java - 如何修复有关 switch 语句的输出？

Question

我的程序输出遇到一些问题。应该发生的是用户要选择他们想要使用的方法。然后，程序将根据该选择执行 switch 语句，并为用户输入的任何方法和值输出相应的和或乘积。

但是，程序显示菜单选择，但仅输出错误消息。我遇到这个错误已经有一段时间了，但老实说我不知道​​我做错了什么。请您原谅我的无知，因为我编程时间不长。

import java.util.Scanner;
public class Sum {
    public static void main(String[] args)
    {
        String sum = "S";
        String factorial = "F";

        Scanner scan = new Scanner(System.in);
        System.out.println("Enter " + sum + " to use the sum method or " + factorial + " to use the factorial method.");
        String userSelection = scan.next();

        switch (userSelection.toLowerCase()) {
            case "sum":
                System.out.println("Enter the number you want to use: ");
                int userValue = scan.nextInt();
                int userSum = sum(userValue);
                System.out.println("The resulting value for the sum method is: " + userSum);
                break;

            case "fact":
                System.out.println("Enter the number you want to use: ");
                int userFact = scan.nextInt();
                int userFactorial = fact(userFact);
                System.out.println("The resulting value for the factorial method is: " + userFactorial);
                break;
            default:
                System.out.println("ERROR: Invalid choice.");
            }
            scan.close();
    }
    public static int sum(int num) {
        int sum = 0;
        for (int i = 1; i <= num; i++) {
            sum = i + sum;
        }
        return sum;
    }
    public static int fact(int num) {
        int fact = 1;
        for (int i = 1; i <= num; i++) {
            fact = i * fact;
        }
        return fact;
    }

Answer 1

您正在将用户输入与错误的字符串进行比较。您要求他输入“S”或“F”，然后将此输入与“sum”和“fact”进行比较。