNumericValue(num1.charAt(num1.length() - 1 - i)) * Character.getNumericValue(num2.charAt(num2.length() - 1 - j));

product += carry;

//carry = product / 10;

int index = i + j;

// if it does not exit in the stringBuilder, insert it.

if (index >= rst.length()) {

rst.insert(0, product);

} else {

int sum = product + Character.getNumericValue(rst.charAt(rst.length() - 1 - index));

carry = sum / 10;

sum = sum % 10;

rst.setCharAt(rst.length() - 1 - index, (char)('0' + sum));

if (index >= rst.length() - 1 && carry > 0) {

rst.insert(0, carry);

carry = 0;

}

}

}

}

return rst.toString();

}

}