public static void main(String[] strs) {

multiply("9", "9");

}

public static String multiply(String num1, String num2) {

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

return null;

}

int len1 = num1.length();

int len2 = num2.length();

int[] product = new int[len1 + len2];

// 计算相应位置的product.

for (int i = 0; i < len1; i++) {

for (int j = 0; j < len2; j++) {

product[i + j] = (num1.charAt(len1 - 1 - i) - '0') * (num2.charAt(len2 - 1 - j) - '0');

}

}

StringBuilder ret = new StringBuilder();

int carry = 0;

for (int i = 0; i < len1 + len2; i++) {

product[i] = product[i] + carry;

int digit = product[i] % 10;

carry = digit / 10;

ret.insert(0, digit);

}

while (ret.length() > 1 && ret.charAt(0) == '0') {

ret.deleteCharAt(0);

}

return ret.toString();

}