java - 程序从 switch java 中断

Question

由于某种原因，当我在切换菜单上输入“1”时，没有任何反应，但程序不会终止。与选项 2-5 相同。默认选项工作得很好。谁能帮我这个？谢谢

代码:

import java.util.Scanner;
public class ConversionTrial {
public static void main(String[] args) {
    double pound;
    double euro;
    double dollars;
    double yen;
    double rupees;
    double poundEuro;
    double poundDollars;
    double poundYen;
    double poundRupees;
    int choice;

    Scanner input = new Scanner(System.in);
    Scanner exchange = new Scanner(System.in);

    menu: while(true) { 
        System.out.println("Please choose an option:");
        System.out.println("1.  Enter values");
        System.out.println("2.  Euros (1GBP = 1.28EUR)");
        System.out.println("3.  Dollars (1GBP = 1.51USD)");
        System.out.println("4.  Yen (1GBP = 179.80JPY)");
        System.out.println("5.  Rupees (1GBP = 95.60INR)");
        System.out.println("6.  Exit");

        choice = input.nextInt();

        switch(choice){
            case -1:
            case 6:
                break menu;
            case 1:
                    pound = exchange.nextDouble();  
                    System.out.print("Please enter your values you would like to exchange:");
                break;
            case 2:
                pound = exchange.nextDouble();
                euro = 1.28;
                poundEuro = pound * euro; 
                System.out.println("Your amounts in Euros are" + poundEuro);
            case 3:
                pound = exchange.nextDouble();
                dollars = 1.51;
                poundDollars = pound * dollars; 
                System.out.println("Your amounts in Dollars are" + poundDollars);
            case 4:
                pound = exchange.nextDouble();
                yen = 1.28;
                poundYen = pound * yen; 
                System.out.println("Your amounts in Yen are" + poundYen);
            case 5:
                pound = exchange.nextDouble();
                rupees = 1.28;
                poundRupees = pound * rupees; 
                System.out.println("Your amounts in Rupees are" + poundRupees);
            default:
                System.out.println("You must enter an option between 1 and 6!");

        }
    }
    input.close();
    exchange.close();
}
}

Answer 1

首先，不要创建两个扫描仪对象。只需创建一个并使用它即可。

其次，在选项 1-5 中，您在向用户输出任何内容之前等待输入，因此这可能就是它似乎不起作用的原因。您应该添加预期值的提示。

第三，在案例 2-5 的末尾缺少 break;。

第四，使用标签通常不是最好的做事方式。它最终可能会产生一些难以阅读的代码。更好的方法是使用一个标志变量，boolean exit = false;。然后，您的 while 循环将基于它不为 true 进行循环，while(!exit)。在你的情况6:中，exit = true;

第五，当用户没有选择退出时，为什么会有 -1 退出？我会删除它。

import java.util.Scanner;
public class ConversionTrial{
    public static void main(String[] args) {
        double pound;
        double euro;
        double dollars;
        double yen;
        double rupees;
        double poundEuro;
        double poundDollars;
        double poundYen;
        double poundRupees;
        int choice;

        Scanner input = new Scanner(System.in);

        boolean exit = false;

        while(!exit) { 
            System.out.println("Please choose an option:");
            System.out.println("1.  Enter values");
            System.out.println("2.  Euros (1GBP = 1.28EUR)");
            System.out.println("3.  Dollars (1GBP = 1.51USD)");
            System.out.println("4.  Yen (1GBP = 179.80JPY)");
            System.out.println("5.  Rupees (1GBP = 95.60INR)");
            System.out.println("6.  Exit");

            choice = input.nextInt();

            switch(choice){
                case 6:
                    exit = true;
                    break;
                case 1:
                    System.out.print("Please enter your values you would like to exchange: ");
                    pound = input.nextDouble();
                    break;
                case 2:
                    System.out.print("Please enter your values you would like to exchange: ");
                    pound = input.nextDouble();
                    euro = 1.28;
                    poundEuro = pound * euro; 
                    System.out.println("Your amounts in Euros are " + poundEuro);
                    break;
                case 3:
                    System.out.print("Please enter your values you would like to exchange: ");
                    pound = input.nextDouble();
                    dollars = 1.51;
                    poundDollars = pound * dollars; 
                    System.out.println("Your amounts in Dollars are " + poundDollars);
                    break;
                case 4:
                    System.out.print("Please enter your values you would like to exchange: ");
                    pound = input.nextDouble();
                    yen = 1.28;
                    poundYen = pound * yen; 
                    System.out.println("Your amounts in Yen are " + poundYen);
                    break;
                case 5:
                    System.out.print("Please enter your values you would like to exchange: ");
                    pound = input.nextDouble();
                    rupees = 1.28;
                    poundRupees = pound * rupees; 
                    System.out.println("Your amounts in Rupees are " + poundRupees);
                    break;
                default:
                    System.out.println("You must enter an option between 1 and 6!");
                    break;
            }
        }
        input.close();
    }
}

编辑:作为奖励，我还注意到选项 1 实际上没有执行任何操作。这是故意的吗？为了使代码更清晰，我会在定义变量时初始化转换变量的值，而不是每次使用它们时。您还可以在菜单中使用这些值，因此如果值发生变化，只需更改一次。