Programming Forums - JLabel question

i am trying to display the output in jlabels for this shipping cost program i have coded. I have created individual jlabels for the package ID, weight, and shipping cost and tried to display them with the mainPanel. I had no problems making labels for the input in my designFrame method, but when I try to make new labels for the mainPanel in displayoutput they simply do not display. here is the code:

import javax.swing.*;

import java.awt.event.*;







public class shippingCharge extends JFrame implements ActionListener

{



        //objects and data types created here

        JPanel mainPanel = new JPanel();

        JTextField packageIdentificationTextField = new JTextField(6);

        JTextField poundsTextField = new JTextField(10);

        JTextField ouncesTextField = new JTextField(10);

        JButton displayButton = new JButton("Calculate  ");

        

        

        //Variables

        String packageIdentificationString;

        double poundDouble, ouncesDouble, poundToOunceOuncesDouble, shippingCostDouble;





        public static void main(String[] args)

        {

                shippingCharge shippingTotal = new shippingCharge();

        }

                

        public shippingCharge()

        {

                designFrame();

                setSize(500,500);

                setVisible(true);

                

        }

        

        public void designFrame()

        {

                mainPanel.add(new JLabel("Package ID"));

                mainPanel.add(packageIdentificationTextField);

                mainPanel.add(new JLabel("Pounds"));

                mainPanel.add(poundsTextField);

                mainPanel.add(new JLabel("Ounces"));

                mainPanel.add(ouncesTextField);

                mainPanel.add(displayButton);

                

                

        add(mainPanel);

                //add listener to the  object

        packageIdentificationTextField.addActionListener(this);

                displayButton.addActionListener(this);

                

        }

        

        public void getInput()

        {

                packageIdentificationString = packageIdentificationTextField.getText();

                poundDouble = Double.parseDouble(poundsTextField.getText()); 

                ouncesDouble = Double.parseDouble(ouncesTextField.getText());

                

        }

        

        public void calculateShipping()

        {

                final double SHIPPING_RATE = .12;

                final double OUNCES_PER_POUND = 16;

                poundToOunceOuncesDouble = poundDouble * OUNCES_PER_POUND;

                shippingCostDouble = (poundToOunceOuncesDouble + ouncesDouble) * SHIPPING_RATE;

                

        }



        public void actionPerformed(ActionEvent evt)

        {

                getInput();

                calculateShipping();

                displayOutput();

                

                

        }

        

        public void displayOutput()

        {

                

        mainPanel.add(new JLabel("Package ID:" + packageIdentificationString));

        mainPanel.add(new JLabel("Weight:" + poundDouble + "lbs" + ouncesDouble + "oz."));

        mainPanel.add(new JLabel("Shipping Cost:" + shippingCostDouble));

        

                        

        }

}