leetcode 上的问题，字符串有size方法？

[qq_28273155]

Given a column title as appear in an Excel sheet, return its corresponding column number.

For example:

A -> 1
B -> 2
C -> 3
...
Z -> 26
AA -> 27
AB -> 28

还是leetcode上面的问题，小白我完全蒙了，看答案是这样的：

public class Solution {
public int titleToNumber(String s) {
int sum = 0;
char c;
String tmp = s;
int slength = 0;
while(!tmp.isEmpty()) {
slength = tmp.length();
c = tmp.charAt(0);
tmp = tmp.subString(1,slength);
sum = sum*26+(c-'A'+1);
}
return s;
}
}

char 在这里为什么可以做加减运算？