具有特定方法和多个类的简单计算器 - Java

问：

我正在尝试制作一个使用特定方法和类的计算器。给出了以下方法：

public class App {

    private static final Scanner inputScanner = new Scanner(System.in);

    private static String getString(String prompt) {  }

    private static double getDouble(String prompt) {  }
}

它必须有一个单独的类，其中包含 add、sub、mult、divide 的代码。Calculator2

该程序应如下所示：

What would you like to do? 
Add
Subtract 
Multiply 
Divide
Exit
*user inputs*
What is the first number:
*user inputs*
What is the second number:
*user inputs*
Result:

我真的在为如何调用类而苦苦挣扎，尤其是 String 方法的特定要求。我尝试了多种方法（、、只要求第一个字母使其成为字符等），但没有任何效果。我最近的尝试是尝试获取用户输入并将其转换为 char，以便我可以使用开关方法。任何帮助将不胜感激。Calculator2else ifswitch

import java.util.Scanner;


public class App {

    private static final Scanner inputScanner = new Scanner(System.in);

    public static void main(String[] args) {
        System.out.println("What would you like to do?");
        System.out.println("add");
        System.out.println("subtract");
        System.out.println("multiply");
        System.out.println("divide");
        System.out.println("exit");
        String operator = inputScanner.nextLine();
        Double userInput = getDouble();
        Double math = getString();
    }

    private static double getString() {
        char choice = operator.charAt(0);
        switch (choice) {
            case 'a':
                calculator2Object.add();
                System.out.println();
                return result = operator;
                break;
            case 's':
                calculator2Object.subtract();
                System.out.println();
                break;
            case 'm':
                calculator2Object.multiply();
                System.out.println();
                break;
            case 'd':
                calculator2Object.divide();
                System.out.println();
                break;
            case 'e':
                System.out.println("Goodbye");
                System.exit(0);
        }
    }

    private static double getDouble() {
       System.out.println("Enter the first number: ");
       double firstNumber = inputScanner.nextDouble();
       System.out.println("Enter the second number: ");
       double secondNumber = inputScanner.nextDouble();
       return firstNumber + secondNumber;
    }
}

这是课程：Calculator2

public class Calculator2 {
        double add(double firstNumber, double secondNumber) {
            double result = firstNumber + secondNumber;
            System.out.println(result);
            return result;
        }

        double subtract(double firstNumber, double secondNumber) {
            double result = firstNumber - secondNumber;
            System.out.println(result);
            return result;
        }

        double multiply(double firstNumber, double secondNumber) {
            double result = firstNumber * secondNumber;
            System.out.println(result);
            return result;
        }

        double divide(double firstNumber, double secondNumber) {
            double result = firstNumber / secondNumber;
            System.out.println(result);
            return result;
        }
    }

这些是我的错误：

cannot find symbol variable operator
cannot find symbol variable calculator2Object
cannot find symbol variable result