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I made a two classes, one text file, and a code that should print everything out, but it still becomes null no matter what I do? I'm trying to read the code from the text file by using the toString method from the Book class and the printInventory from the Inventory class in the InventoryTester.

My first code derives on what should be printed. I made a constructor and I used setters and getters to make this. Then ended it with a toString() method. I made a Book class that should print everything to the printInventory in the InventoryTester.

Book class:

import java.math.RoundingMode;
import java.text.DecimalFormat;
import java.lang.*;
public class Book {
   
   // Initiate variables
   // Note: I used the DecimalFormat so every price is always going to have 2 decimal spaces
   private static DecimalFormat df2 = new DecimalFormat("#.##");
   private String ISBN;
   private String author;
   private String title;
   private int edition;
   private String pubCode;
   private int quantity;
   private double price;
   
   //constructor
   public Book(String isbn, String auth, String ti, int ed, String pc, int qty, double pr)
   {
      ISBN = isbn;
      author = auth;
      title = ti;
      edition = ed;
      pubCode = pc;
      quantity = qty;
      price = pr;
   }
   
   //getters
   public String getTitle(){return title;}
   public String getAuthor(){return author;}
   public double getPrice(){return price;}
   public int getEdition(){return edition;}
   public String getISBN(){return ISBN;}
   public String getpubCode(){return pubCode;}
   public int getQuantity(){return quantity;}
   
   //setters
   public void changePrice(double newPrice){price = newPrice;}
   public boolean changeQuantity(int changeAmt){
   changeAmt = quantity + changeAmt;
   if (changeAmt < 0) {
   return false;
   } 
   else {
   return true;
   }
   }

   //This prints out all of the information once the object 'Book' is used
   public String toString() 
   {
      String subAuthor = "";
      String subTitle = "";
      subAuthor = author.substring(0,16);
      subTitle = title.substring(0,32);
      return "ISBN: " +ISBN + "\nAuthor: " + subAuthor + "\nTitle: " + subTitle + "\nEdition: " + edition + "\nPublisher Code: " + pubCode +"\nQuantity: " +quantity+ "\nPrice: $ " +df2.format(price) + "\n\n";
   }
}

Then I made a class called Inventory to manage instances of the Book class. However, I'm still not sure if most of my code is correct since I compiled it without any errors. What I mean is the constructor itself, the addBook method and the changeQuantity method.

The constructor takes one argument, which is the size of the books array. It should create the empty array of the specified size, and initializes the nextEntry field to be 0. I use the printInventory method here. This method should print the entire inventory from the Book class, using the toString()method on each book.

Inventory class:

import java.math.RoundingMode;
import java.text.DecimalFormat;
import javax.swing.JOptionPane;
import javax.swing.UIManager;
import java.util.Arrays;

public class Inventory {

   private Book[] books;
   private int nextEntry;
   int questionAmt;
   
   public Inventory(int size){
      size = 7; 
      books = new Book[size];
      nextEntry = 0; 
   }
   
   public boolean addBook(Book theBook) {
   if (nextEntry > 27)
   { return false;}
   else{
   nextEntry++;
   books[nextEntry] = theBook;
   return true;
   }
  }
   
   public boolean changeQuantity(String isbn, int changeAmt) {
      if (changeAmt < 0) {
         return false;
      } 
      else {
         return true;
      }
      }
   public void printInventory()
   {
   for  (int i=0; i<books.length; i++) {
   System.out.println(books[i].toString()); 
   
   return;
   }
   }

Then I added the text file, which is called inventory. This is what should be shown in the code and I should be able to edit the quantity it inside the console. I also added a 28 because it needs 28 Books.

inventory.txt:

28
013478796X_Tony Gaddis_Starting Out with Java: From Control Structures through Data Structures_4_PE_10_121.75
0321409493_John Lewis_Java Software Solutions: Foundations of Program Design_5_AW_12_94.05
0023606924_Richard Johnsonbaugh_Algorithms_1_PH_1_109.00
0134743350_Harvey Dietel_Java: How to Program, Early Objects_12_PE_12_134.84
0131474340_Ralph Morelli_Java, Java, Java, Object-Oriented Problem Solving_3_PH_4_95.25
0596100469_Alex Martelli_Python in a Nutshell_2_OR_6_39.99
0134802217_Tony Gaddis_Starting Out with Java: From Control Structures through Objects_7_PE_8_118.67
1403946876_Sally Fincher_Studying Programming_1_PM_3_26.59
0596510047_Andy Oram_Beautiful Code: Leading Programmers Explain How They Think_1_OR_5_44.99
0143037889_Ray Kurzweil_The Singularity is Near: When Humans Transcend Biology_1_PG_20_17.70
0135205972_John Lewis_Java Foundations: Introduction to Program Design and Data Structures_5_PE_5_129.99
0131872893_Wanda Dann_Learning to Program with Alice_1_PH_12_47.50
159413962X_Dave Eggers_The Circle_1_AW_4_3.99
1887902996_John Zelle_Python Programming: An Introduction to Computer Science_1_FB_2_26.40
0133356728_Rafael Gonzales_Digital Image Processing_4_PE_3_248.17
1592400876_Lynne Truss_Eats, Shoots & Leaves_1_PG_5_17.70
0072823798_William Collins_Data Structures and the Java Collections Framework_2_MH_6_105.57
0072866098_Allen Tucker_Programming Languages: Principles and Paradigms_2_MH_1_127.50
0534950973_Michael Sipser_Introduction to the Theory of Computation_2_CT_3_98.90
0131496710_Francis Hill_Computer Graphics Using OpenGL 3rd Edition_3_PH_4_112.00
0321173486_Dave Shreiner_OpenGL Programming Guide_5_AW_1_24.00
0072865512_Steven Schach_Object Oriented and Classical Software Engineering_6_MH_9_123.44
0321228383_Michael Kifer_Database Systems: An Application-Oriented Approach_2_AW_3_112.86
1416587787_Cliff Stoll_The Cuckoo's Egg_1_PG_3_13.32
1400032717_Mark Haddon_The Curious Incident of the Dog in the Night-Time_1_VI_10_13.95
006025492X_Maurice Sendak_Where the Wild Things Are_1_HC_6_17.95
0694003611_Margaret Brown_Goodnight Moon_1_HC_138_8.99
069401298X_Arnold Lobel_Frog and Toad Together_1_HC_27_11.55

Last, but not least, I made an InventoryTester file to read the text file and print it using the code from both classes.

InventoryTester:

import javax.swing.JOptionPane;
import javax.swing.UIManager;
import java.awt.Font;
import java.util.Scanner;
import java.io.*;
public class InventoryTester {
   public static void main(String[] args) throws IOException {
      Scanner inFile= new Scanner(new File("inventory.txt"));
      Book books;
      String inLine= inFile.nextLine();
      int size= Integer.parseInt(inLine);
      Inventory myInventory= new Inventory(size);
      
      while (inFile.hasNext()) {
      inLine= inFile.nextLine();
      String ISBN = inFile.next();       
      String author = inFile.next(); 
      String title = inFile.next();
      int edition = inFile.nextInt();
      String pubCode = inFile.next();
      int quantity = inFile.nextInt();
      double price = inFile.nextDouble();
      myInventory.addBook(new Book(ISBN, author, title, edition, pubCode, quantity, price));
      String[] tokens  = inLine.split("_");
      myInventory.printInventory(); 
      }
      
      inFile.close();
      }
      }

I was trying to make Java understand that there are strings, ints, and doubles. I'm trying to convert the text file using arrays into Strings, ints, and doubles. It should print, but the code considers it to be null. Am I missing something?

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "Book.toString()" because "this.books[i]" is null
    at Inventory.printInventory(Inventory.java:42)
    at InventoryTester.main(InventoryTester.java:28)

 ----jGRASP wedge2: exit code for process is 1.
user15
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  • 1
  • Use something better than an array for your book inventory, an ArrayList is a very good option that is easy to use. Regarding the error, you add 1 book and then you try to print 7 books. – Joakim Danielson Mar 07 '21 at 14:03
  • Does this answer your question? [What is a NullPointerException, and how do I fix it?](https://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) – Joakim Danielson Mar 07 '21 at 14:04
  • If you use an ArrayList to hold your Inventory, you don't need the magic number 28. – NomadMaker Mar 07 '21 at 14:30

1 Answers1

0

You could make a class to manage your inventory with an arraylist containing books. And then save that class in a binary file.

The class could look like this:

public class BookList implements Serializable
{
  private ArrayList<Book> books;

  public BookList()
  {
    books = new ArrayList<>();
  }

  public void addBook(Book book)
  {
    this.books.add(book);
  }

  public void removeBook(Book book)
  {
    this.books.remove(book);
  }

  public ArrayList<Book> getAllBooks()
  {
    return books;
  }

And then save and load the file like this.

File file = new File("inventory.bin")

public void save(BookList bookList){
        FileOutputStream out = new FileOutputStream(file);
        ObjectOutputStream oos = new ObjectOutputStream(out);
        oos.writeObject(bookList);
}

public BookList load(){
        BookList bookList = null;
        
        FileInputStream in = new FileInputStream(file);
        ObjectInputStream ois = new ObjectInputStream(in);

        bookList = (BookList) ois.readObject();
        in.close();
        ois.close();
    }

Then you shouldn't have any trouble reading from the file.

REMEMBER! Both the list class and book class needs to implement Serializable.

Firrez
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  • 2