Question

我是Java的新手，对所有的错误感到抱歉！

我创建了一个由4个类组成的库程序：Library，Book，BookInterface＆amp;顾客。

在Book类中，我有一个方法可以打印出库中的所有书籍及其状态（输入或输出）。相反，我不断得到这样的东西：

 Great Gatsby: null                                            

 Withering Heights: null

是否与setStatus（）方法有关？每次用户添加新书时，它都会创建一个新的Book实例，然后我会设置setStatus（“IN”）。那么为什么它不是保存而是打印出null？

非常感谢您的帮助!!

图书课程：

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Book implements BookInterface
{

    Scanner input = new Scanner(System.in);
    static ArrayList <String> UserList = new ArrayList<String>(); 
    static ArrayList <String> BookList = new ArrayList <String> (); //display just titles// use when checking out books
    static ArrayList <String> OrigBookList = new ArrayList <String> (); //keep track of all titles ever entered


    public String title; 
    public String author;
    public String book;
    public boolean checkIn;

    private String status;
    private String borrower; 

    public Book(String t, String a)
    {
        title = t; 
        author = a; 
    }

    //constructor create new book
    public Book(String newTitle)
    {
        title = newTitle;   
    }


    public String toString()
    {
        return title + " " + author; 
    }

    public String getTitle() 
    {
        return title;
    }

    public void setTitle(String title) 
    {
        this.title = title;
    }

    public String getAuthor() 
    {
        return author;
    }

    public void setAuthor(String author) 
    {
        this.author = author;
    }

    public String getStatus(String book)
    {
        return status; 
    }

    public void setStatus(String status)    
    {
        this.status = status; 
    }

    public void setBorrower(String borrower)
    {
        this.borrower = borrower; 
    }

    public String getBorrower(String checkPatron)
    {
        return borrower; 
    }

    public String getBook(String checkPatron)
    {
        return book; 
    }

    public void setBook(String bookCheckOut)
    {
        this.book = bookCheckOut;  
    }

    public void addBook()
    {
        Scanner input = new Scanner(System.in);
        Scanner inputread = new Scanner(System.in);
        System.out.print("Enter book title: ");
        String title1 = inputread.nextLine();

        Scanner input1 = new Scanner(System.in);
        System.out.print("Enter book author: ");
        String author1 = inputread.next(); 

        Book fullBook = new Book(title1, author1);  //create constructor w/ title & author
        Book book1 = new Book(title1);              //constructor w/ just title to be used to display all books
        OrigBookList.add(title1);
        book1.setStatus("IN");  
        System.out.println("-----------------------------------------------");
        System.out.println("-----" + title1 + " is now in the library!-----");
        System.out.println("-----------------------------------------------");
    }

    public void editBook()
    {
        Scanner inputread = new Scanner(System.in);
        System.out.println("Enter original book title: ");
        String origTitle = inputread.nextLine(); 
        System.out.println("Enter edited book title: ");
        String editedTitle = inputread.nextLine();
        Collections.replaceAll(Book.UserList, origTitle, editedTitle);
        System.out.println("------------------------------------------------------");
        System.out.println(origTitle + " has been changed to " + editedTitle + "!");
        System.out.println("------------------------------------------------------");

    }

    public void libraryInventory()
    {
        System.out.println("------------------ Library Inventory: ---------------");
        for(int i =0; i<= OrigBookList.size()-1; i++)
        {
            //Book Title: checked in/out
            System.out.println(OrigBookList.get(i) + ":" + getStatus(OrigBookList.get(i)));         
        }
        System.out.println("-----------------------------------------------------");
    }

}

Answer 1

getStatus(OrigBookList.get(i))会忽略您传递给它的参数，只返回您调用Book方法的libraryInventory的状态。显然，Book实例没有初始化状态字段，但即使它已经初始化，它也会给你一个Book的状态。

您可能应该维护一个书籍清单（static ArrayList <String>），或者甚至更好地将该列表放在一个单独的类中（而不是拥有书名的静态列表（static ArrayList <Book>）（你可以称之为Library）。

libraryInventory之类的方法不应该在Book类中（如果你坚持将它们保存在Book类中，请将它们设为静态，因为它们没有引用到单个Book实例。）

Answer 2

您的整个程序似乎在类Book的实例中运行。在其中，您正在制作并放弃Book的新实例，名为fullBook和book1，而fullBook则设置其状态。当您在程序中的主getStatus上调用Book时，它只返回自己的状态，该状态从未设置为任何内容。

如果要保存Book的一系列实例，则需要将实例放在某处，而不是仅实例化它们，然后将标题添加到列表中。

ArrayList的循环打印出null

2 个答案: