怎么用JAVA做加减混合运算?

没分数..叫人怎么给你做..

import java.util.Scanner;

public class HybridOperation {

public static void main(String[] args) {

Scanner input = new Scanner(System.in);
System.out.println("输入一个表达式:");
String s = input.nextLine().trim();
int n = s.length();

int sum = 0;
int p = -1;
char a = '+';

for (int i = 1; i < n; i++) {
char ch = s.charAt(i);
if (ch == '+' || ch == '-') {
if (a == '+') {
sum += Integer.parseInt(s.substring(p + 1, i));
a = ch;
p = i;
} else if (a == '-') {
sum -= Integer.parseInt(s.substring(p + 1, i));
a = ch;
p = i;
}
}
}
if (a == '+') {
sum += Integer.parseInt(s.substring(p + 1));
}
if (a == '-') {
sum -= Integer.parseInt(s.substring(p + 1));
}
System.out.println(s+" = "+sum);
} <