java - 全局扫描仪可以工作一次，但在第二次运行时会变得有点毛茸茸的

Question

我知道这个网站对于像我这样的菜鸟来说可能相当苛刻，但我已经尝试了三天来解决这个问题。这是期末项目作业的一部分。我遇到的问题是，当我多次使用案例 1 时，第一次它按计划工作，但第二次它似乎跳过了以下几行。

System.out.println("Please enter the title: ");
        String title = two.nextLine();

我关注的领域是 add() 方法中的 Bookstore 类和主类中的情况 1。当尝试添加第二本书时，代码立即跳过书名并直接转到国际标准书号 (ISBN)。

import java.util.Scanner;
public class MIS_Tester {
public static Scanner two = new Scanner(System.in);
public static Scanner three = new Scanner(System.in);
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        Bookstore bookseller = new Bookstore();
        Order addBook = new Order();
        Sale sell = new Sale();
        boolean finished = false;
        while(!finished){
            System.out.println("Select: (0) output; (1) add book; (2) delete book; (3) order book;"
                    + "(4) sell a book; (5) exit");
            int option = in.nextInt();
            switch(option){
            case 0: bookseller.ouput();break;
            case 1: bookseller.add(two);break;
            case 2: bookseller.delete(two);break;
            case 3: addBook.add(three);break;
            case 4: sell.sellBook(two);break;
            default: finished = true; break;
            }
        }   
        in.close();
    }
}

class Bookstore{
    private Book[] catalog;
    private int numBooks;
    private final int MAXBOOKNUM = 100;
    Scanner nine = new Scanner(System.in);

    public Bookstore(){
        numBooks = 0;
        catalog = new Book[MAXBOOKNUM];
    }
    public void ouput(){
        for (int i = 0; i < numBooks; i++) catalog[i].output();
    }
    public void add(Scanner two){
        System.out.println("Please enter the title: ");
        String title = two.nextLine();
        System.out.println("Enter the ISBN: ");
        int isbn = two.nextInt();
        System.out.println("Enter the publication date: ");
        int month = two.nextInt();int day = two.nextInt();int year = two.nextInt();
        catalog[numBooks++] = new Book(title, isbn, new Date(month, day, year));
    }
    public void delete(Scanner two){
        System.out.println("enter the one to delete");
        int option = two.nextInt();
        int numOfElements = catalog.length - (option +1);
        System.arraycopy(catalog, option + 1, catalog, option, numOfElements);
    }
}

class Book{
    private String title;
    private Date pubDate;
    private int isbn;
    private int orderNumber;

    public Book(String title, int isbn, Date pubDate){
        this.title = title;
        this.isbn = isbn;
        this.pubDate = pubDate;
    }

    public Book(String title, int isbn, Date pubDate, int orderNumber){
        this.title = title;
        this.isbn = isbn;
        this.pubDate = pubDate;
        this.orderNumber = orderNumber;
    }
    public void output(){
        System.out.println("Title: " + "\"" +title + "\"");
        System.out.println("ISBN: " + isbn);
        pubDate.output();
    }
    public void orderOut(){
        System.out.println("The book " + "\"" + title + "\"" + " has been ordered.");
    }
}

控制台日志:

Select: (0) output; (1) add book; (2) delete book; (3) order book;(4) sell a book; (5) exit
1
Please enter the title: 
All About Dogs
Enter the ISBN: 
1234
Enter the publication date: 
12
29
1978
Select: (0) output; (1) add book; (2) delete book; (3) order book;(4) sell a book; (5) exit
1
Please enter the title: 
Enter the ISBN: 

Answer 1

所以在java api中位于 here.

nextLine() - Advances this scanner past the current line and returns the input that was skipped.

因此，此方法将扫描仪前进到新行并返回跳过的输入。

nextInt() - Scans the next token of the input as an int.

此方法仅获取输入的下一个标记作为 int。它不消耗换行符。

所以基本上发生的事情是:

当您使用 System.out.println 时，您正在创建换行符但 nextInt() 方法不消耗换行符，因此它们保留在输入缓冲区中。因此，当您第二次调用 nextLine() 时，它仅返回输入缓冲区中下一个的换行符(剩余的)。我希望这是有道理的，如果我应该详细说明的话，请告诉我。

解决方法:- 添加two.nextLine() //after each call to two.nextInt()

因此，更新后的 add() 方法将如下所示:

public void add(Scanner two){
    System.out.println("Please enter the title: ");
    String title = two.nextLine();
    System.out.println("Enter the ISBN: ");
    int isbn = two.nextInt();
    two.nextLine(); // consume the left over newline token in the buffer
    System.out.println("Enter the publication date: ");
    int month = two.nextInt();int day = two.nextInt();int year = two.nextInt();
    two.nextLine(); // consume the left over newline token in the buffer
    catalog[numBooks++] = new Book(title, isbn, new Date(month, day, year));
}

请注意，此代码未经测试。