Como 2 classes controlar o mesmo container?

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

/**
 *
 * @author Spyman
 */


public class Main extends JFrame {
    
    JPanel panel;
    JLabel msg, msg2, msg3;    
    
    Container container = getContentPane();
    
    
    /** Creates a new instance of Main */
    public Main() {
        super("BluePhoto");
        
       container.setLayout(new FlowLayout());
        
        //Configura menu Principal
        JMenu princMenu = new JMenu("Principal");
        princMenu.setMnemonic('P');
        
        //Configura item Novo Album no menu Principal      
        JMenuItem novoItem = new JMenuItem ("Novo Album");
        novoItem.setMnemonic('X');
        novoItem.addActionListener(
                new ActionListener() {
                    public void actionPerformed (ActionEvent event){
                        //NovoAlbum();
                        Teste amora = new Teste();
                    //JOptionPane.showMessageDialog(null, "" + x);
                    }//Fim actionPerformed
                }//fim ActionListener
        ); //fim chamada addActionListener
        princMenu.add(novoItem);
        
        //Configura item Sair no menu Principal      
        JMenuItem sairItem = new JMenuItem ("Sair");
        sairItem.setMnemonic('X');
        sairItem.addActionListener(
                new ActionListener() {
                    public void actionPerformed (ActionEvent event){
                        System.exit(0);
                    }//Fim actionPerformed
                }//fim ActionListener
        ); //fim chamada addActionListener
        princMenu.add(sairItem);
        
        
        //Configura menu Ajuda
        JMenu ajudaMenu = new JMenu("Ajuda");
        ajudaMenu.setMnemonic('A');
        
        //Configura item Sobre no menu Ajuda      
        JMenuItem sobreItem = new JMenuItem ("Sobre");
        sobreItem.setMnemonic('S');
        sobreItem.addActionListener(
                new ActionListener() {
                    public void actionPerformed (ActionEvent event){
                    }//Fim actionPerformed
                }//fim ActionListener
        ); //fim chamada addActionListener
        ajudaMenu.add(sobreItem);
        
        
        //Cria Menu e insere menus Principal, Ajuda e Sair
        JMenuBar bar = new JMenuBar();
        setJMenuBar(bar);
        bar.add(princMenu);
        bar.add(ajudaMenu); 
        
    }

    public void Janela()
     {        
         panel = new JPanel();
         panel.setLayout(new GridLayout(3, 1, 30, 30));
         
         panel.setBackground(Color.LIGHT_GRAY);
         
         JLabel msg = new JLabel ("Bem Vindo ao BluePhoto! Comece já a criar seu álbum de fotos particular!", SwingConstants.CENTER);
         JLabel msg2 = new JLabel ("Para iniciar a criação do álbum acesse o menu Principal",SwingConstants.CENTER);
         JLabel msg3 = new JLabel ("Boa Diversão!",SwingConstants.CENTER);
         
         msg.setForeground(Color.BLACK);
         msg.setFont(new Font("Verdana", Font.BOLD, 15));
         
         msg2.setForeground(Color.BLACK);
         msg2.setFont(new Font("Verdana", Font.CENTER_BASELINE, 10));
         
         msg3.setForeground(Color.BLACK);
         msg3.setFont(new Font("Verdana", Font.CENTER_BASELINE, 10));
         
         panel.add(msg);
         panel.add(msg2);         
         panel.add(msg3);
         
         container.setBackground(Color.LIGHT_GRAY);
         container.add(panel);
         
         
         setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
         
         setVisible(true);
         setSize(1024,768);
    }
    
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Main application = new Main();
        application.Janela();
   //     application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
/**
 *
 * @author Spyman
 */
public class Teste extends Main {    


    public Teste() {
        container.setBackground(Color.BLACK);
        container.removeAll();
        container.repaint();
        setVisible(true);
        panel = new JPanel();
        panel.setLayout(new GridLayout(3, 1, 30, 30));
    }
}

Main() {
         super("BluePhoto");
         
        container.setLayout(new FlowLayout());

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. codigo