java - 返回整数而不是 Scanner.NextInt() 时出错

Question

我正在编写一个菜单驱动的 Java 程序来实现维吉尼亚密码。 对于菜单功能，我编写了以下代码:

    public static int printmenu(){
        Scanner scan = new Scanner(System.in);
        System.out.println("Welcome to the Vigenere Cipher!");
        System.out.println("What would you like to do?");
        System.out.println("1.Encrypt a message");  
        System.out.println("2.Decrypt a message");
        System.out.println("Enter your choice:");
        return scan.nextInt();
    }

现在这可以正确完成工作，但我因没有关闭我的扫描仪类对象“scan”而受到教授的批评。 因此，我对代码进行了以下编辑:

    public static int printmenu(){
            int a;
            Scanner scan = new Scanner(System.in);
            System.out.println("Welcome to the Vigenere Cipher!");
            System.out.println("What would you like to do?");
            System.out.println("1.Encrypt a message");  
            System.out.println("2.Decrypt a message");
            System.out.println("Enter your choice:");
            a = scan.nextInt();
            scan.close();
            return a;
    }

但是，这会返回以下错误:

Exception in thread "main" java.util.NoSuchElementException: No line found
    at java.util.Scanner.nextLine(Unknown Source)
    at vigenere.Ciphermain.main(Ciphermain.java:30)

我不明白我做错了什么。谁能帮我吗？

编辑:这是 Ciphermain 类的源代码:

public class Ciphermain extends JFrame{

    public static int printmenu(){
        int a;
        Scanner scan = new Scanner(System.in);
        System.out.println("Welcome to the Vigenere Cipher!");
        System.out.println("What would you like to do?");
        System.out.println("1.Encrypt a message");  
        System.out.println("2.Decrypt a message");
        System.out.println("Enter your choice:");
        a = scan.nextInt();
        scan.close();
        return a;
    }

    public static void main(String[] args) {
        String message, key;
        int choice;
        choice = printmenu();
        Scanner scan = new Scanner(System.in);
        if(choice == 1){
            System.out.println("Enter the message to be encrypted:");
            message = scan.nextLine();
            System.out.println("Enter the secret key:");
            key = scan.next();
            Cipher ciph = new Cipher(key);
            System.out.println("Encrypted message is: " + ciph.encrypt(message));
        }
        else{
            System.out.println("Enter the message to be decrypted:");
            message = scan.nextLine();
            System.out.println("Enter the secret key:");
            key = scan.next();
            Cipher ciph = new Cipher(key);
            System.out.println("Decrypted message is: " + ciph.decrypt(message));
        }
    scan.close();
    }
}

Answer 1

关闭扫描器将关闭底层流(System.in)。因此，下次您在已关闭的 System.in 上创建扫描程序时，您将得到这样的异常。

在程序开始时创建扫描仪，在任何地方使用它并在程序结束时关闭它。

喜欢:

static Scanner scan;

static int displayMenu(){
    // display menu.
    return scan.nextInt();
}

public static void main(String [] args){
    scan = new Scanner(System.in);

    // start work.

    // end of work.

    scan.close();
}

检查这个:http://docs.oracle.com/javase/7/docs/api/java/util/Scanner.html#close()