Java Add Button GUI Grow Array

Question

I am trying to create an add button in a GUI interface which I have achieved, but I need this "add button" to prompt the user to enter data into the supplied fields and the application place all of the existing inventory as well as the new book created into a new array. The error I am currently getting is when trying to run it after it compiles.

"Exception in thread "main" java.lang.NullPointerException
at Bookstore.calculateInventoryTotal(Bookstore.java:198)
at Bookstore.main(Bookstore.java:232)"

Please know that because of this exception the GUI doesn't launch at all. I have pasted all of the code below, and thank you for your time taking a look at this!

import java.util.Arrays;
import java.text.NumberFormat;
import java.util.Locale;
import java.text.DecimalFormat;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;





// Begin class Book
class Book
{
private String isbn;
private String title;
private String authorName;
private int yearPublished;
private String publisherName;
private double price;

NumberFormat usCurrency = NumberFormat.getCurrencyInstance(Locale.US);

public Book (String isbn, String title, String authorName, int yearPublished, String publisherName, double price)
{
    this.isbn = isbn;
    this.title = title;
    this.authorName = authorName;
    this.yearPublished = yearPublished;
    this.publisherName = publisherName;
    this.price = price;
}


///////////////////////////////////////////////
public void setISBN (String ISBN) //set ISBN
{
    this.isbn = ISBN;
}
public String getISBN () //get ISBN
{
    return isbn;
}
//////////////////////////////////////////////
public void setTitle (String Title) //set Title
{
    this.title = Title;
}
public String getTitle () //get Title
{
    return title;
}
///////////////////////////////////////////////
public void setAuthorName (String AuthorName) //set AuthorName
{ 
    this.authorName = AuthorName;
}
public String getAuthorName () //get AuthorName
{
    return authorName;
}
///////////////////////////////////////////////
public void setYearPublished (int YearPublished)//set YearPublished
{
    this.yearPublished = YearPublished;
}
public int getYearPublished () //get YearPublished
{
    return yearPublished;
}
///////////////////////////////////////////////
public void setPublisherName (String PublisherName)
{
    this.publisherName = PublisherName;
}
public String getPublisherName ()
{
    return publisherName;
}
///////////////////////////////////////////////
public void setPrice (double Price)
{
    this.price = Price;
}
public double getPrice ()
{
    return price;
}


//toString method
public String toString ()
{
    return "ISBN:" + "\t\t\t" + isbn + "\n" +
           "Title:" + "\t\t\t" + title + "\n" +
           "Author's Name:" + "\t \t" + authorName + "\n" +
           "Year Published:" + "\t \t" + yearPublished + "\n" +
           "Publisher's Name:" + "\t\t" + publisherName + "\n" +
           "Price" + "\t\t\t" + usCurrency.format(price) + "\n";
}
} // end class Book


//Begin class EBook
class EBook extends Book
{
private String webSite;


// constructor
public EBook (String isbn, String title, String authorName, int yearPublished, String publisherName, double price, String webSite)
{
    super(isbn, title, authorName, yearPublished, publisherName, price);
    setWebsite(webSite);
}

//accessor methods
public void setWebsite(String webSite)
{
    this.webSite = webSite;
}
public String getWebsite ()
{
    return webSite;
}


public double discount ()
{
    return (super.getPrice()) * .10; // EBook discount of 10%
}

public String toString ()
{
    return super.toString() + "Website:" + "\t\t\t" + webSite + "\n" +
                    "EBook Discount:" + "\t\t" + usCurrency.format(discount()) + "\n";
}

} //end EBook class



public class Bookstore
{
private static Book inventoryBook[] = new Book[5];
private static NumberFormat usCurrency = NumberFormat.getCurrencyInstance(Locale.US);
static int bookIndex = 0;

public static JTextArea prepareDisplay (Book myBook, JTextArea myTextArea)
{
    myTextArea.setText("");

    myTextArea.append(myBook.toString());

    return myTextArea;
}

public static Book [] sortArray(Book[] books)
{
    // Step1
    String[] titles = new String[books.length];

    // Step2
    Book[] sortedBooks = new Book [books.length];

    // Step3
    for (int i = 0; i < books.length; i++)
    {
        titles[i] = books[i].getTitle();
    }

    // Step4
    Arrays.sort(titles, String.CASE_INSENSITIVE_ORDER);

    // Step5
    for (int i = 0; i < books.length; i++)
    {
        for (int j = 0; j < titles.length; j++)
        {
            if (books[i].getTitle().equalsIgnoreCase(titles[j]))
            {
                sortedBooks[j] = books[i];
                break;
            }
        }
    }

    return sortedBooks;
}

public static double calculateInventoryTotal(Book[] books)
{
    double total = 0;

    for (int i = 0; i < books.length; i++)
    {
        total += books[i].getPrice();
    }

    return total;
}



public static void main ( String args [])
{




    //initial array of Bookstore before anything is added
    inventoryBook [0] = new EBook ("0075260012", "David goes to School", "David Shannon", 2010, "Shannon Rock", 11.98, "http://www.tinyurl.qqwert67o9");
    inventoryBook [1] = new Book ("7423540089", "No David!", "David Shannon", 2009, "Shannon Rock", 12.99);
    inventoryBook [2] = new Book ("0743200616", "Simple Abundance", "Sarah Breathnach", 2009, "Scribner", 14.99);
    inventoryBook [3] = new EBook ("78137521819", "The very hungry caterpillar", "Eric Carle", 2005, "Philomel Books", 13.99, "http://www.tinyurl.fguopt8u90");
    inventoryBook [4] = new Book ("9781416987116", "We are going on a bear hunt", "Michael Rosen", 2009, "McElderry", 15.99);

    final Book [] newBookInventory = new Book [inventoryBook.length + 1];


    for (int i = 0; i < inventoryBook.length; i++)
    {
        newBookInventory[i] = inventoryBook[i];
    }

    inventoryBook = newBookInventory;


    //inventoryBook = sortArray(inventoryBook); 

    final double inventoryTotal = calculateInventoryTotal(newBookInventory);

    final JTextArea textArea = new JTextArea(30, 30);
    textArea.setText("");
    textArea.setEditable(false);

    JPanel buttonPanel = new JPanel();
    buttonPanel.setLayout(new GridLayout(1,3));

    JButton firstButton = new JButton("First");
    buttonPanel.add(firstButton);
    firstButton.addActionListener(new ActionListener()
    {
        public void actionPerformed(ActionEvent e)
        {
            bookIndex = 0;
            prepareDisplay(inventoryBook[bookIndex], textArea);
            textArea.append("\n Total Inventory Value: " + "\t\t" + usCurrency.format(inventoryTotal));
        }
    });

    JButton previousButton = new JButton("Previous");
    buttonPanel.add(previousButton);
    previousButton.addActionListener(new ActionListener()
    {
        public void actionPerformed(ActionEvent e)
        {
            if(bookIndex == 0) 
            {
                bookIndex = inventoryBook.length - 1;
            } 
            else 
            {
                bookIndex = bookIndex - 1;
            }
            prepareDisplay(inventoryBook[bookIndex], textArea);
            textArea.append("\n Total Inventory Value: " + "\t\t" + usCurrency.format(inventoryTotal));
        }
    });

    JButton nextButton = new JButton("Next");
    buttonPanel.add(nextButton);
    nextButton.addActionListener(new ActionListener()
    {
        public void actionPerformed(ActionEvent e)
        {
            if(bookIndex == inventoryBook.length - 1) 
            {
                bookIndex = 0;
            } 
            else 
            {
                bookIndex = bookIndex + 1;
            }
            prepareDisplay(inventoryBook[bookIndex], textArea);
            textArea.append("\n Total Inventory Value: " + "\t\t" + usCurrency.format(inventoryTotal));
        }
    });

    JButton lastButton = new JButton("Last");
    buttonPanel.add(lastButton);
    lastButton.addActionListener(new ActionListener()
    {   
        public void actionPerformed(ActionEvent e)
        {
            bookIndex = (inventoryBook.length - 1);
            prepareDisplay(inventoryBook[bookIndex], textArea);
            textArea.append("\n Total Inventory Value: " + "\t\t" + usCurrency.format(inventoryTotal));
        }
    });


    JButton searchButton = new JButton("Search");
    buttonPanel.add(searchButton);
    searchButton.addActionListener(new ActionListener()
    {
        public void actionPerformed(ActionEvent e)
        {
            boolean matchFound = false;
            String searchCriteria = JOptionPane.showInputDialog("Enter Book Title");

            for (int i = 0; i < inventoryBook.length; i++)
            {
                if (inventoryBook[i].getTitle().equalsIgnoreCase(searchCriteria))
                {
                    matchFound = true;
                    bookIndex = i;
                    break;
                }
            }

            if (matchFound)
            {
                prepareDisplay(inventoryBook[bookIndex], textArea);
            }
            else
            {
                JOptionPane.showMessageDialog( null, "Book Title " + searchCriteria + " does not exist.");
            }
        }
    });

    JButton modifyButton = new JButton("Modify");
    buttonPanel.add(modifyButton);
    modifyButton.addActionListener(new ActionListener ()
    {
        public void actionPerformed(ActionEvent e)
        {
            String title = JOptionPane.showInputDialog(null, "Enter Book Title", inventoryBook[bookIndex].getTitle());
            if (title != null)
            {
                String isbn = JOptionPane.showInputDialog(null, "Enter ISBN", inventoryBook[bookIndex].getISBN());
                if (isbn != null)
                {
                    String authorName = JOptionPane.showInputDialog(null, "Enter Author's Name", inventoryBook[bookIndex].getAuthorName());
                    if (authorName != null)
                    {
                        String yearPublished = JOptionPane.showInputDialog(null, "Enter Year Published", inventoryBook[bookIndex].getYearPublished());
                        if (yearPublished != null)
                        {
                            String publisherName = JOptionPane.showInputDialog(null, "Enter Publisher Name", inventoryBook[bookIndex].getPublisherName());
                            if (publisherName != null)
                            {
                                String price = JOptionPane.showInputDialog(null, "Enter Price", inventoryBook[bookIndex].getPrice());
                                if (price != null)
                                {
                                    inventoryBook[bookIndex].setTitle(title);
                                    inventoryBook[bookIndex].setISBN(isbn);
                                    inventoryBook[bookIndex].setAuthorName(authorName);
                                    inventoryBook[bookIndex].setYearPublished(Integer.parseInt(yearPublished));
                                    inventoryBook[bookIndex].setPublisherName(publisherName);
                                    inventoryBook[bookIndex].setPrice(Double.parseDouble(price));

                                    prepareDisplay(inventoryBook[bookIndex], textArea);
                                }
                            }
                        }
                    }
                }
            }
        }
    });


    JButton addButton = new JButton("Add");
    buttonPanel.add(addButton);
    addButton.addActionListener(new ActionListener ()
    {
        public void actionPerformed(ActionEvent e)
        {
            String title = JOptionPane.showInputDialog(null, "Enter Title");
            if (title != null)
            {
                String isbn = JOptionPane.showInputDialog(null, "Enter ISBN");
                if (isbn != null)
                {
                    String authorName = JOptionPane.showInputDialog(null, "Enter Author's Name");
                    if (authorName != null)
                    {
                        String yearPublished = JOptionPane.showInputDialog(null, "Enter Year Published");
                        if (yearPublished != null)
                        {
                            String publisherName = JOptionPane.showInputDialog(null, "Enter Publisher Name");
                            if (publisherName != null)
                            {
                                String price = JOptionPane.showInputDialog(null, "Enter Price");
                                if (price != null)
                                {


                                    Book newBook = new Book (title, isbn, authorName, (Integer.parseInt(yearPublished)), publisherName,(Double.parseDouble(price)));
                                    inventoryBook[newBookInventory.length - 1] = newBook;
                                    prepareDisplay(inventoryBook[bookIndex], textArea);
                                }
                            }
                        }
                    }
                }
            }
        }
    });




    JLabel logoLabel = new JLabel (new ImageIcon("GoblinBooks.jpg"));
    JPanel logoPanel = new JPanel();
    logoPanel.add(logoLabel);

    JPanel centerPanel = new JPanel();
    centerPanel.setLayout(new BoxLayout(centerPanel, BoxLayout.Y_AXIS));
    centerPanel.add(prepareDisplay(inventoryBook[bookIndex], textArea));




    //for (int i = 0; i < inventoryBook.length; i++ )
    //{
    //  textArea.append(inventoryBook[i] + "\n");
    //}



    textArea.append("Total Inventory Value: " + "\t\t" + usCurrency.format(inventoryTotal));

    JFrame frame = new JFrame();
    frame.setLayout(new BorderLayout());
    frame.add(logoPanel, BorderLayout.NORTH);
    frame.add(buttonPanel, BorderLayout.SOUTH);
    frame.add(centerPanel, BorderLayout.CENTER);
    frame.getContentPane().add(new JScrollPane(textArea));
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
}
} // end class Bookstore

Mad, thank you again, the null itself I understand, I am not sure if I am to null the books[i] value, what would I return here instead. It should look like this, but I am uncertain to how make it calculate the original inventory prior to any adding. Any and all help is much appreciated!

public static double calculateInventoryTotal(Book[] books)
{


    double total = 0;

    for (int i = 0; i < books.length; i++)
    {
        total += books[i].getPrice();
        if(books[i]!= null) return ???;
    }

    return total;
}

Answer 1

The problem is one of the elements in the books array in the calculateInventoryTotal is null

This caused by this

final Book [] newBookInventory = new Book [inventoryBook.length + 1];

You then populate this array with contents from inventoryBook, but the last element is null

Add a null check in your calculateInventoryTotal Method before trying to access the element in the array

Basically an array is a series of "buckets" which "may" contain a reference to an object. You need to be checking that the element you are trying to inspect is not null before you try to access any it's properties, for example

for (int i = 0; i < books.length; i++)
{
    if(books[i]!= null) {
        total += books[i].getPrice();
    }
}