Algumas duvidas Noob

import java.awt.event.WindowEvent;
import java.awt.event.WindowListener;

import javax.swing.JButton;
import javax.swing.JDesktopPane;
import javax.swing.JFrame;
import javax.swing.JInternalFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class Desktop extends JFrame {

	private JDesktopPane desktop;

	private JInternalFrame cadastro;

	private JPanel painel;

	private JButton botao;

	private JLabel label;

	private JTextField campo;

	public Desktop() {

		this.setSize(800, 600);
		
		desktop = new JDesktopPane();
		cadastro = new JInternalFrame();
		painel = new JPanel();
		botao = new JButton();
		label = new JLabel("CPF:");
		campo = new JTextField();
		
		label.setSize(50,20);
		label.setLocation(10,10);
		
		campo.setSize(150,20);
		campo.setLocation(50,10);
		
		cadastro.setLayout(null);
		cadastro.setSize(300, 300);
		cadastro.setLocation(50, 50);
		cadastro.add(label);
		cadastro.add(campo);
		
		botao.setSize(100, 40);
		botao.setText("Cadastro");

		painel.setLayout(null);
		painel.add(botao);

		this.add(desktop);
		this.add(cadastro);
		this.add(painel);

		botao.addActionListener(new java.awt.event.ActionListener() {
			public void actionPerformed(java.awt.event.ActionEvent evt) {
				cadastro();
			}
		});
	   this.addWindowListener(new WindowListener() {
               public void windowClosing(WindowEvent windowEvent) {
                   sair();
               }
               public void windowActivated(WindowEvent windowEvent) { }
               public void windowClosed(WindowEvent windowEvent) {
                   return ;
               }
               public void windowDeactivated(WindowEvent windowEvent) {}
               public void windowDeiconified(WindowEvent windowEvent) {}
               public void windowIconified(WindowEvent windowEvent) {}
               public void windowOpened(WindowEvent windowEvent) {}
           });
	   
	   //this.pack();
	   this.setVisible(true);
	}

	private void cadastro() {

		cadastro.setVisible(true);
	}
	
	private void sair(){
		JOptionPane.showMessageDialog(null, "Tchau...");
		System.exit(0);
	}

	public static void main(String[] args) {

		new Desktop();
	}
}