Java SE (Java Platform, Standard Edition)

hello all
I have read that statement "frame.setLocationRelativeTo(null);" is used to center frame
but i have tried insert the statement in line 73 in the code below it does'nt center the frame at all
any explaination why ?

thank you

denny

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.net.URL;
import javax.swing.AbstractButton;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

//source  http://www.javabeginner.com/java-swing/java-jbutton-class-example

public class JButtonDemo extends JPanel implements ActionListener {

	protected static JButton jbnLeft, jbnMiddle, jbnRight;
	public JButtonDemo() {
		// Create Icons that can be used with the jButtons
		ImageIcon leftButtonIcon = createImageIcon("jbutton\\rightarrow.jpg");//bukan absolut address,absolutnya D:\JavaApp\jbutton\rightarrow.jpg
        ImageIcon middleButtonIcon = createImageIcon("jbutton\\java-swing-tutorial.JPG");
		ImageIcon rightButtonIcon = createImageIcon("jbutton\\leftarrow.jpg");
		jbnLeft = new JButton("Disable centre button", leftButtonIcon);
		jbnLeft.setVerticalTextPosition(AbstractButton.CENTER);
		jbnLeft.setHorizontalTextPosition(AbstractButton.LEADING);
		jbnLeft.setMnemonic(KeyEvent.VK_D);	// Alt-D clicks the button
		jbnLeft.setActionCommand("disable");
		jbnLeft.setToolTipText("disable the Centre button."); // Adding Tool tips
		jbnMiddle = new JButton("Centre button", middleButtonIcon);
		jbnMiddle.setVerticalTextPosition(AbstractButton.BOTTOM);
		jbnMiddle.setHorizontalTextPosition(AbstractButton.CENTER);
		jbnMiddle.setMnemonic(KeyEvent.VK_M);// Alt-M clicks the button
		jbnMiddle.setToolTipText("Centre button");
		jbnRight = new JButton("Enable centre button", rightButtonIcon);
		// Use the default text position of CENTER, TRAILING (RIGHT).
		jbnRight.setMnemonic(KeyEvent.VK_E);// Alt-E clicks the button
		jbnRight.setActionCommand("enable");
		jbnRight.setEnabled(false);
		// Disable the Button at creation time
		// Listen for actions on Left and Roght Buttons
		jbnLeft.addActionListener(this);
		jbnRight.addActionListener(this);
		jbnRight.setToolTipText("Enable the Centre button.");
		// Add Components to the frame, using the default FlowLayout.
		add(jbnLeft);
		add(jbnMiddle);
		add(jbnRight);
	}
	public void actionPerformed(ActionEvent e) {
		if ("disable".equals(e.getActionCommand())) {
			jbnMiddle.setEnabled(false);
			jbnLeft.setEnabled(false);
			jbnRight.setEnabled(true);
		} else {
			jbnMiddle.setEnabled(true);
			jbnLeft.setEnabled(true);
			jbnRight.setEnabled(false);
		}
	}
	// Returns an ImageIcon, or null if the path was invalid.
	protected static ImageIcon createImageIcon(String path) {
		URL imgURL = JButtonDemo.class.getResource(path);
		if (imgURL != null) {
			return new ImageIcon(imgURL);
		} else {
			System.err.println("Couldn't find image in system: " + path);
			return null;
		}
	}
	// Create the GUI and show it.
	private static void createGUI() {
		JFrame.setDefaultLookAndFeelDecorated(true);
		// Create and set up the frame.
		JFrame frame = new JFrame("jButton usage demo");
		frame.setLocationRelativeTo(null);  //to center
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		
		// Create and set up the content pane.
		JButtonDemo buttonContentPane = new JButtonDemo();
		buttonContentPane.setOpaque(true); // content panes must be opaque
		frame.getRootPane().setDefaultButton(jbnLeft);
		frame.setContentPane(buttonContentPane);
		// Display the window.
		frame.pack();
		frame.setVisible(true);
	}
	public static void main(String[] args) {
		javax.swing.SwingUtilities.invokeLater(new Runnable() {

			public void run() {
				createGUI();
			}
		});
	}
}