如何从另一个范围调用变量？

Question

我正在学习 Java。我来自 python，所以范围不是最大的问题（至少对我来说是直截了当的）。我正在尝试制作一个简单的收据程序。我需要在收据末尾的方法 ratingService() 中调用 qualityOfService（我在我想要的地方添加了评论）。我该怎么做呢？我已经阅读了有关范围的其他地方，但这并没有真正的帮助。对不起我的格式，我知道阅读写得很糟糕的代码是多么烦人。提前致谢。

import java.util.Scanner;

class Main 
    {
    public static void main(String[] args) 
    {
        Scanner userInput = new Scanner(System.in);

        System.out.println("Enter your first name: ");
        String firstName = userInput.nextLine();

        System.out.println("Enter the cost of your meal: ");
        double mealCost = userInput.nextDouble();
        String dummy = userInput.nextLine();

        System.out.println("Enter the percentage of tip you want to leave: ");
        double tipPercentage = userInput.nextDouble();
        String dummy1 = userInput.nextLine();

        System.out.println("Enter the quality of service: ");
        int qualityOfService = userInput.nextInt();

        // calculate values
        double tipAmount = (double)mealCost * ((double)tipPercentage/100);
        double totalCost = (double)mealCost + (double)tipAmount;
        int numberOfTwentyDollarBills = (int)totalCost / 20;   

        // 
        System.out.println("Thank you " + firstName + " for choosing our resturant!");
        System.out.println("=========================================");
        System.out.println("Cost of Meal = " + mealCost);
        System.out.println("Percentage of tip = " + tipPercentage);
        System.out.println("Tip amount = " + tipAmount);
        System.out.println("Total cost of meal = " + totalCost);
        System.out.println("Total number of twenty dollar bills = " + numberOfTwentyDollarBills);
        System.out.println("Rating of your service = " + qualityOfService);
        System.out.println("=========================================");
        // calling ratingService() goes here.
    }

    // service rating method 
    public static void ratingservice() 
    {
        int qualityOfService = 1;
        if (qualityOfService == 1) {
            System.out.println("We are sorry you weren't happy with your serivce. Please let staff know if you have any concerns");
        }
        else if (qualityOfService == 2) {
            System.out.println("We are sorry you weren't happy with your serivce. Please let staff know if you have any concerns");
        }
        else if (qualityOfService == 3) {
            System.out.println("We are happy that you enjoyed your stay somewhat, please let staff know if you have any concerns.");
        }
        else if (qualityOfService == 4) {
            System.out.println("We are glad you enjoyed your stay! Please let our staff know if you have any concerns");
        }
        else {
            System.out.println("We are glad you enjoyed your stay! Please let our staff know if you have any concerns");
        }
    }
}

Answer 1

您需要在ratingservice 方法上设置qualityOfService 参数。

public static void ratingservice(int qualityOfService) {...}

然后像这样在第一个方法中调用它：

ratingService(qualityOfService)

---

这是因为在 Java 中，我们必须将方法所需的所有变量作为参数传递给方法。每个方法都有自己的变量作用域，为了将变量从一个作用域（第一种方法）传递到另一个作用域（第二种方法），您必须将其作为参数传递。

另一种解决方案是在您的类中创建一个实例变量，但由于这些是静态方法，因此不起作用。一个实例变量可以被一个类的所有实例（非静态）方法使用。

Answer 2

这是因为您在main() 方法中声明 qualityOfService，因此它仅在该方法中可用，如果您希望它在其他地方可用，则必须通过它（这个是“范围”的意思）。建议的解决方案的另一个选择是将其设置在类级别，如下所示：

class Main {

    static int qualityOfService;

    public static void main(String[] args) 
    {
    ...stuff...
    qualityOfService = userInput.nextInt();
    ...
    }

    public static void ratingservice() 
    {
        if (qualityOfService == 1) {
            System.out.println("We are sorry you weren't happy with your serivce. Please let staff know if you have any concerns");
        }
        ...etc...
    }
}

在这种情况下，因为您在类级别声明它，即使直到您在 main() 方法中才初始化它，它也是可在类的任何（静态）方法中使用。