Need more help with a GUI

Question

It's the ever popular Inventory program again! I'm creating a GUI to display the information contained within an array of objects which, in this case, represent compact discs. I've received some good help from other's posts on this project since it seems there's a few of us working on the same one but now, since each person working on this project has programmed theirs differently, I'm stuck. I'm not sure how to proceed with my ActionListeners for the buttons I've created.
Here's my code:
// GUICDInventory.java
// uses CD class
import java.awt.*;
import javax.swing.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
public class GUICDInventory extends JFrame
     protected JPanel panel; //panel to hold buttons
     protected JPanel cdImage; // panel to hold image
     int displayElement = 0;
     public String display(int element)
               return CD2[element].toString();
          }//end method
     public static void main( String args[] )
          new GUICDInventory();
     }// end main
     public GUICDInventory()
          CD completeCDInventory[] = new CD2[ 5 ]; // creates a new 5 element array
         // populates array with objects that implement CD
         completeCDInventory[ 0 ] = new CD2( "Sixpence None the Richer" , "D121401" , 12 , 11.99, 1990 );
         completeCDInventory[ 1 ] = new CD2( "Clear" , "D126413" , 10 , 10.99, 1998 );
         completeCDInventory[ 2 ] = new CD2( "NewsBoys: Love Liberty Disco" , "2438-51720-2" , 10 , 12.99, 1999 );
         completeCDInventory[ 3 ] = new CD2( "Skillet: Hey You, I Love Your Soul" , "D122966" , 9 , 9.99, 1998 );
         completeCDInventory[ 4 ] = new CD2( "Michael Sweet: Real" , "020831-1376-204" , 15 , 12.99, 1995 );
         //declares totalInventoryValue variable
             double totalInventoryValue = CD.calculateTotalInventory( completeCDInventory );
          final JTextArea textArea = new JTextArea(display(displayElement));
          final JButton prevBtn = new JButton("Previous");
          final JButton nextBtn = new JButton("Next");
          final JButton lastBtn = new JButton("Last");
          final JButton firstBtn = new JButton("First");
          prevBtn.addActionListener(new ActionListener()
                public void actionPerformed(ActionEvent ae)
                    displayElement = (//what goe here? ) % //what goes here?
                    textArea.setText(display(displayElement));// <--is this right?
          nextBtn.addActionListener(new ActionListener()
                public void actionPerformed(ActionEvent ae)
                      displayElement = (//what goes here? ) % //what goes here?
                    textArea.setText(display(displayElement));// <--is this right?
          firstBtn.addActionListener(new ActionListener()
                public void actionPerformed(ActionEvent ae)
                    displayElement = 0;
                    textArea.setText(display(displayElement));// <--is this right?
          lastBtn.addActionListener(new ActionListener()
                public void actionPerformed(ActionEvent ae)
                    displayElement = //what goes here?;
                    textArea.setText(display(displayElement));// <--is this right?
          JPanel panel = new JPanel(new GridLayout(1,2));
          panel.add(firstBtn); panel.add(nextBtn); panel.add(prevBtn); panel.add(lastBtn);
          JPanel cdImage = new JPanel(new BorderLayout());
          cdImage.setPreferredSize(new Dimension(600,400));
          JFrame      cdFrame = new JFrame();
          cdFrame.getContentPane().add(new JScrollPane(textArea),BorderLayout.CENTER);
          cdFrame.getContentPane().add(panel,BorderLayout.SOUTH);
          cdFrame.getContentPane().add(new JLabel("",new ImageIcon("cd.gif"),JLabel.CENTER),BorderLayout.NORTH);
          cdFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
          cdFrame.pack();
          cdFrame.setSize(500,200);
          cdFrame.setTitle("Compact Disc Inventory");
          cdFrame.setLocationRelativeTo(null);
          cdFrame.setVisible(true);
     }//end GUICDInventory constructor
} // end class GUICDInventory
// CD2.java
// subclass of CD
public class CD2 extends CD
     protected int copyrightDate; // CDs copyright date variable declaration
     private double price2;
     // constructor
     public CD2( String title, String prodNumber, double numStock, double price, int copyrightDate )
          // explicit call to superclass CD constructor
          super( title, prodNumber, numStock, price );
          this.copyrightDate = copyrightDate;
     }// end constructor
     public double getInventoryValue() // modified subclass method to add restocking fee
        price2 = price + price * 0.05;
        return numStock * price2;
    } //end getInventoryValue
    // Returns a formated String contains the information about any particular item of inventory
    public String displayInventory() // modified subclass display method
          return String.format("\n%-22s%s\n%-22s%d\n%-22s%s\n%-22s%.2f\n%-22s%s%.2f\n%-22s%s%.2f\n%-22s%s%.2f\n  \n" ,
                                   "CD Title:", title, "Copyright Date:", copyrightDate, "Product Number:", prodNumber , "Number in Stock:",
                                   numStock , "CD Price:" , "$" , price , "Restocking fee (5%):", "$", price*0.05, "Inventory Value:" , "$" ,
                                   getInventoryValue() );
     } // end method
}//end class CD2
// CD.java
// Represents a compact disc object
import java.util.Arrays;
class CD implements Comparable
    protected String title; // CD title (name of product)
    protected String prodNumber; // CD product number
    protected double numStock; // CD stock number
    protected double price; // price of CD
    protected double inventoryValue; //number of units in stock times price of each unit
    // constructor initializes CD information
    public CD( String title, String prodNumber, double numStock, double price )
        this.title = title; // Artist: album name
        this.prodNumber = prodNumber; //product number
        this.numStock = numStock; // number of CDs in stock
        this.price = price; //price per CD
    } // end constructor
    public double getInventoryValue()
        return numStock * price;
    } //end getInventoryValue
    //Returns a formated String contains the information about any particular item of inventory
    public String displayInventory()
          //return the formated String containing the complete information about CD
        return String.format("\n%-22s%s\n%-22s%s\n%-22s%.2f\n%-22s%s%.2f\n%-22s%s%.2f\n%-22s%s%.2f\n  \n" ,
                                   "CD Title:", title, "Product Number:", prodNumber , "Number in Stock:",
                                   numStock , "CD Price:" , "$" , price , "Restocking fee (5%):", "$", price*0.05, "Inventory Value:" , "$" ,
                                   getInventoryValue() );
    } // end method
    //method to calculate the total inventory of the array of objects
    public static double calculateTotalInventory( CD completeCDInventory[] )
        double totalInventoryValue = 0;
        for ( int count = 0; count < completeCDInventory.length; count++ )
             totalInventoryValue += completeCDInventory[count].getInventoryValue();
        } // end for
        return totalInventoryValue;
    } // end calculateTotalInventory
     // Method to return the String containing the Information about Inventory's Item
     //as appear in array (non-sorted)
    public static String displayTotalInventory( CD completeCDInventory[] )
          //string which is to be returned from the method containing the complete inventory's information
          String retInfo = "\nInventory of CDs (unsorted):\n";
          //loop to go through complete array
          for ( int count = 0; count < completeCDInventory.length; count++ )
               retInfo = retInfo + "Item# " + (count + 1);          //add the item number in String
               retInfo = retInfo + completeCDInventory[count].displayInventory();     //add the inventory detail in String
          }// end for
          return retInfo;          //return the String containing complete detail of Inventory
    }// end displayTotalInventory
     public int compareTo( Object obj ) //overlaod compareTo method
          CD tmp = ( CD )obj;
          if( this.title.compareTo( tmp.title ) < 0 )
               return -1; //instance lt received
          else if( this.title.compareTo( tmp.title ) > 0 )
               return 1; //instance gt received
          return 0; //instance == received
          }// end compareTo method
     //Method to return the String containing the Information about Inventory's Item
     // in sorted order (sorted by title)
     public static String sortedCDInventory( CD completeCDInventory[] )
          //string which is to be returned from the method containing the complete inventory's information
          String retInfo = "\nInventory of CDs (sorted by title):\n";
          Arrays.sort( completeCDInventory ); // sort array
          //loop to go through complete array
          for( int count = 0; count < completeCDInventory.length; count++ )
               retInfo = retInfo + "Item# " + (count + 1);     //add the item number in String
               retInfo = retInfo + completeCDInventory[count].displayInventory(); //add the inventory detail in String
          return retInfo;     //return the String containing complete detail of Inventory
     } // end method sortedCDInventory
} // end class CD

Answer 1

nextBtn.addActionListener(new ActionListener()
     public void actionPerformed(ActionEvent ae)
               displayElement = (//what goes here? ) % //what goes here?
               textArea.setText(display(displayElement));// <--is this right?
});Above is your code for the "Next" button.
You ask the question "What goes here"? Well what do you think goes there?
If you are looking at item 1 and you press the "Next" button do you not want to look at item 2?
So the obvious solution would be to add 1 to the previous item value. Does that not make sense? What problem do you have when you try that code?????
Next you say "Is this right"? Well how are we supposed to know? You wrote the code. We don't know what the code is supposed to do. Again you try it. If it doesn't work then describe the problem you are having. I for one can't execute your code because I don't use JDK5. So looking at the code it looks reasonable, but I can't tell by looking at it what is wrong. Only you can add debug statements in the code to see whats happening.
If you want help learn to as a proper question and doen't expect us to spoon feed the code to you. This is your assignment, not ours. We are under no obligation to debug and write the code for you. The sooner you learn that, the more help you will receive.