一些 if 在 switch-case 块中

Question

我有一个关于如何在开关盒内执行多项检查的问题？我需要在案例 2 中进行一些检查，但是添加第二个 if 块，我的应用程序什么也不做，它只是挂起。我做错了什么？

BufferedReader inputCommand = new BufferedReader(new InputStreamReader(System.in));

        while (true) {
                    System.out.println("Instruction:");
                    System.out.println();
                    System.out.println("1 -- Show all product at the store");
                    System.out.println("2 -- Add the product at the client basket");
                    System.out.println("3 -- Show client basket");
                    System.out.println();
                    switch (inputCommand.readLine()) {
                        case "1":
                            basketCommand.get();
                            System.out.println();
                            break;
                        case "2":
                            System.out.println();
                            System.out.println("Select product to add into your basket");
                            if (inputCommand.readLine().equals("su")){
                                basketCommand.addIntoBasket(productContainer.productList.get("su"));
                            }
                            if (inputCommand.readLine().equals("an")){
                                basketCommand.addIntoBasket(productContainer.productList.get("an"));
                            }
                            break;
    }

Answer 1

在第二个case语句中，你应该只读取下一个输入：

BufferedReader inputCommand = new BufferedReader(new InputStreamReader(System.in));

while (true) {
    System.out.println("Instruction:");
    System.out.println();
    System.out.println("1 -- Show all product at the store");
    System.out.println("2 -- Add the product at the client basket");
    System.out.println("3 -- Show client basket");
    System.out.println();
    switch (inputCommand.readLine()) {
    case "1":
        basketCommand.get();
        System.out.println();
        break;
    case "2":
        System.out.println();
        System.out.println("Select product to add into your basket");

        String next = inputCommand.readLine();
        if (next.equals("su")) {
            basketCommand.addIntoBasket(productContainer.productList.get("su"));
        }
        else if (next.equals("an")) {
            basketCommand.addIntoBasket(productContainer.productList.get("an"));
        }
        break;
    }
}

Answer 2

您的第一个 if 语句正在吞噬输入行，因此第二个 if 语句没有可读取的内容。

读取一次后存储该行：

                String readLine = inputCommand.readLine();

                if (readLine.equals("su")){
                    basketCommand.addIntoBasket(productContainer.productList.get("su"));
                }
                else if (readLine.equals("an")){
                    basketCommand.addIntoBasket(productContainer.productList.get("an"));
                }

Answer 3

每次您执行inputCommand.readLine() 时，程序都会等待您的输入。您无法像在此示例中那样比较单个输入。最好的方法是将输入保存在变量中，然后执行检查。像这样的东西会起作用（未经测试）：

BufferedReader inputCommand = new BufferedReader(new InputStreamReader(System.in));
    while (true) {
                System.out.println("Instruction:");
                System.out.println();
                System.out.println("1 -- Show all product at the store");
                System.out.println("2 -- Add the product at the client basket");
                System.out.println("3 -- Show client basket");
                System.out.println();
                switch (inputCommand.readLine()) {
                    case "1":
                        basketCommand.get();
                        System.out.println();
                        break;
                    case "2":
                        System.out.println();
                        System.out.println("Select product to add into your basket");
                        String input = inputCommand.readLine();
                        if (input.equals("su")){
                            basketCommand.addIntoBasket(productContainer.productList.get("su"));
                        }
                        if (input.equals("an")){
                            basketCommand.addIntoBasket(productContainer.productList.get("an"));
                        }
                        break;
}

Answer 4

将 if 替换为 else if

case "2":
     System.out.println();
     System.out.println("Select product to add into your basket");
     if (inputCommand.readLine().equals("su")){
        basketCommand.addIntoBasket(productContainer.productList.get("su"));
     }
     else if (inputCommand.readLine().equals("an")){
     basketCommand.addIntoBasket(productContainer.productList.get("an"));
     } 
     break;

Answer 5

我认为这是一个用于学习目的的基本程序，所以我尽量保持简单。

1.不要忘记关闭 Scanner 类。

2.始终将数字输入转换为小写（考虑“q”与“Q”的大小写）

3.每次你执行“scanner.nextLine()”你的程序将停止并等待来自源的输入（在这种情况下 System.in 配置为从键盘输入）

4.System.out.println 将在其输入结束时将“\n”打印到屏幕上，从而创建一个新行，您可以在字符串中使用此字符来保存代码行。

完整代码：

import java.util.Scanner;
import java.io.IOException;

public class MyClass {

public static void ShowMenu()
{
      System.out.println("\nInstruction:");
      System.out.println("1 -- Show all product at the store");
      System.out.println("2 -- Add the product at the client basket");
      System.out.println("3 -- Show client basket");
      System.out.println("q -- Quit Program");


}

public static void ProductMenu()
{
      System.out.println("Select product to add into your basket");
      System.out.println();
      System.out.println("ba -- Basketball");
      System.out.println("fi -- Fiat 500");
      System.out.println("ip -- Iphone");
      System.out.println();
}

public static void Exit(Scanner sc) {
    sc.close();
    System.out.println("You've exited the program. goodbye!");
    System.exit(1);
}

public static void main(String args[]) {
    Scanner scanner = new Scanner(System.in);
    String inputCommand = null;

    while (true) {
       ShowMenu();
        switch (inputCommand = scanner.nextLine().toLowerCase()) {
            case "1":
                System.out.println("you entered 1");
                break;
            case "2":
                ProductMenu();
                inputCommand = scanner.nextLine(); //should block

                if (inputCommand.equals("ba")){
                    System.out.println("Basketball added.");
                }
                if (inputCommand.equals("fi")){
                    System.out.println("Fiat 500 added.");


                }
                if (inputCommand.equals("ip")){
                    System.out.println("Iphone added.");

                }
                break;

            case "q":
                Exit(scanner);
                break;
            default:
                System.out.println("Invalid Input");
                break;
        }
   }

}

}