How do I incorporate a scroll bar after a few instances of actionListener? - jscrollpane

Question

Ok. so I'm writing a grocery checkout system code and on the right side of the frame, I'm displaying a Jpanel with Labels. Every time I click the 'Scan' button, I add a new label into the JPanel so that everytime I click, it displays the output. However, I can only fit so many entries in my window, how do I incorporate a scroll bar so that I can scroll through the groceries?
I tried inputting a scroll bar EAST with a BorderLayout and all the labels CENTER, however, everytime I click the button, it only reprints it at the same exact spot instead of top/down so that I can scroll. (Please, bare in mind that this is a very very rough draft. I'll be incorporating the logic beneath the code once I get the nitty-gritty with the GUI)
Here's my code:
public class Checkout extends JFrame implements ActionListener
{
private int numOfItems = 21;
private Integer[] itemCode = new Integer[numOfItems];
private StringBuffer[] itemDesc = new StringBuffer[numOfItems];
private Double[] unitPrice = new Double[numOfItems];
private Integer[] taxCode = new Integer[numOfItems];
private Integer[] quantity = new Integer[numOfItems];
private Integer[] reorderLevel = new Integer[numOfItems];
private double subTotal, tax, grandTotal;
private JButton scan = new JButton("Scan");
private JButton pay = new JButton("Finish & Pay");
private JButton readFile = new JButton("Read from File");
private JTextField itemNumber = new JTextField(4);
private JTextField itemQuantity = new JTextField(4);
private JLabel displayReceipt = new JLabel();
private JPanel p4;
public Checkout()
{
setValues();
JPanel p2 = new JPanel(new FlowLayout());
p2.add(scan);
p2.add(pay);
p2.add(readFile);
JPanel p1 = new JPanel(new FlowLayout());
p1.add(new JLabel("Item Number:"));
p1.add(itemNumber);
p1.add(new JLabel("Item Quantity:"));
p1.add(itemQuantity);
p1.setBorder(new TitledBorder("Scan Items First. Then Hit 'Finish and Pay'"));
JPanel p3 = new JPanel(new BorderLayout(5, 5));
p3.add(p1, BorderLayout.NORTH);
p3.add(p2, BorderLayout.CENTER);
p4 = new JPanel(new GridLayout(0, 1, 0, 0));
JScrollPane scroll = new JScrollPane(p4);
add(p3, BorderLayout.WEST);
add(p4, BorderLayout.CENTER);
scan.addActionListener(this);
pay.addActionListener(this);
readFile.addActionListener(this);
}
public void setValues()
{
String[] tokens = new String[6];
String[][] multi = new String[numOfItems][6];
try{BufferedReader textReader = new BufferedReader(new FileReader("inventory.txt"));
for(int i = 0; i < numOfItems; i++) // Split the file
{
tokens = textReader.readLine().split("\t");
for(int j = 0; j < tokens.length; j++)
{
multi[i][j] = tokens[j];
}
}
for(int i = 0; i < numOfItems; i++)
{
itemCode[i] = (Integer.parseInt(multi[i][0]));
itemDesc[i] = new StringBuffer(multi[i][1]);
unitPrice[i] = (Double.parseDouble(multi[i][2]));
taxCode[i] = (Integer.parseInt(multi[i][3]));
quantity[i] = (Integer.parseInt(multi[i][4]));
reorderLevel[i] = (Integer.parseInt(multi[i][5]));
}}
catch(Exception e){}
}
public void setQuantity(int item, int itemCount)
{
for(int i = 0; i < numOfItems; i++)
{
if(itemCode[i].equals(item))
{
quantity[i] = quantity[i] - itemCount;
subTotal = itemCount * unitPrice[i];
if(quantity[i] < reorderLevel[i])
itemDesc[i] = itemDesc[i].append("**MARKED FOR REORDER**\t");
}
}
}
public String toString()
{
System.out.println("CUST TICKET");
//for(int i = 0; i < numOfItems; i++)
//return (itemCode[i] + "\t" + itemDesc[i] + "\t" + unitPrice[i] + "\t" + taxCode[i] + "\t" + quantity[i] + "\t" + reorderLevel[i]);
System.out.println("SUBTOTAL\t" + subTotal);
System.out.println("TAX\t\t" + tax);
System.out.println("GRAND TOTAL\t" + grandTotal);
return "";
}
public static void main(String[] args) throws Exception
{
Checkout frame = new Checkout(); // Frame layout
frame.setSize(600, 135);
frame.setTitle("Computer Science 202: Final Project - Supermarket Checkout System");
frame.setLocationRelativeTo(null);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
/*Scanner input = new Scanner(System.in);
System.out.print("Please enter item number: ");
Integer itemNumber = Integer.parseInt(input.nextLine());
System.out.print("Please enter quantity: ");
Integer quantity = Integer.parseInt(input.nextLine());
System.out.println(frame.stockCheck(itemNumber, quantity));*/
}
public void actionPerformed(ActionEvent e)
{
if(e.getSource() == scan)
{
try
{
Integer quantityOrdered = 1;
Integer item = Integer.parseInt(itemNumber.getText());
quantityOrdered = Integer.parseInt(itemQuantity.getText());
for(int i = 0; i < numOfItems; i++)
{
if(itemCode[i].equals(item))
{
if(quantityOrdered > 1)
{
p4.add(new JLabel(itemDesc[i] + "" + unitPrice[i]));
p4.add(new JLabel(quantityOrdered + " # " + unitPrice[i]));
p4.revalidate();
p4.repaint();
}
else
{
p4.add(new JLabel(itemDesc[i] + "\t" + unitPrice[i]));
p4.revalidate();
p4.repaint();
}
}
}
setQuantity(item, quantityOrdered);
}
catch(Exception ex){}
}
else if(e.getSource() == pay)
{
displayReceipt.setText(toString());
}
else if(e.getSource() == readFile)
{
System.out.println("READ FILE");
}
}
}