异常-扫描器读取用户输入

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The problem is

When a Scanner is closed, it will close its input source if the source implements the Closeable interface.

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

Thus scan.close() closes System.in.

To fix it you can make

Scanner scan static and do not close it in PromptCustomerQty. Code below works.

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);
}
}

On a side note, you shouldn't use == for String comparision, use .equals instead.

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最佳答案

This has really puzzled me for a while but this is what I found in the end.

When you call, sc.close() in first method, it not only closes your scanner but closes your System.in input stream as well. You can verify it by printing its status at very top of the second method as :

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

So, now when you re-instantiate, Scanner in second method, it doesn't find any open System.in stream and hence the exception.

I doubt if there is any way out to reopen System.in because:

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.**

The only good solution for your problem is to initiate the Scanner in your main method, pass that as argument in your two methods, and close it again in your main method e.g.:

main method related code block:

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();

Your Methods:

 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
}

Hope this gives you some insight about the failure and possible resolution.

小开

You need to remove the scanner closing lines: scan.close();

It happened to me before and that was the reason.

小开

the reason of the exception has been explained already, however the suggested solution isn't really the best.

You should create a class that keeps a Scanner as private using Singleton Pattern, that makes that scanner unique on your code.

Then you can implement the methods you need or you can create a getScanner ( not recommended ) and you can control it with a private boolean, something like alreadyClosed.

If you are not aware how to use Singleton Pattern, here's a example:

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();
}
}

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For anyone who arrived here while taking an online exam on a site like HackerRank-

You might receive this if you are trying to test your (possibly) perfectly fine code by clicking the button to execute main() with custom input.

In this case, you need to be clicking the other button, something like "Run Unit Tests". You're likely only being evaluated on whether the code passes the unit tests they wrote- not your ability to refactor code to fewer LOC or your coding style.