计算器 输入式子计算结果 （字符串、抛异常）

import java.util.Scanner;

public class JiSuanQi {
    public static void execute(int first_number, int second_number)
            throws MyException {
        // 手写异常不能让输入的数为负数
        if (first_number < 0 || second_number < 0) {
            throw new MyException("参数不能为负数");
        }
    }

    public static void main(String[] args) {
        for (;;) {
            Scanner input = new Scanner(System.in);
            System.out.println("请输入式子");
            String shizi = input.nextLine();
            String[] allfuhao = { "+", "-", "*", "/" };
            jiSuan(shizi, allfuhao);

        }
    }

    private static void jiSuan(String shizi, String[] allfuhao) {
        for (int i = 0; i < allfuhao.length; i++) {
            for (int j = 0; j < shizi.length(); j++) {
                String fu = String.valueOf(shizi.charAt(j));
                if (allfuhao[i].equals(fu)) {
                    try {
                        String first = shizi.substring(0, j);
                        Integer first_number = Integer.parseInt(first);
                        String second = shizi.substring(j + 1);
                        Integer second_number = Integer.parseInt(second);
                        execute(first_number, second_number);
                        if (i == 0) {
                            System.out.println(shizi + "="
                                    + (first_number + second_number));
                        } else if (i == 1) {
                            System.out.println(shizi + "="
                                    + (first_number - second_number));
                        } else if (i == 2) {
                            System.out.println(shizi + "="
                                    + (first_number * second_number));
                        } else if (i == 3) {

                            System.out.println(shizi + "="
                                    + (first_number / second_number));
                        }

                    } catch (MyException e) {
                        System.out.println("不允许使用负数");
                    } catch (NumberFormatException e) {
                        System.out.println("不是数字");
                    } catch (ArithmeticException e) {
                        System.out.println("除数不能为0");
                    }
                    return;
                }

            }

        }
        System.out.println("没找到运算符号，重新输入");
    }
}

public class MyException extends Exception {
    
    public MyException(){
        super();
    }
    public MyException(String Exception ){
        super(Exception);
        
    }
}