I'm writing a program using GUI, radio buttons to change color of the background and text. I'm running to some error as I'm compiling it. here are some errors I got from Java:

Exception in thread "main" java.lang.NullPointerException

at ColorFactory.buildTopPanel(ColorFactory.java:64)

at ColorFactory.(ColorFactory.java:31)

at ColorFactory.main(ColorFactory.java:133)

Please check my program.

import java.awt.BorderLayout;

import java.awt.*;

import java.awt.event.*;

import javax.swing.*;

public class ColorFactory extends JFrame{

private final int WINDOW_WIDTH = 500; //window width

private final int WINDOW_HEIGHT= 300; //window height

private JButton redButton; //change background color to red

private JButton orangeButton; // change background color to orange

private JButton yellowButton; //change background color to yellow

private JRadioButton greenButton; //change text color to green

private JRadioButton blueButton; //change text color to blue

private JRadioButton cyanButton; //change text color to cyan

private ButtonGroup radioButtonGroup; //to group radio buttons

private JPanel topPanel; // top panel that holds 3 buttons

private JPanel bottomPanel; // bottom panel that holds radio buttons

private JLabel messageLabel; //A message to user

//constructor

public ColorFactory () {

//set title

setTitle("Color Factory");

//set size

setSize(WINDOW_WIDTH, WINDOW_HEIGHT);

//specify an action for the close button

setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

//set Layout

setLayout(new BorderLayout());

//build the top panel and add it to the frame

buildTopPanel();

add(topPanel, BorderLayout.NORTH);

//build the bottom panel and ad it to the frame

buildBottomPanel();

add(bottomPanel, BorderLayout.SOUTH);

//create a label that contains the message

messageLabel = new JLabel ("Top buttons change the panel color and bottom raido buttons change the text color");

add(messageLabel, BorderLayout.CENTER);

//display the window

setVisible(true);

}

private void buildTopPanel() {

//set Flow Layout

setLayout (new FlowLayout());

//set background

setBackground(Color.WHITE);

//create the label for buttons and set button's background

redButton = new JButton ("Red");

redButton.setBackground(Color.RED);

orangeButton = new JButton ("Orange");

orangeButton.setBackground(Color.ORANGE);

yellowButton = new JButton ("Yellow");

yellowButton.setBackground(Color.YELLOW);

//set action command

redButton.setActionCommand("R");

orangeButton.setActionCommand("O");

yellowButton.setActionCommand("Y");

// add listener

redButton.addActionListener(new RedButtonListener());

orangeButton.addActionListener (new OrangeButtonListener());

yellowButton.addActionListener (new YellowButtonListener());

topPanel.add(redButton);

topPanel.add(orangeButton);

topPanel.add(yellowButton);

}

private void buildBottomPanel() {

//set Flow Layout

setLayout (new FlowLayout());

//set background

setBackground(Color.WHITE);

//create label for radio buttons and set buttons' foreground

greenButton = new JRadioButton ("Green");

greenButton.setForeground(Color.GREEN);

blueButton = new JRadioButton ("Blue");

blueButton.setForeground(Color.BLUE);

cyanButton = new JRadioButton ("Cyan");

cyanButton.setForeground(Color.CYAN);

// grouping radio button

radioButtonGroup = new ButtonGroup();

radioButtonGroup.add(greenButton);

radioButtonGroup.add(blueButton);

radioButtonGroup.add(cyanButton);

//add action listeners to the radio buttons

greenButton.addActionListener(new RadioButtonListener());

blueButton.addActionListener(new RadioButtonListener());

cyanButton.addActionListener(new RadioButtonListener());

}

/**

* private inner classes that handle the event when the user click Red, Orange, Yellow buttons

*/

private class RedButtonListener implements ActionListener{

public void actionPerformed (ActionEvent e) {

messageLabel.setBackground(Color.RED);

}

}

private class OrangeButtonListener implements ActionListener{

public void actionPerformed (ActionEvent e) {

messageLabel.setBackground(Color.ORANGE);

}

}

private class YellowButtonListener implements ActionListener{

public void actionPerformed (ActionEvent e) {

messageLabel.setBackground(Color.YELLOW);

}

}

/**

* private inner class that handle the event when the user choose one of the radio buttons

*/

private class RadioButtonListener implements ActionListener {

public void actionPerformed (ActionEvent e) {

if (e.getSource() == greenButton) {

messageLabel.setForeground(Color.GREEN);

}

else if (e.getSource() == blueButton) {

messageLabel.setForeground(Color.BLUE);

}

else if (e.getSource() == cyanButton) {

messageLabel.setForeground(Color.CYAN);

}

}

}

/**

* the main method creates an instance of the ColorFactory class, displaying its window

* @param args

*/

public static void main (String[]args) {

new ColorFactory();

}

}