java - 对象数组的空指针异常

Question

我正在创建的以下程序将允许用户输入值来创建视频数组。每个视频都包含多个数据字段(编号、标题、发布者、持续时间和日期)。该程序主要通过开关菜单进行控制。但是在创建数组后，选择另一个选项，例如 Show videos 将抛出 Null Pointer Exception。这些异常通常发生在您没有为数组赋值时，但应该已经在第一个选项 createLibrary() 中完成了。 Search videos 和 Change videos 中的其他函数也有同样的问题，每次使用 get 方法时都会出现。

任何能提供合理实用答案的人都会有很大帮助。

这是我的一段代码:

import java.util.*;
public class EnterLibrary
{
public final int MAX_ITEMS = 5;
public Library[] videos;
public int size = 0;

public EnterLibrary()
    {
        videos = new Library[MAX_ITEMS];
        java.util.Scanner scannerObject =new java.util.Scanner(System.in);
        LibraryMenu Menu = new LibraryMenu();
        Menu.displayMenu();
        switch (scannerObject.nextInt())
        {
            case 1:
            System.out.println ("1 - Add Videos");
                if (size < MAX_ITEMS) {
                Library video = createLibrary();
                videos[size++] = video;
                }
            new EnterLibrary();
            break;
            case 2:
            System.out.println ("2 - Show Videos");
            printVidLibrary(videos);
            new EnterLibrary();
            break;
            case 3:
            System.out.println ("3 - Search Videos");
            searchLibrary(videos);
            new EnterLibrary();
            break;
            case 4:
            System.out.println ("4 - Change Videos");
            changeLibrary(videos);
            break;      
            case 5:
            System.out.println ("5 - Delete Videos");
            deleteVideo(videos);
            new EnterLibrary();
            break;
            default:
            System.out.println ("Unrecognized option - please select options 1-5 ");
            break;
        }
    }

public Library createLibrary()
{
    Library video = new Library();
    java.util.Scanner scannerObject =new java.util.Scanner(System.in); 
    for (int i = 0; i < videos.length; i++)
    {
    //User enters values into set methods within the Library class
    System.out.print("Enter video number: " + (i+1) + "\n");
    String number = scannerObject.nextLine();
    System.out.print("Enter video title: " + (i+1) + "\n");
    String title = scannerObject.nextLine();
    System.out.print("Enter video publisher: " + (i+1) + "\n");
    String publisher = scannerObject.nextLine();
    System.out.print("Enter video duration: " + (i+1) + "\n");
    String duration = scannerObject.nextLine();
    System.out.print("Enter video date: " + (i+1) + "\n");
    String date= scannerObject.nextLine();
    System.out.print("VIDEO " + (i+1) + " ENTRY ADDED " + "\n \n");
    //Initialize arrays
    videos[i] = new Library ();
    videos[i].setVideo( number, title, publisher, duration, date  );
    }
    return video;
}

public void printVidLibrary( Library[] videos)
{
    //Get methods to print results
    System.out.print("\n***VIDEO CATALOGUE*** \n");
    for (int i = 0; i < videos.length; i++)
    {
    System.out.print("Video number " + (i+1) + ": \n" + videos[i].getNumber() + "\n ");
    System.out.print("Video title " + (i+1) + ": \n" + videos[i].getTitle() + "\n ");
    System.out.print("Video publisher " + (i+1) + ": \n" + videos[i].getPublisher() + "\n ");
    System.out.print("Video duration " + (i+1) + ": \n" + videos[i].getDuration() + "\n ");
    System.out.print("Video date " + (i+1) + ": \n" + videos[i].getDate() + "\n ");
    }
}
//Code for other functions not displayed here

public static void main(String[] args)
    {

    new EnterLibrary();
    }
}

异常错误:

Exception in thread "main" java.lang.NullPointerException
        at EnterLibrary.printVidLibrary(EnterLibrary.java:80)
        at EnterLibrary.<init>(EnterLibrary.java:26)
        at EnterLibrary.<init>(EnterLibrary.java:22)
        at EnterLibrary.<init>(EnterLibrary.java:22)
        at EnterLibrary.<init>(EnterLibrary.java:22)
        at EnterLibrary.<init>(EnterLibrary.java:22)
        at EnterLibrary.main(EnterLibrary.java:202)

Answer 1

您的代码中至少有两个严重错误:

1. 等待发生堆栈溢出:您在 EnterLibrary() 中调用 EnterLibrary()，这将产生无限递归
2. 虽然您已经实例化了 videos 数组，但您还没有初始化它，因为它充满了空对象；确保在 EnterLibrary 的构造函数开始时调用 createLibrary()，在调用 printVidLibrary() 之前，它遍历 videos 的所有内容并假定它已完全初始化