java.util.NoSuchElementException - 扫描程序读取用户输入

Question

我刚接触使用Java，但我之前有过使用C＃的经验。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

我有两个用户输入的函数：

• PromptCustomerQty
• PromptCustomerPayment

如果我没有调用PromptCustomerQty，那么我就不会收到此错误，这让我误以为我在扫描器上做错了什么。以下是我的完整代码示例。我感谢任何帮助。

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() == "yes") {
        customer.setPaidInFull(true);
    }
    else {
        customer.setPaidInFull(false);
    }

    // Cleanup
    sc.close(); 
}

Answer 1

这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拨打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，现在当你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，因此也就是异常。

我怀疑是否有任何出路重新开放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.**

解决问题的唯一方法是在main方法中启动Scanner，在两种方法中将其作为参数传递，然后在主方法中再次关闭它，例如：

main方法相关的代码块：

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
 }

希望这能让您对失败和可能的解决方案有所了解。

Answer 2

问题是

  

当扫描仪关闭时，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。

要解决此问题，您可以

Scanner scan static 并且不要在PromptCustomerQty中关闭它。以下代码有效。

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);
}
}

另外，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();

以前发生在我身上，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实际上并不是最好的。

您应该使用Singleton Pattern创建一个使Scanner保持私有的类，从而使该Scanner在您的代码中唯一。

然后，您可以实现所需的方法，也可以创建getScanner（不推荐使用），并可以使用私有布尔值（例如已经关闭）对其进行控制。

如果您不知道如何使用Singleton Pattern，请看以下示例：

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();
    }   
}