Java GUI FullScreen window with smaller window inside

I am working on a Java desktop application. It uses MySQL database to store all data etc. I use swing for the GUI.

The GUI of this application is layed out as follows:

1. Main Window taking the entire screen size (with image in the background)


2. Internal Window 800 x 600 centered within the Main Window (that holds current content that can be switched between using menu and/or event within the application.

LoginPanel.java:

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

public class LoginPanel {

    private JPanel loginPanel;

    public void loginForm()
    {
        JButton loginSubmit = new JButton("Login");

        loginPanel = new JPanel();
        loginPanel.add(loginSubmit);
        loginPanel.setSize(800, 600);
    }

    public JComponent getGUI() 
    {
        return loginPanel;
    }

    public static void main(String[] args)
    {

    }


}

Main.java:

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

public class Main {

    public static void main(String[] args)
    {
        JFrame mainFrame;
        mainFrame = new JFrame();
        mainFrame.setLayout(new BorderLayout());
        mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mainFrame.setTitle("Caledonian Library System");     
        LoginPanel loginObj = new LoginPanel();
        mainFrame.add(loginObj.getGUI());
        mainFrame.pack();
        mainFrame.setVisible(true);      
    }

}

Should I maybe use box layout? any suggestions?