Java SE (Java Platform, Standard Edition)

Hi Guys,

I need to get componets(JPanel, JSplitPane, etc) size in main JFrame.
First the main frame default restore size is set by
"mainFrame.setSize(100,100);"
And then it is full-screened by
"mainFrame.setExtendedState(Frame.MAXIMIZED_BOTH);"
And when I get the size of jpanel inside the main frame,
the size is coming as default restore frame not full screen frame.
My purpose is to get the size when the frame is full size when the frame is opening(initiating).
You may understand if see the below code.
Thank you in advance.

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

class GetComponentSize implements ActionListener
{
	JFrame mainFrame;
	JPanel subPanel;
	JButton bt;

	public GetComponentSize()
	{
		mainFrame = new JFrame();
		subPanel = new JPanel();
		bt = new JButton("Get Panel Size");
		bt.addActionListener(this);
		subPanel.add(bt);
		mainFrame.getContentPane().setLayout(new BorderLayout());
		mainFrame.getContentPane().add(subPanel, BorderLayout.CENTER);
		mainFrame.setSize(100,100);//Default Restore size
		mainFrame.setExtendedState(Frame.MAXIMIZED_BOTH);//Make frame full screen
		mainFrame.setVisible(true);

		System.out.println("First size:"+subPanel.getSize());//Print component JPanel size
	}

	public void actionPerformed(ActionEvent e)
	{
		System.out.println("Second size:"+subPanel.getSize());
	}

	public static void main(String arg[])
	{
		new GetComponentSize();
	}
};