在此Java代碼中無法找到NullPointerException的源

Question

當我嘗試運行下面的代碼塊（應該打印出存儲在txt文件中的書籍列表）時，出現Null指針異常。 （故意在解析之前首先具有book對象的“回旋”實現）

空指針異常的出現在代碼中指示（基本上是引用BookPrint的任何地方）

有什么想法嗎？

     import java.io.*;
import java.util.Scanner;

/** there is a basic Book.java file that stores a title and a category that are set by a *constructor and has get methods for each and a "toString" Method for a Book object that *prints out the book's title and category
**/

/**

** BookPrint prints out all the books in the txt file as stored in an array of book objects
**derived by parsing the text file

**/

public class BookPrint
{
    private Book[] b_books;
    private Scanner defaultFR;
    private Book bookPerLine;


    /** 
      * main takes a txt file with book titles on separate lines in title category format  like "The Lion King, Children"/n "Yellow Sun, Fiction" /n etc
     */
    public static void main(String[] argv)
                  throws FileNotFoundException
    {
        BookPrint bk;
/**
The following declaration gives a NullPointer exception
**/
        bk = new BookPrint(new FileReader("C:\\Users\\Owner\\workspace\\Book\\src\\hotBooks.txt")); 
        Book[] books;

        books = bk.getBooks();

        // 
        //take each book in  and print out
        for(int i =0; i<50; i++){
            books[i].toString();
        }
    }

    /** constructor populates BookPrint 
     *
     */
    public BookPrint(FileReader fr)
           throws ParseError
    {
        defaultFR = new Scanner(fr);

//Null pointer exception when the parseAll method is called
        this.parseAll(defaultFR);             

    }

    /** Return array of books
     */
    public Book[] getBooks()
    {
        return b_books;
    }

    /** Parse all.
     *Null Pointer Exception here as well
     * 
     */
    private void parseAll(Scanner scn)
                 throws ParseError
    {
        //open scanner object that reads file
        //for all books in array, if there is next line parseOne, and store in book array

        try{
        while(scn.hasNext()){
            for(int i=0; i< 50; i++){
                b_books[i]= parseOne(scn.nextLine());
            }           
        }
        }
        finally{
      scn.close();
        }
    }

    /** Parse one 
     *
     * 
     */
    private Book parseOne(String line)
                  throws ParseError
    {
        Scanner scn = new Scanner(line);

        //parse line by "," , store each value as book.title and book.category and return book
        scn.useDelimiter(",");
        if (scn.hasNext() ){
          String title = scn.next();
          String category = scn.next();
          bookPerLine = new Book(title, category);

          }

        else {
          System.out.println("Empty or invalid line. Unable to process.");
        }
        scn.close();
        return bookPerLine;
    }
}

Answer 1

您要在parseAll()分配給b_books[i] ，而不創建數組。

private Book[] b_books = new Book[50];