这确实让我困惑了一段时间，但这是我最终的发现。

当你在第一个方法中调用sc.close()时，它不仅关闭了你的扫描仪，也关闭了你的System.in输入流。你可以通过在第二个方法的最顶端打印它的状态来验证它：

System.out.println(System.in.available());

所以，现在当你在第二个方法中重新示例化Scanner时，它找不到任何打开的System.in流，因此出现异常。
我怀疑是否有办法重新打开System.in，因为：
public void close() throws IOException --> Closes this input stream and releases any system resources associated with this stream. The general contract of close is that it closes the input stream. A closed stream cannot perform input operations and **cannot be reopened.**
解决这个问题的唯一好办法是在main方法中初始化Scanner，在两个方法中将其作为参数传递，然后在main方法中再次关闭它，例如：
main方法相关代码块：

Scanner scanner = new Scanner(System.in);  

// Ask users for quantities 
PromptCustomerQty(customer, ProductList, scanner );

// Ask user for payment method
PromptCustomerPayment(customer, scanner );

//close the scanner 
scanner.close();

您的方法：

public static void PromptCustomerQty(Customer customer, 
                             ArrayList<Product> ProductList, Scanner scanner) {

    // no more scanner instantiation
    ...
    // no more scanner close
 }

 public static void PromptCustomerPayment (Customer customer, Scanner sc) {

    // no more scanner instantiation
    ...
    // no more scanner close
 }

希望这能帮助您了解故障和可能的解决方案。

问题是
当扫描器关闭时，如果输入源实现Closeable接口，则扫描器将关闭该输入源。
http://docs.oracle.com/javase/1.5.0/docs/api/java/util/Scanner.html
因此scan.close()闭合System.in。
要修复它，您可以
Scanner scanstatic并且在PromptCustomerQty中不关闭。下面的代码可以工作。

public static void main (String[] args) {   

// Create a customer
// Future proofing the possabiltiies of multiple customers
Customer customer = new Customer("Will");

// Create object for each Product
// (Name,Code,Description,Price)
// Initalize Qty at 0
Product Computer = new Product("Computer","PC1003","Basic Computer",399.99); 
Product Monitor = new Product("Monitor","MN1003","LCD Monitor",99.99);
Product Printer = new Product("Printer","PR1003x","Inkjet Printer",54.23);

// Define internal variables 
// ## DONT CHANGE 
ArrayList<Product> ProductList = new ArrayList<Product>(); // List to store Products
String formatString = "%-15s %-10s %-20s %-10s %-10s %n"; // Default format for output

// Add objects to list
ProductList.add(Computer);
ProductList.add(Monitor);
ProductList.add(Printer);

// Ask users for quantities 
PromptCustomerQty(customer, ProductList);

// Ask user for payment method
PromptCustomerPayment(customer);

// Create the header
PrintHeader(customer, formatString);

// Create Body
PrintBody(ProductList, formatString);   
}

static Scanner scan;

public static void PromptCustomerQty(Customer customer, ArrayList<Product> ProductList)               {
// Initiate a Scanner
scan = new Scanner(System.in);

// **** VARIABLES ****
int qty = 0;

// Greet Customer
System.out.println("Hello " + customer.getName());

// Loop through each item and ask for qty desired
for (Product p : ProductList) {

    do {
    // Ask user for qty
    System.out.println("How many would you like for product: " + p.name);
    System.out.print("> ");

    // Get input and set qty for the object
    qty = scan.nextInt();

    }
    while (qty < 0); // Validation

    p.setQty(qty); // Set qty for object
    qty = 0; // Reset count
}

// Cleanup

}

public static void PromptCustomerPayment (Customer customer) {
// Variables
String payment = "";

// Prompt User
do {
System.out.println("Would you like to pay in full? [Yes/No]");
System.out.print("> ");

payment = scan.next();

} while ((!payment.toLowerCase().equals("yes")) && (!payment.toLowerCase().equals("no")));

// Check/set result
if (payment.toLowerCase() == "yes") {
    customer.setPaidInFull(true);
}
else {
    customer.setPaidInFull(false);
}
}

另外，不应该使用==进行字符串比较，而应该使用.equals。

您需要删除扫描仪关闭行：scan.close();
我以前也遇到过，这就是原因。

例外的原因已经解释过了，但是建议的解决方案实际上并不是最好的。
您应该创建一个类，使用Singleton模式将Scanner保持为私有的，这使得该扫描器在您的代码中是唯一的。
然后，您可以实现所需的方法，或者创建一个getScanner（不推荐），并使用私有布尔值（如alreadyClosed）来控制它。
如果你不知道如何使用Singleton模式，这里有一个例子：

public class Reader {
    
    
    private Scanner reader;
    private static Reader singleton = null;
    private boolean alreadyClosed;
    
    private Reader() {
        alreadyClosed = false;
        reader = new Scanner(System.in);
    }
    
    public static Reader getInstance() {
        if(singleton == null) {
            singleton = new Reader();
        }
        return singleton;
    }
    
    public int nextInt() throws AlreadyClosedException {
        if(!alreadyClosed) {
            return reader.nextInt();
        }
        throw new AlreadyClosedException(); //Custom exception
    }
    
    public double nextDouble() throws AlreadyClosedException {
        if(!alreadyClosed) {
            return reader.nextDouble();
        }
        throw new AlreadyClosedException();
    }
    
    public String nextLine() throws AlreadyClosedException {
        if(!alreadyClosed) {
            return reader.nextLine();
        }
        throw new AlreadyClosedException();
    }
    
    public void close() {
        alreadyClosed = true;
        reader.close();
    }   
}

对于任何在HackerRank这样的网站上参加在线考试的人来说-
如果您试图通过单击按钮以使用自定义输入执行main（）来测试（可能）完美的代码，则可能会收到此消息。
在这种情况下，你需要点击另一个按钮，比如“运行单元测试”。你可能只会被评估代码是否通过了他们编写的单元测试，而不是你重构代码以减少LOC的能力或你的编码风格。