如何在选定的场景中实现按值传递？

Question

我在项目中实现传递值时遇到了一点问题。在buyOption、fillOption 和takeOption 的情况下。

我不知道如何使更改的值应用于整个程序而不仅仅是方法。任何提示/解决方案/反馈将不胜感激。

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 总是按值传递。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，您使用的每个操作，比如说，“牛奶”将只存在于这个方法中。你的参数被命名为 milk，但它与在你的类顶部声明的“milk”没有什么可共享的。它们的名字相同只是巧合。 您不能使用 this.milk （或 CoffeeMachine.milk，因为它是一个静态字段）来引用您的 static milk，但是由于您的代码是目前编写的，如果不进一步编辑您的方法，我认为没有用处。

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

像这样编辑您的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。

你必须在你的方法中直接引用静态变量：

所以不要仅仅使用

water -= 250;

仅影响参数（本地范围）

你必须使用：

CoffeeMachine.water -= 250;

正如已经指出的那样，字段不应该是静态的，但我假设你现在正在学习 java。

注意： 出于您的目的，您的方法中可能不需要参数，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)

很容易变成：

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

使用新名称，您可以只使用变量 water 来表示您的静态变量，同时使用 localWater 来表示您的参数

这样就可以了：

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!");
            }