如何在选定的场景中实现按值传递？

Question

我在项目中实现按值传递时遇到一个小问题。在 buyOption、fillOption 和 takeOption 的情况下。

我不知道如何将更改后的值应用于整个程序而不仅仅是方法。任何 tips/solution/feedback 将不胜感激。

public class CoffeeMachine {

    static int water = 400;
    static int milk = 540;
    static int beans = 120;
    static int cups = 9;
    static int money = 550;
    static boolean exit = false;

    public static void main(String[] args) {
        do {
            printMenu(water, milk, beans, cups, money);
        } while (!exit);
    }

    private static void printMenu(int water, int milk, int beans, int cups, int money) {
        Scanner scanner = new Scanner(System.in);

        System.out.println("Write action (buy, fill, take, remaining, exit): ");
        String input = scanner.nextLine();

        switch (input) {
            case "buy":
                buyOption(water, milk, beans, cups, money);
                break;

            case "fill":
                fillOption(water, milk, beans, cups, money);
                break;

            case "take":
                takeOption(water, milk, beans, cups, money);
                break;

            case "remaining":
                contentOption(water, milk, beans, cups, money);
                break;

            case "exit":
                exit = true;
                System.exit(0);
                break;

            default:
                System.out.println("Wrong option!");
                break;
        }
    }

    private static void contentOption(int water, int milk, int beans, int cups, int money) {
        System.out.println("The coffee machine has: ");
        System.out.println(water + " of water");
        System.out.println(milk + " of milk");
        System.out.println(beans + " of coffee beans");
        System.out.println(cups + " of disposable cups");
        System.out.println(money + " of money");
    }

    private static void buyOption(int water, int milk, int beans, int cups, int money) {
        Scanner scanner = new Scanner(System.in);

        String temp = null;

        System.out.println("What do you want to buy? 1 - espresso, 2 - latte, 3 - cappuccino: ");

        String input = scanner.nextLine();

        switch (input) {
            case "1":
                if (water >= 250 && beans >= 16 && cups >= 1) {
                    water -= 250;
                    beans -= 16;
                    cups -= 1;
                    money += 4;
                    System.out.println("I have enough resources, making you a coffee!");
                } else {
                    if (water < 250) {
                        temp = "water";
                    } else if (beans < 16) {
                        temp = "beans";
                    } else if (cups < 1) {
                        temp = "cups";
                    }
                    System.out.println("Sorry, not enough " + temp + "!");
                }
                break;

            case "2":
                if (water >= 350 && milk >= 75 && beans >= 20 && cups >= 1) {
                    water -= 350;
                    milk -= 75;
                    beans -= 20;
                    cups -= 1;
                    money += 7;
                    System.out.println("I have enough resources, making you a coffee!");
                } else {
                    if (water < 350) {
                        temp = "water";
                    } else if (milk < 75) {
                        temp = "milk";
                    } else if (beans < 20) {
                        temp = "beans";
                    } else if (cups < 1) {
                        temp = "cups";
                    }
                    System.out.println("Sorry, not enough " + temp + "!");
                }
                break;

            case "3":
                if (water >= 200 && milk >= 100 && beans >= 12 && cups >= 1) {
                    water -= 200;
                    milk -= 100;
                    beans -= 12;
                    cups -= 1;
                    money += 6;
                    System.out.println("I have enough resources, making you a coffee!");
                } else {
                    if (water < 200) {
                        temp = "water";
                    } else if (milk < 100) {
                        temp = "milk";
                    } else if (beans < 12) {
                        temp = "beans";
                    } else if (cups < 1) {
                        temp = "cups";
                    }
                    System.out.println("Sorry, not enough " + temp + "!");
                }
                break;

            case "back":
                break;

            default:
                System.out.println("Wrong option!");
        }
    }

    private static void fillOption(int water, int milk, int beans, int cups, int money) {
        Scanner scanner = new Scanner(System.in);

        System.out.println("Write how many ml of water do you want to add: ");
        water = water + scanner.nextInt();
        System.out.println("Write how many ml of milk do you want to add: ");
        milk = milk + scanner.nextInt();
        System.out.println("Write how many grams of coffee beans do you want to add: ");
        beans = beans + scanner.nextInt();
        System.out.println("Write how many disposable cups of coffee do you want to add: ");
        cups = cups + scanner.nextInt();
    }

    private static void takeOption(int water, int milk, int beans, int cups, int money) {
        System.out.println("I gave you " + money);
        money = 0;
    }
}

Answer 1

Java 总是 pass-by-value。这里的问题是您正在使用隐藏静态成员的局部变量（在每个方法中声明的参数）。您可以简单地省略方法声明中的所有参数：这样您将“使用”您在开始时声明的静态成员。

编辑

private static void takeOption(int water, int milk, int beans, int cups, int money) {
   //sample code
   milk = milk+1;
}

在这种情况下，您使用比方说“牛奶”进行的每个操作都只会存在于此方法中。您的参数名为 milk，但它与在您的 class 上声明的“milk”没有任何共享。他们的名字相同只是巧合。您可以使用 this.milk（或 CoffeeMachine.milk，因为它是一个静态字段）来引用您的 static milk，但是由于您目前正在编写代码，所以如果不编辑您的代码，我看不出有什么用方法进一步。

private static void takeOption() {
   //code here
}

像这样编辑方法签名将使您使用静态变量。

Answer 2

为了影响您为 CoffeeMachine 设置的静态变量。

您必须在方法中直接引用静态变量：

所以不只是使用

water -= 250;

只影响参数（局部作用域）

您将不得不使用：

CoffeeMachine.water -= 250;

如前所述，字段不应该是静态的，但我假设您此时正在学习 java。

注意： 出于您的目的，您可能不需要方法中的参数，因为您正在影响初始静态变量，但如果您确实需要参数：

您可以更改参数的名称：

private static void buyOption(int water, int milk, int beans, int cups, int money)

本可以很容易地变成：

private static void buyOption(int localWater, int localMilk, int localBeans, int localCups, int localMoney)

有了新名称，您可以只使用变量 water 来表示您的静态变量，同时使用 localWater代表你的参数

那么这会起作用：

if (localWater >= 250 && localBeans >= 16 && localCups >= 1) {
                water -= 250;
                beans -= 16;
                cups -= 1;
                money += 4;
                System.out.println("I have enough resources, making you a coffee!");
            }

如何在选定的场景中实现按值传递？

How do I implement pass-by-value in selected scenario?

java

pass-by-value