Add Binary

public class Solution {    public String addBinary(String a, String b) {        if (a == null || b == null) {    throw new IllegalArgumentException("");    }    int i = a.length() - 1, j = b.length() - 1, carry = 0;    StringBuilder sb = new StringBuilder();    while (i >=0 || j >= 0 || carry != 0) {    int numA = 0;    if (i >= 0)     numA = a.charAt(i) - '0';    int numB = 0;    if (j >= 0)     numB = b.charAt(j) - '0';    int sum = (numA + numB + carry) % 2;    carry = (numA + numB + carry) / 2;    sb.insert(0, sum);    i--;    j--;    }    return sb.toString();    }}