我不明白这段代码最后那个运算判断的意义

Question

import java.util.Scanner;
    public class Calc{
        public static void main(String[]args){
            Scanner input=new Scanner(System.in);//create the Scanner objects
            System.out.println("------Welcome to the calculator------");
            System.out.println("Please enter the first number:");
            int firstNumber=input.nextInt();
            System.out.println("Please enter the second number:");
            int secondNumber=input.nextInt();
            System.out.println("Please enter the operators(+,-,*,/,%):");
            String oper=input.next();
            int result=0;
            boolean flag=true;

            switch(oper){
                case"+":
                    result=firstNumber+secondNumber;
                    break;
                case"-":
                    result=firstNumber-secondNumber;
                    break;
                case"*":
                    result=firstNumber*secondNumber;
                    break;
                case"/":
                    if(secondNumber!=0){
                    result=firstNumber/secondNumber;
                    }else{
                    flag=false;
                    }
                    break;
                case"%":
                    result=firstNumber%secondNumber;
                    break;
                default:
                    flag=false;
                    System.out.println("Sorry, the operator you enter is wrong");
                    break;

            }
            //判断是否可以计算，并输出运算结果
            if(flag){
                System.out.println(firstNumber+oper+secondNumber+"="+result);
            }
        }
    }

为啥最后要加一个if判断类

Answer 1

如果输入的符号在加减乘除等之外，比如输入1=2，则进入flag=0的分支，输出无法判断
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