在 switch 语句中读取用户输入时出现 java.util.NoSuchElementException

Question

我正在为计算机科学类(class)做的这个项目遇到了错误，希望有人能让我深入了解可能发生的情况。代码中发生的情况的一些背景知识:

在下面的代码中，我的程序开始显示 UI 菜单并要求用户输入，并使用 switch 语句根据用户输入运行适当的方法。

    /**
     * Start of program
     */
    public static void main(String[] args) {

        Library library = new Library();

        char userChoice; //Stores what menu item the user chose
        Scanner userMenuChoiceScanner = new Scanner(System.in);
        boolean loop = true;

        displayMenu(); //Displays main UI menu
        userChoice = Character.toLowerCase(userMenuChoiceScanner.nextLine().charAt(0)); //Gets user's menu choice

        /*Loops UI menu afer user has made menu choice and exits loop if user choses 'q' from menu.*/
        while(loop) {

            /* Switch descides what menu choice user has made and whether to prin menu UI again, load, 
             * search, print, or analyze a music catalog, or to quit program.
             */
            switch(userChoice) {
            case 'm':
                System.out.println();
                displayMenu();
                userChoice = Character.toLowerCase(userMenuChoiceScanner.nextLine().charAt(0));
                break;
            case 'p':
                System.out.println();

                if(library.getBooks().size() != 0) {
                    printLibrary(library);
                } else {
                    System.out.println("There are no books in the library. Please add a book first.");
                }

                System.out.print("Please enter a command (press 'm' for Menu):");
                userChoice = Character.toLowerCase(userMenuChoiceScanner.nextLine().charAt(0));
                break;
            case 'a':
                System.out.println();
                addBookToLibrary(library);
                System.out.print("Please enter a command (press 'm' for Menu):");
                userChoice = Character.toLowerCase(userMenuChoiceScanner.nextLine().charAt(0));
                break;
            case 'd':

                System.out.println();

                if(library.getBooks().size() != 0) {
                    deleteBookFromLibrary(library);
                } else {
                    System.out.println("There are no books in the library. Please add a book first.");
                }

                System.out.print("Please enter a command (press 'm' for Menu):");
                userChoice = Character.toLowerCase(userMenuChoiceScanner.nextLine().charAt(0));
                break;
            case 'r':           
                System.out.println();

                if(library.getBooks().size() != 0) {
                    readBookFromLibrary(library);
                } else {
                    System.out.println("There are no books in the library. Please add a book first.");
                }

                System.out.print("Please enter a command (press 'm' for Menu):");
                userChoice = Character.toLowerCase(userMenuChoiceScanner.nextLine().charAt(0));
                break;
            case 'q':
                System.out.println();
                System.out.println("Thank you! Goodbye!");
                loop = false;
                break;
            default:
                System.out.println();
                System.out.println("Invalid selection!");
                System.out.print("Please enter a command (press 'm' for Menu):");
                userChoice = Character.toLowerCase(userMenuChoiceScanner.nextLine().charAt(0));
                System.out.println();

            }
        }

        userMenuChoiceScanner.close();



    }

发生的情况是，当用户做出选择时，它会进入适当的方法并在那里完成其任务，然后当它返回到 switch 语句以获取新的用户输入时，它会抛出 java.util.NoSuchElementException错误就像扫描仪流已关闭一样，但我直到最后才关闭扫描仪(流应该仍然打开)。

switch 语句的设置方式是必须先添加一本书(用户必须首先选择“a”)，然后才能选择任何其他选项。这是他的代码 addBookToLibrary()方法有几个打开和关闭的扫描仪。我认为关闭这些扫描仪可能会导致问题？

    /**
     * Adds new book to library
     * @param library ArrayList object which holds book objects
     */
    public static void addBookToLibrary(Library library) {
        Scanner addBookScanner = new Scanner(System.in);
        String title = "";
        String author = "";
        String genre = "";
        String filename = "";
        boolean addStatus = false;


        /*Asks user for book details */
        System.out.println("Enter the details of the book you want to add to the library:");
        System.out.println("What is the book title?");
        title = addBookScanner.nextLine();
        System.out.println("What is the author's name?");
        author = addBookScanner.nextLine();
        System.out.println("What is book genre?");
        genre = addBookScanner.nextLine();
        System.out.println("What is the book's filename?");
        filename = addBookScanner.nextLine();
        addBookScanner.close();

        Book newBook = new Book(author, title); //creates new book with author and title set

        newBook.setGenre(genre); //sets book genre

        File bookFile = new File(System.getProperty("user.dir") + '\\' + filename); //used to check if file user provided exists
        System.out.println(bookFile);
        /*Checks to see if file user provided actually exists*/
        if(bookFile.exists()) {

            try {

                newBook.setFilename(filename);

            }catch(Exception e) {

            }

        }
        else {

            Scanner getNewFilenameScanner = new Scanner(System.in);

            //Continues to ask user for new filename if file doesn't exist
            do {
            System.out.println("I'm sorry, but the file you specified does not exist.");
            System.out.print("Enter a new file name:");
            bookFile = new File(getNewFilenameScanner.nextLine());

            }while (!(bookFile.exists()));

            getNewFilenameScanner.close();
        }

        addStatus = library.addBook(newBook); //adds book to library

        if(addStatus) {
            System.out.println("Book was successfully added!");
        }
        else {
            System.out.println("Book was not successfully added. Please try again.");
        }
    }

这段代码在以前的项目中运行得很好，所以我不确定为什么它现在不起作用。任何帮助将不胜感激。

非常感谢

Answer 1

在您的 addBookToLibrary 方法中，您可以在使用完 Scanner 后关闭它们。然而，这些扫描仪连接到标准输入流System.in。根据documentation，关闭这些扫描仪也会关闭标准输入流。 .

还有什么连接到标准输入流？ userMenuChoiceScanner!标准输入流关闭后，userMenuChoiceScanner 无法从中读取数据，因此会引发异常。

请注意，尽管 userMenuChoiceScanner 直到非常结束才关闭，但它正在读取的流已关闭。

事实上，您不需要在这里创建多个扫描仪。您只需要使用一台扫描仪，并将其传递给不同的方法。例如，addBookToLibrary 可以接受 Scanner:

public static void addBookToLibrary(Library library, Scanner s) {

它只会使用 s 来读取输入。您可以将您的 userMenuChoiceScanner 传递给它:

addBookToLibrary(library, userMenuChoiceScanner);

作为一般规则，您不应关闭任何非您打开的内容。您没有打开标准输入流(JVM 已经打开了)，因此您不应该关闭它。