Thread: Null pointer exception in inventory program

import java.util.Arrays;
import java.awt.event.*;
import java.text.NumberFormat;
import java.util.Locale;
import java.io.*;
import java.awt.*;
import javax.swing.*;
 
class InventoryGUI extends JFrame {
 
	private static final long serialVersionUID = 1L;
//Navigate inventory
	private static final int NAVIGATION_MODE = 0;
 
//Add new Items
	private static final int ADDITION_MODE = 1;
//Change items
	private static final int MODIFICATION_MODE = 2;
//Stores status of application
	private int currentMode;
        JTextField itemNumber = new JTextField();
	JTextField ISBN = new JTextField();
	JTextField title = new JTextField();
	JTextField authorName = new JTextField();
	JTextField year = new JTextField();
	JTextField pubName = new JTextField();
	JTextField price = new JTextField();
	JTextField valueOfInventory = new JTextField();
 
	JButton next = new JButton("Next >");
	JButton previous = new JButton("< Previous");
 
	JButton first = new JButton("<< First");
	JButton last = new JButton("Last >>");
 
	JButton add = new JButton("Add");
	JButton delete = new JButton("Delete");
	JButton modify = new JButton("Modify");
 
//Save or apply changes
	JButton saveOrApply = new JButton("Save");
 
//Search or cancel changes
	JButton searchOrCancel = new JButton("Search");
 
 
	private Inventory inventory;
 
//Current index
	private int currentIndex = 0;
 
	public InventoryGUI(Inventory inventory) {
	//Create GUI
		super();
		this.inventory = inventory;
 
		setTitle("Bookstore Part 6");
		setBounds(0, 0, 860, 500);
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
		createGUI();
		pack();
 
		setLocationRelativeTo(null);
 
		first.addActionListener(new ActionListener() {
 
			public void actionPerformed(ActionEvent e) {
				showFirst();
			}
		});
 
		previous.addActionListener(new ActionListener() {
 
			public void actionPerformed(ActionEvent e) {
				showPrevious();
			}
		});
 
		next.addActionListener(new ActionListener() {
 
			public void actionPerformed(ActionEvent e) {
				showNext();
			}
		});
 
		last.addActionListener(new ActionListener() {
 
			public void actionPerformed(ActionEvent e) {
				showLast();
			}
		});
 
		add.addActionListener(new ActionListener() {
 
			public void actionPerformed(ActionEvent e) {
				addItem();
			}
		});
 
		delete.addActionListener(new ActionListener() {
 
			public void actionPerformed(ActionEvent e) {
				deleteItem();
			}
		});
 
		modify.addActionListener(new ActionListener() {
 
			public void actionPerformed(ActionEvent e) {
				modifyItem();
			}
		});
 
		saveOrApply.addActionListener(new ActionListener() {
 
			public void actionPerformed(ActionEvent e) {
				if (getCurrentMode() == NAVIGATION_MODE) {
					saveInventory();
				} else {
					apply();
				}
			}
		});
 
		searchOrCancel.addActionListener(new ActionListener() {
 
			public void actionPerformed(ActionEvent e) {
				if (getCurrentMode() == NAVIGATION_MODE) {
					searchInventory();
				} else {
					cancel();
				}
			}
		});
 
		updateGUI();
	}
 
//Prepares to add new item
	protected void addItem() {
		clearFields();
 
		Integer newItemNumber = inventory.getMaxItemNumber() + 1;
		itemNumber.setText(newItemNumber.toString());
 
		title.requestFocus();
 
 
 
		setCurrentMode(ADDITION_MODE);
	}
 
//Updates item
	protected void apply() {
             int newItem = Integer.valueOf(itemNumber.getText());
 
 
		String newISBN = ISBN.getText();
		String newTitle = title.getText();
		String newauthorName = authorName.getText();
		int newYear;
                try {
                    newYear = Integer.parseInt(year.getText());
                } catch (Exception e){
                    showErrorMessage("Invalid year");
                    year.requestFocus();
                    return;
                }
		String newpubName = pubName.getText();
 
		double newPrice;
		try {
			newPrice = Double.parseDouble(price.getText());
		} catch (Exception e) {
			showErrorMessage("Invalid value for price.");
			price.requestFocus();
			return;
		}
 
 
 
		if (currentMode == ADDITION_MODE) {
 
			Book uf = new Book (newItem, newISBN, newTitle, newauthorName,
					newYear, newpubName, newPrice);
 
			inventory.addItem(uf);
			currentIndex = inventory.getItemCount() - 1;
 
		} else if (currentMode == MODIFICATION_MODE) {
 
			Book uf = inventory.getItem(currentIndex);
 
                        uf.setISBN(newISBN);
			uf.setTitle(newTitle);
			uf.setauthorName(newauthorName);
			uf.setYear(newYear);
			uf.setpubName(newpubName);
			uf.setPrice(newPrice);
		}
 
		setCurrentMode(NAVIGATION_MODE);
		updateGUI();
	}
//Sets GUI for navigation
	protected void cancel() {
		setCurrentMode(NAVIGATION_MODE);
		updateGUI();
	}
 
	//Clears editable fields
	private void clearFields() {
                itemNumber.setText("");
		ISBN.setText("");
		title.setText("");
		authorName.setText("");
		year.setText("");
		pubName.setText("");
		price.setText("");
 
	}
 
//Creates GUI
	private void createGUI() {
		JPanel itemPanel = new JPanel(new GridLayout(0, 4, 5, 5));
                itemPanel.add(new JLabel("Number of Items"));
                itemPanel.add(itemNumber);
 
		itemPanel.add(new JLabel("ISBN:"));
		itemPanel.add(ISBN);
 
		itemPanel.add(new JLabel("Title:"));
		itemPanel.add(title);
 
		itemPanel.add(new JLabel("Author: "));
		itemPanel.add(authorName);
 
		itemPanel.add(new JLabel("Year: "));
		itemPanel.add(year);
 
		itemPanel.add(new JLabel("Publisher: "));
		itemPanel.add(pubName);
 
		itemPanel.add(new JLabel("Price: $"));
		itemPanel.add(price);
 
 
		itemPanel.add(new JLabel("Value of Entire Inventory: $"));
		itemPanel.add(valueOfInventory);
		valueOfInventory.setEditable(false);
 
		JPanel inventoryPanel = new JPanel(new BorderLayout(5, 5));
		inventoryPanel.setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));
 
		inventoryPanel.add(new LogoPanel(getWidth(), 80), BorderLayout.NORTH);
 
		inventoryPanel.add(itemPanel, BorderLayout.CENTER);
 
		JPanel bottomPanel = new JPanel(new GridLayout(2, 1, 5, 5));
 
		JPanel navigationPanel = new JPanel(new GridLayout(1, 7, 5, 5));
		navigationPanel.add(first);
		navigationPanel.add(previous);
		navigationPanel.add(add);
		navigationPanel.add(delete);
		navigationPanel.add(modify);
		navigationPanel.add(next);
		navigationPanel.add(last);
		bottomPanel.add(navigationPanel);
 
		JPanel extraPanel = new JPanel(new GridLayout(0, 2, 5, 5));
		extraPanel.add(saveOrApply);
		extraPanel.add(searchOrCancel);
		bottomPanel.add(extraPanel);
 
		inventoryPanel.add(bottomPanel, BorderLayout.SOUTH);
		setContentPane(inventoryPanel);
 
		setCurrentMode(NAVIGATION_MODE);
	}
//Deletes items
	protected void deleteItem() {
		if (isInventoryEmpty()) {
			return;
		}
		if (JOptionPane.showConfirmDialog(this,
				"Do you really want to delete this item?", "Confirmation",
				JOptionPane.YES_NO_OPTION) == JOptionPane.YES_OPTION) {
			inventory.deleteItem(currentIndex);
			currentIndex = inventory.getItemCount() - 1;
			updateGUI();
		}
	}
 
	public int getCurrentMode() {
		return currentMode;
	}
 
//Checks if inventory is empty
	private boolean isInventoryEmpty() {
		if (inventory.getItemCount() == 0) {
			JOptionPane.showMessageDialog(this, "The Inventory is Empty!");
			return true;
		}
		return false;
	}
 
//Prepares for modifications
	protected void modifyItem() {
		if (isInventoryEmpty()) {
			return;
		}
		setCurrentMode(MODIFICATION_MODE);
	}
 
//Saves inventory
	protected void saveInventory() {
		if (isInventoryEmpty()) {
			return;
		}
		String[] items = inventory.serialize();
 
		File file;
 
		file = new File("c:\\data");
		if (!file.exists()) {
			file.mkdir();
		}
 
		file = new File("c:\\data\\inventory.dat");
		if (file.exists()) {
			file.delete();
		}
 
		try {
			file.createNewFile();
		} catch (IOException e) {
			showErrorMessage("Failed to create the Inventory file.");
			return;
		}
 
		try {
			FileWriter fw = new FileWriter(file);
			for (int i = 0; i < items.length; i++) {
				fw.write(items[i] + "\n");
			}
			fw.close();
 
			JOptionPane.showMessageDialog(this, "File successfully saved!");
		} catch (IOException e) {
			showErrorMessage("Failed to write the Inventory file.");
		}
	}
 
	//Searches inventory
	protected void searchInventory() {
		if (isInventoryEmpty()) {
			return;
		}
		String itemName = JOptionPane.showInputDialog(this,
				"Enter the Item Name", "Search Item",
				JOptionPane.QUESTION_MESSAGE);
 
		if (itemName != null) {
			int item = inventory.getItemByName(itemName);
			if (item != -1) {
				currentIndex = item;
				updateGUI();
			} else {
				showErrorMessage(String.format("Item \"%s\" not found.",
						itemName));
			}
		}
	}
 
	//Changes status of app
	public void setCurrentMode(int currentMode) {
		this.currentMode = currentMode;
 
		switch (currentMode) {
 
		case ADDITION_MODE:
		case MODIFICATION_MODE:
			first.setEnabled(false);
			previous.setEnabled(false);
			add.setEnabled(false);
			modify.setEnabled(false);
			delete.setEnabled(false);
			next.setEnabled(false);
			last.setEnabled(false);
			saveOrApply.setText("Apply");
			searchOrCancel.setText("Cancel");
			setFieldsEditable(true);
			break;
 
		case NAVIGATION_MODE:
			first.setEnabled(true);
			previous.setEnabled(true);
			add.setEnabled(true);
			modify.setEnabled(true);
			delete.setEnabled(true);
			next.setEnabled(true);
			last.setEnabled(true);
			saveOrApply.setText("Save");
			searchOrCancel.setText("Search");
			setFieldsEditable(false);
			break;
		}
	}
//Sets fields editable
	protected void setFieldsEditable(boolean editable) {
		ISBN.setEditable(editable);
		title.setEditable(editable);
		authorName.setEditable(editable);
		year.setEditable(editable);
		pubName.setEditable(editable);
		price.setEditable(editable);
	}
 
//Shows error
	private void showErrorMessage(String message) {
		JOptionPane.showMessageDialog(this, message, "Error",
				JOptionPane.ERROR_MESSAGE);
	}
//Displays first item
	protected void showFirst() {
		if (isInventoryEmpty()) {
			return;
		}
		currentIndex = 0;
		updateGUI();
	}
 
//displays last
	protected void showLast() {
		if (isInventoryEmpty()) {
			return;
		}
		currentIndex = inventory.getItemCount() - 1;
		updateGUI();
	}
 
//Displays next
	private void showNext() {
		if (isInventoryEmpty()) {
			return;
		}
		if (currentIndex < inventory.getItemCount() - 1) {
			currentIndex++;
			updateGUI();
		} else {
			showFirst();
		}
	}
 
//Shows previous
	private void showPrevious() {
		if (isInventoryEmpty()) {
			return;
		}
		if (currentIndex > 0) {
			currentIndex--;
			updateGUI();
		} else {
			showLast();
		}
	}
 
//Updates fields
	public void updateGUI() {
		if (currentIndex != -1) {
			Book  item = inventory.getItem(currentIndex);
 
			ISBN.setText(String.valueOf(item.getISBN()));
			title.setText(item.getTitle());
			authorName.setText(String.valueOf(item.getauthorName()));
			year.setText(String.format("%d", item.getYear()));
			pubName.setText(String.format( item.getpubName()));
			price.setText(String.format("%.2f", item.getPrice()));
 
		} else {
			clearFields();
		}
 
		valueOfInventory.setText(String.format("%.2f", inventory.getValueOfInventory()));
	}
}
public class Bookstore6{
 
	public static void main(String args[]) {
 
		Book  p1 = new Book  (1,"AIF435", "Java Programming", "Bob Oracle", 2012, "Sue Oracle", 8.74);
		Book  p2 = new Book (1,"ABR43F", "Cats 101", "Walter White", 2011, "Skyler White", 9.46);
		Book  p3 = new Book  (1,"AIF435", "Dogs 201", "Jesse Pinkman", 2004, "Bob Joe", 10.00);
		Book  p4 = new Book (1,"YUI46", "Adventure Books", "Laurie Smith", 2011, "Joe Smith", 8.97);
		Book  p5 = new Book  (1,"AGVI345", "Candy Corn", "Jimmy Johnson", 2010, "Sue Zumba", 5.55);
		Book  p6 = new Book  (1,"UIFE45", "Childrens Books", "Brittany Anderson", 2012, "Alex Moona", 12.75);
 
 
		Inventory inv = new Inventory();
		inv.addItem(p1);
		inv.addItem(p2);
		inv.addItem(p3);
		inv.addItem(p4);
		inv.addItem(p5);
		inv.addItem(p6);
		inv.sortItems();
 
		new InventoryGUI(inv).setVisible(true);
	}
}
 class LogoPanel extends JPanel {
 
	private static final long serialVersionUID = 1L;
 
	public LogoPanel(int width, int height) {
		super();
		setPreferredSize(new Dimension(width, height));
	}
 
 
	public void paint(Graphics g) {
		g.setColor(Color.BLACK);
		g.fill3DRect(0, 0, getWidth(), getHeight(), true);
 
		int midX = getWidth() / 2;
		int midY = getHeight() / 2;
 
		g.setColor(Color.RED);
		g.fillOval(midX - 100, midY - 25, 50, 50);
 
		g.setColor(Color.WHITE);
		g.setFont(new Font("Verdana", Font.BOLD, 28));
		g.drawString("Lauren's Logo", midX - 70, midY + 10);
	}
 
}
 
class Book {
NumberFormat nf = NumberFormat.getCurrencyInstance(Locale.US);
private int itemNumber;
private String ISBN;
private String title;
private String authorName;
private int year;
private String pubName;
private double price;
 
public Book ( int itemNumber, String ISBN, String title, String authorName, int year, String pubName, double price)
{
this.itemNumber = itemNumber;
this.ISBN = ISBN;
this.title = title;
this.authorName = authorName;
this.year = year;
this.pubName = pubName;
this.price = price;
 
}
 
public int getItemNumber() {
    return itemNumber;
 }
public void setItemNumber() {
    this.itemNumber = itemNumber;
}
public String getISBN() {
	return ISBN;
}
public void setISBN (String identNum){
	this.ISBN = ISBN;
}
public String getTitle () {
	return title;
}
public void setTitle (String title) {
	this.title = title;
}
public String getauthorName () {
	return authorName;
}
public void setauthorName (String authorName) {
	this.authorName = authorName;
}
public String getpubName () {
	return pubName;
}
public void setpubName (String pubName) {
	this.pubName = pubName;
}
public int getYear () {
	return year;
}
public void setYear (int year) {
	this.year = year;
}
public double getPrice () {
	return price;
}
 
public  void setPrice (double bookPrice){
	this.price = price;
}
	public String serialize() {
		return String.format("%d|%s|%s|%s|%d|%s|%.2f",itemNumber, ISBN, title, authorName,
				year, pubName,price);
	}
 
}
 
 
 
class Inventory {
	private Book[] bookList;
	private int itemCount = 0;
 
	public Inventory() {
		bookList = new Book[100];
	}
        	public void add(Book p) {
		Book[] temp = new Book[bookList.length+1];
		for (int i = 0; i < bookList.length; i++) {
			temp[i] = bookList[i];
		}
		temp[temp.length-1] = p; // add it at the end
		bookList = temp;
		itemCount++;
	}
 
	public void addItem(Book p) {
		bookList[itemCount++] = p;
	}
 
	public void deleteItem(int currentIndex) {
		for (int i = currentIndex; i < (getItemCount() - 1); i++) {
			bookList[i] = bookList[i + 1];
		}
		bookList[getItemCount() - 1] = null;
		itemCount--;
	}
 
	public Book getItem(int i) {
		return bookList[i];
	}
 
	public int getItemByName(String title) {
		int item = -1;
		if (itemCount > 0) {
			for (int i = 0; i < itemCount; i++) {
				Book uf = bookList[i];
				if (title.equalsIgnoreCase(uf.getTitle())) {
					item = i;
					break;
				}
			}
		}
		return item;
	}
 
	public int getItemCount() {
		return itemCount;
	}
 
public double getValueOfInventory (){
		double total = 0;
	for (int i = 0; i < bookList.length; i++){
 
			total += bookList[i].getPrice();
		}
 
		return total;
	}
 
	public int getMaxItemNumber() {
		int max = 0;
		if (itemCount > 0) {
			for (int i = 0; i < itemCount; i++) {
				Book uf = bookList[i];
				if (uf.getItemNumber() > max) {
					max = uf.getItemNumber();
				}
			}
		}
		return max;
	}
 
 
	public void sortItems() {
 
		int n = getItemCount();
		for (int search = 1; search < n; search++) {
			for (int i = 0; i < n - search; i++) {
				if (bookList[i].getTitle().compareToIgnoreCase(
						bookList[i + 1].getTitle()) > 0) {
 
					Book temp = bookList[i];
					bookList[i] = bookList[i + 1];
					bookList[i + 1] = temp;
				}
			}
		}
	}
 
	public String[] serialize() {
		String[] items = new String[getItemCount()];
		for (int i = 0; i < itemCount; i++) {
			items[i] = getItem(i).serialize();
		}
		return items;
	}
}

Exception in thread "main" java.lang.NullPointerException
at bookstoreprogrampart5.Inventory.getValueOfInventor y(BookstoreProgramPart5.java:675)
at bookstoreprogrampart5.InventoryGUI.updateGUI(Books toreProgramPart5.java:492)
at bookstoreprogrampart5.InventoryGUI.<init>(Bookstor eProgramPart5.java:143)
at bookstoreprogrampart5.BookstoreProgramPart5.main(B ookstoreProgramPart5.java:516)

Not too sure why I can't solve this. Ive been sick for weeks, so I'm sure that has something to do with it. Any suggestions would be greatly appreciated.