Simple expression parser that supports add and sub.

ParseExp.java

public class ParseExp {
    public static void main(String[] args) {
        int n = 0;
        String exp = "123+44-21";
        int s = 0;
        int result = 0;
        char opCh = '+';
        for(int i = 0; i < exp.length(); ++i){
            char ch = exp.charAt(i);
            boolean num = ch >= '0' && ch <= '9';
            boolean op = ch == '+' || ch == '-';
            switch(s){
                case 0:// first
                    if(!num){
                        System.out.printf("error %s on %d%n", ch, i);
                        return;
                    }
                    n = ch - '0';
                    s = 1;
                    break;
                case 1:// num
                    if(num){
                        n = n * 10 + (ch - '0');
                    } else if (op){
                        if(opCh == '+'){
                            result += n;
                        }else if(opCh =='-'){
                            result -= n;
                        } else {
                            System.out.printf("error %s on %d%n", ch, i);
                            return;
                        }
                        opCh = ch;
                        s = 0;
                    } else {
                        System.out.printf("error %s on %d%n", ch, i);
                        return;
                    }
                    break;
            }
        }
        switch(s){
            case 0:
                System.out.println("error terminated.");
                return;
            case 1:
                if(opCh == '+'){
                    result += n;
                }else if(opCh == '-'){
                    result -= n;
                }
                break;
        }
        System.out.println(123+44-21);
        System.out.println(result);
    }
}