java.util.NoSuchElementException - 扫描器读取用户输入

Question

我是使用 Java 的新手，但我以前使用过 C#。我遇到的问题是从控制台读取用户输入。

我在这部分代码中遇到了“java.util.NoSuchElementException”错误：

payment = sc.next(); // PromptCustomerPayment function

我有两个获取用户输入的函数：

• 提示客户数量
• 提示客户付款

如果我不调用 PromptCustomerQty，那么我不会收到此错误，这让我相信我在使用扫描仪时做错了。以下是我的完整代码示例。感谢您的帮助。

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);   
}

public static void PromptCustomerQty(Customer customer, ArrayList<Product> ProductList) {
    // Initiate a Scanner
    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
    scan.close();
}

public static void PromptCustomerPayment (Customer customer) {
    // Initiate Scanner 
    Scanner sc = new Scanner(System.in);
    
    // Variables
    String payment = "";

    // Prompt User
    do {
    System.out.println("Would you like to pay in full? [Yes/No]");
    System.out.print("> ");
    
    payment = sc.next();
    
    } while ((!payment.toLowerCase().equals("yes")) && (!payment.toLowerCase().equals("no")));
    
    // Check/set result
    if (payment.toLowerCase().equals("yes")) {
        customer.setPaidInFull(true);
    }
    else {
        customer.setPaidInFull(false);
    }
    
    // Cleanup
    sc.close(); 
}

Answer 1

这确实让我困惑了一段时间，但这是我最终发现的。

当您在第一种方法中调用sc.close() 时，它不仅会关闭您的扫描仪，还会关闭您的System.in 输入流。您可以通过在第二种方法的最顶部打印其状态来验证它：

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

所以，现在当您在第二种方法中重新实例化 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.**

解决您的问题的唯一好方法是在您的 main 方法中启动 Scanner，将其作为参数传递给您的两个方法，然后在您的 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();

你的方法：

 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
 }

希望这能让您对失败和可能的解决方案有所了解。

Answer 2

问题是

当 Scanner 关闭时，如果源实现了 Closeable 接口，它将关闭其输入源。

http://docs.oracle.com/javase/1.5.0/docs/api/java/util/Scanner.html

因此scan.close() 关闭System.in。

要修复它，您可以制作

Scanner scanstatic 并且不要在 PromptCustomerQty 中关闭它。下面的代码有效。

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);
}
}

附带说明，您不应该使用== 进行字符串比较，而是使用.equals。

Answer 3

您需要删除扫描仪结束行：scan.close();

我以前也遇到过，这就是原因。

Answer 4

已经解释了异常的原因，但是建议的解决方案并不是最好的。

您应该创建一个使用单例模式将 Scanner 保持为私有的类，这使得该扫描仪在您的代码中是唯一的。

然后你可以实现你需要的方法，或者你可以创建一个 getScanner（不推荐），你可以用一个私有的布尔值来控制它，比如 alreadyClosed。

如果你不知道如何使用单例模式，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();
    }   
}

Answer 5

对于在 HackerRank 等网站上参加在线考试时到达这里的任何人-

如果您尝试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，您可能会收到此消息。

在这种情况下，您需要单击另一个按钮，例如“运行单元测试”。您可能只会被评估代码是否通过了他们编写的单元测试 - 而不是您将代码重构为更少 LOC 的能力或您的编码风格。