找到不兼容的类型：java.lang.String

Question

我正在学习java，我写了这个程序，它在我的大学电脑上编译成功，但在我的家庭电脑上没有编译。有谁能帮助我吗？

import java.util.Scanner;

public class Calculator{

    public static void main(String[] args){
    
        Scanner input = new Scanner(System.in);
        
        System.out.println("Please Enter 2 Values");
        
        int userInput1 = input.nextInt();
        int userInput2 = input.nextInt();
        
        System.out.println("Please Enter an Operation");
        
        String operator = input.next();
        if(operator == null){
            return;
        }
        
        int answer = 0;
        
        switch (operator){
            case "+":
                answer = Add(userInput1,userInput2);    
            break;
            case "*":
                answer = Multiply(userInput1,userInput2);       
            break;          
            case "-":
                answer = Subtract(userInput1,userInput2);       
            break;          
            case "/":
                answer = Divide(userInput1,userInput2);     
            break;          
            default:
                System.out.println("Invalid Operator");
                System.exit(0);
            break;
        }
        
    
        System.out.println("The answer is " + answer);
        
    }

    public static int Add(int num1, int num2)
    {
        return num1 + num2;
    }
    
    
    public static int Subtract(int num1, int num2)
    {
        return num1 - num2;
    }

    public static int Multiply(int num1, int num2)
    {   
        return num1 * num2;
    }

    
    public static int Divide(int num1, int num2)
    {   
        return num1 / num2;
    }
    
    
    
    
}

           switch (operator){

                   ^

Answer 1

Java7中引入了在字符串上使用开关。确保您在另一台机器上使用JDK7+来编译程序。

如注释中所述，如果由于某种原因无法升级JDK，则可以使用char:

String operator = input.next();
if(operator == null || operator.isEmpty()) {
   return;
}
int answer = 0;
switch (operator.charAt(0)) {
    case '+':
        answer = Add(userInput1, userInput2);
        break;
    case '*':
        answer = Multiply(userInput1, userInput2);
        break;
    ...