need help with java program

// created by Nicholas Baatz on July 24,2007
  import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;



public class Inventory2
{
//main method begins execution of java application
public static void main(final String args[])
{

int i; // varialbe for looping
double total = 0; // variable for total inventory
final int dispProd = 0; // variable for actionEvents

// Instantiate a product object
final ProductAdd[] nwProduct = new ProductAdd[5];
// Instantiate objects for the array
for (i=0; i<5; i++)
{
nwProduct[0] = new ProductAdd("Paper", 101, 10, 1.00, "Box");
nwProduct[1] = new ProductAdd("Pen", 102, 10, 0.75, "Pack");
nwProduct[2] = new ProductAdd("Pencil", 103, 10, 0.50, "Pack");
nwProduct[3] = new ProductAdd("Staples", 104, 10, 1.00, "Box");
nwProduct[4] = new ProductAdd("Clip Board", 105, 10, 3.00, "Two Pack");
}



for (i=0; i<5; i++)
total += nwProduct.length; // calculate total inventory cost



final JButton firstBtn = new JButton("First"); // first button
final JButton prevBtn = new JButton("Previous"); // previous button
final JButton nextBtn = new JButton("Next"); // next button
final JButton lastBtn = new JButton("Last"); // last button
final JLabel label; // logo
final JTextArea textArea; // text area for product list
final JPanel buttonJPanel; // panel to hold buttons

//JLabel constructor for logo
Icon logo = new ImageIcon("C:/logo.jpg"); // load logo
label = new JLabel(logo); // create logo label
label.setToolTipText("Company Logo"); // create tooltip


buttonJPanel = new JPanel(); // set up panel
buttonJPanel.setLayout( new GridLayout(1, 4)); //set layout
// add buttons to buttonPanel
buttonJPanel.add(firstBtn);
buttonJPanel.add(prevBtn);
buttonJPanel.add(nextBtn);
buttonJPanel.add(lastBtn);


textArea = new JTextArea(nwProduct[3]+"\n"); // create textArea for product display

// add total inventory value to GUI
textArea.append("\nTotal value of Inventory "+new java.text.DecimalFormat("$0.00").format(total)+"\n\n");
textArea.setEditable(false); // make text uneditable in main display
JFrame invFrame = new JFrame(); // create JFrame container
invFrame.setLayout(new BorderLayout()); // set layout
invFrame.getContentPane().add(new JScrollPane(textArea), BorderLayout.CENTER); // add textArea to JFrame
invFrame.getContentPane().add(buttonJPanel, BorderLayout.SOUTH); // add buttons to JFrame
invFrame.getContentPane().add(label, BorderLayout.NORTH); // add logo to JFrame
invFrame.setTitle("Office Min Inventory"); // set JFrame title
invFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); // termination command
//invFrame.pack();
invFrame.setSize(400, 400); // set size of JPanel
invFrame.setLocationRelativeTo(null); // set screem location
invFrame.setVisible(true); // display window



// assign actionListener and actionEvent for each button
firstBtn.addActionListener(new ActionListener()
{
public void actionPerformed(ActionEvent ae)
{
textArea.setText(nwProduct[0]+"\n");
} // end firstBtn actionEvent

}); // end firstBtn actionListener
textArea.setText(nwProduct[4]+"n");

// prevBtn.addActionListener(new ActionListener()

// {
// public void actionPerformed(ActionEvent ae)
// {
// dispProd = (nwProduct.length+dispProd-1) % nwProduct.length;
// textArea.setText(nwProduct.display(dispProd)+"\n");
// } // end prevBtn actionEvent
// }); // end prevBtn actionListener





} // end main

} // end class Inventory2

class Product
{
protected String prodName; // name of product
protected int itmNumber; // item number
protected int units; // number of units
protected double price; // price of each unit
protected double value; // value of total units


public Product(String name, int number, int unit, double each) // Constructor for class Product
{
prodName = name;
itmNumber = number;
units = unit;
price = each;

} // end constructor

public void setProdName(String name) // method to set product name
{
prodName = name;
}

public String getProdName() // method to get product name
{
return prodName;
}

public void setItmNumber(int number) // method to set item number
{
itmNumber = number;
}

public int getItmNumber() // method to get item number
{
return itmNumber;
}

public void setUnits(int unit) // method to set number of units
{
units = unit;
}

public int getUnits() // method to get number of units
{
return units;
}

public void setPrice(double each) // method to set price
{
price = each;
}

public double getPrice() // method to get price
{
return price;
}

public double calcValue() // method to set value
{
return units * price;
}



} // end class Product



class ProductAdd extends Product
{
private String feature; // variable for added feature


public ProductAdd(String name, int number, int unit, double each, String addFeat)
{
// call to superclass Product constructor
super(name, number, unit, each);

feature = addFeat;
}// end constructor

public void setFeature(String addFeat) // method to set added feature
{
feature = addFeat;
}

public String getFeature() // method to get added feature
{
return feature;
}

public double calcValueRstk() // method to set value and add restock fee
{
return units * price * 0.05;
}

public String toString()
{
return String.format("Product: %s\nItem Number: %d\nIn Stock: %d\nPrice: $%.2f\nType: %s\nTotal Value of Stock: $%.2f\nRestock Cost: $%.2f\n\n\n",
getProdName(), getItmNumber(), getUnits(), getPrice(), getFeature(), calcValue(), calcValueRstk());
}


} // end class ProductAdd