import java.awt.Color;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.*; //Needed for swing components 

public class PayrollApplication extends JFrame
implements ActionListener
{
//declare your instance objects here
JPanel mainPanel = new JPanel();
JLabel label1 = new JLabel("Fee Calculator");
JLabel IDcode = new JLabel("ID Code: ");
JLabel Weight = new JLabel ("Weight: ");
JLabel Rate = new JLabel ("Rate: ");
JLabel priceCharge = new JLabel ("Price Charge: ");
JTextField label1Field = new JTextField(20);
JTextField IDCodeField = new JTextField(20);
JTextField WeighttField = new JTextField(20);
JTextField RatetField = new JTextField(20);
JTextField PricetField = new JTextField(20);
JButton calculateButton = new JButton("Calculate Shipping Fee");
JTextArea outputTextArea = new JTextArea(5, 20);
JScrollPane outputScrollPane = new JScrollPane(outputTextArea); //For scroll bars around text area

Font bigFont = new Font("Times New Roman", Font.BOLD, 28);

// This is the first method called in a application
//We will create an object of ourselves to call the
//constructor and then set the default close operation for the frame
public static void main(String[] args)
{

PayrollApplication basicGUI = new PayrollApplication();
basicGUI.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

}//end of main

//This is the constructor for this class. It will be called from main
//It will set up our GUI panel with needed components
//and set up appropriate event listening
public PayrollApplication()
{
//call the superclass' constructor and send a title
super("Fee Calculator");

//add GUI components to the appropriate container
mainPanel.add(label1);
label1.setFont(bigFont);
label1.setForeground(Color.RED);
mainPanel.add(IDcode);
mainPanel.add(Weight);
mainPanel.add(Rate);
mainPanel.add(priceCharge);
mainPanel.add(calculateButton);
mainPanel.add(outputScrollPane);
JTextField IDCodeField = new JTextField(20);
JTextField WeighttField = new JTextField(20);
JTextField RatetField = new JTextField(20);
JTextField priceChargetField = new JTextField(20);

//add the JPanel to the JFrame
add(mainPanel);

//call the listener method
addListeners();

//set the properties of the JFrame for display
this.setSize(300, 500);
this.setVisible(true);
}//end of constructor

//use to register the components to the action listener
public void addListeners()
{
calculateButton.addActionListener(this);
}

//Retrieve and convert text fields and calculate and display the gross pay
public void addPerformed(ActionEvent evt)
{
double Weight = 0,Rate,priceCharge;
Rate = 0.12;
priceCharge = (Weight * Rate);
String sign = "$";

String.format("%s%2d", sign, Rate, priceCharge);
}

}//end of class

public PayrollApplication()
{
...
JTextField IDCodeField = new JTextField(20);
JTextField WeighttField = new JTextField(20);
JTextField RatetField = new JTextField(20);
JTextField priceChargetField = new JTextField(20);
...