钥匙串商店 - 当前钥匙串变量中的值未正确更新 - Java

Question

我正在自学Java，刚刚学会了方法。我尝试过这个练习 钥匙扣出售

 // Exercise 109
    import java.util.Scanner;
    public class KeychainShop {
            public static void main(String[] args) {
                Scanner keyboard = new Scanner(System.in);
                int selection, currentKeychains = 0, price = 10;
                System.out.println("Welcome to the Keychain Shop!n");
                do {
                    System.out.println("Select 1, 2, 3, or 4");
                    System.out.println("1. Add Keychains");
                    System.out.println("2. Remove Keychains");
                    System.out.println("3. View Order");
                    System.out.println("4. Checkout");
                    System.out.print("nWhat would you like to do? ");
                    selection = keyboard.nextInt();
                    System.out.println();
                    if (selection == 1) {
                        System.out.println("You now have " + add_keychains(currentKeychains) + " keychains.");
                        System.out.println();
                    } 
                    else if (selection == 2) {
                        System.out.println("You now have " + remove_keychains(currentKeychains) + " keychains.");
                        System.out.println();
                    } 
                    else if (selection == 3) {
                        view_order(currentKeychains, price);
                        System.out.println();
                    } 
                    else if (selection == 4) {
                        checkout(currentKeychains, price);
                    }
                } while (selection != 4);
            }
            public static int add_keychains(int currentKeychains) {
                Scanner keyboard = new Scanner(System.in);
                System.out.print("You have " + currentKeychains + " keychains. How many would you like to add? ");
                int keychainsAdded = keyboard.nextInt();
                currentKeychains += keychainsAdded;
                return currentKeychains;
            }
            public static int remove_keychains(int currentKeychains) {
                Scanner keyboard = new Scanner(System.in);
                System.out.print("You have " + currentKeychains + " keychains. How many would you like to remove? ");
                int keychainsRemoved = keyboard.nextInt();
                currentKeychains -= keychainsRemoved;
                return currentKeychains;
            }
            public static void view_order(int currentKeychains, int price) {
                System.out.println("You are currently buying " + currentKeychains + " keychains.");
                System.out.println("Each keychain costs $" + price + ".");
                int totalCost = currentKeychains * price;
                System.out.println("Your current total is $" + totalCost);
            }
            public static void checkout(int currentKeychains, int price) {
                Scanner keyboard = new Scanner(System.in);
                System.out.print("Please enter your name: ");
                String name = keyboard.nextLine();
                System.out.println("You have bought " + currentKeychains + " keychains.");
                int totalCost = currentKeychains * price;
                System.out.println("Your total is $" + totalCost);
                System.out.println("Thanks for shopping with us today, " + name + ".");
            }
    }

我的程序可以编译，但它没有正确跟踪当前的钥匙串（我知道缺少一些东西，但无法弄清楚）。

如果用户选择"1.添加钥匙串"，要求他输入要添加的钥匙串数量。此数字将添加到存储在当前钥匙串中的值（从 0 开始）。因此，如果他输入 2，则当前的钥匙串现在持有 2。然后，菜单将重新出现，并询问用户的下一个选择。现在，如果用户选择添加钥匙串或删除钥匙串，则当前钥匙串中的值再次为 0（应为 2）。我不明白如何解决这个问题。有些东西我没有看到或理解。另外，我必须在程序中编写Scanner keyboard = new Scanner(System.in);四次代码，一次在main中，一次在add_keychains（），一次在remove_keychains（），一次在checkout（）中。有什么方法可以只键入一次并允许每个方法都能够使用 scanner 类（不确定我的措辞是否正确）？非常感谢帮助！

Answer 1

int是

Java中的原始类型，这意味着它是按值而不是按引用*传递的。当你在add_keychains中执行currentKeychains += keychainsAdded时，它只修改currentKeychains的本地副本——它对main变量的副本没有影响。如果希望更改是永久性的，则应执行以下操作之一：

• 添加currentKeychains作为类变量，例如 private static int currentKeychains ，然后从方法中删除该参数。
• 使用 Integer 而不是 int ，因为这将传递对 Integer 对象的引用，您可以从中设置值。

惯用的方式是前者;将内部状态保留在类变量中通常是要走的路。