Add Binary

public static String addBinary( String a, String b ) {
    StringBuilder sb = new StringBuilder();
    int carry = 0;
    for( int i = a.length()-1, j = b.length()-1; i >= 0 || j >= 0; --i, --j ) {
        int val1 = i >= 0 ? a.charAt(i)-'0' : 0;
        int val2 = j >= 0 ? b.charAt(j)-'0' : 0;
        int sum = val1 + val2 + carry;
        carry = sum / 2;
        sb.append( (char)( (sum%2) + '0' ) );
    }
    if( carry != 0 )
        sb.append( (char)( carry + '0' ) );
    return sb.reverse().toString();
}