java - 如何修复 java.util.NoSuchElementException

Question

我想修复 java.util.NoSuchElementException 错误。我不断收到错误:

Exception in thread "main" java.util.NoSuchElementException: No line found
    at java.util.Scanner.nextLine(Unknown Source)
    at Main.newUser(Main.java:28)
    at Main.main(Main.java:18)

使用此代码

import java.util.Scanner;
import java.io.*;
class Main2
{
    public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);
        input.close();
        newUser();
    }
    private static void newUser()
    {
        try
        {
            Scanner input = new Scanner(System.in);
            System.out.println("Please enter the name for the new user.");
            String userNameNew = input.nextLine();
            System.out.println("Please enter the password for the new user.");
            String userPassWordNew = input.nextLine();
            System.out.println("The new user: " + userNameNew + " has the password: " + userPassWordNew + "." );
            PrintWriter out = new PrintWriter("users.txt");
            out.print(userNameNew + "\r\n" + userPassWordNew);
            out.close();
            input.close();
        } catch (IOException e) { e.printStackTrace(); }
    }
}

你能帮我一下吗？谢谢。

Answer 1

我找到了您遇到此异常的原因。

因此，在您的 main 方法中，您初始化了 Scanner 类对象并立即关闭它。

问题就在这里。因为当scanner调用close()方法时，如果输入源实现了Closeable接口(interface)，它就会关闭它的输入源。

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

https://docs.oracle.com/javase/7/docs/api/java/io/InputStream.html

InputStream 类是您案例中的输入源，它实现了 Closeable 接口(interface)。

https://docs.oracle.com/javase/7/docs/api/java/io/InputStream.html

然后，您将 Scanner 类对象初始化到 newUser() 方法中。这里扫描仪类对象初始化成功，但您的输入源仍然关闭。

所以我的建议是仅关闭扫描仪类对象一次。请找到您的更新代码。

    class Main2
    {
        public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);
        newUser(input); 
      //input.close()
    }
    private static void newUser(Scanner input) 
    {
        try {
            System.out.print("Please enter the name for the new user.");
            String userNameNew = input.nextLine();
            System.out.println("Please enter the password for the new user.");
            String userPassWordNew = input.nextLine();
            System.out.println("The new user: " + userNameNew + " has the password: " + userPassWordNew + "." );
            PrintWriter out = new PrintWriter("users.txt");
            out.print(userNameNew + "\r\n" + userPassWordNew);
            out.close();
        } catch (IOException e) { e.printStackTrace(); }
    }
}