public String multiply(String num1, String num2) {
    	if (num1 ==null || num2 ==null ) return null;
    	if ("".equals(num1.trim()) || "".equals(num2.trim())) return "";
    	if ("0".equals(num1) || "0".equals(num2)) return "0";
    	return doMultiply(num1,num2);
    }