JPanel

package com.gmail.kput4uk.frame;
 
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JTextField;
 
 
 
 
public class MagFrame extends JFrame {
 
    private JPanel current;
    private JPanel mainpanel;
 
    public MagFrame() {
        
        setTitle("Frame");
        setBounds(500, 250, 800, 500);
        mainpanel = new JPanel(new BorderLayout());
        add(mainpanel);
        RegPanel reg = new RegPanel();
        reg.setOwner(this);
        current = reg;
        mainpanel.add(current, BorderLayout.CENTER);
        
    }
    
    public void setPane(JPanel pa) {
        remove(current);
        current = pa;
        mainpanel.add(current, BorderLayout.CENTER);
        mainpanel.revalidate();
    }
 
    public static void main (String[] args) {
        JFrame f = new MagFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setVisible(true);
    }
}
 
class RegPanel extends JPanel {
    
    private JTextField login;
    private JTextField password;
    private MagFrame mf;
    
    public void setOwner(JFrame f) {
        mf = (MagFrame) f;
    }
    
    public RegPanel() {
        
        setLayout(new GridBagLayout());
        setBackground(Color.MAGENTA);
        final JLabel lname = new JLabel("Login");
        final JLabel lpass = new JLabel("Password");
        login = new JTextField(15);
        password = new JTextField(15);
        JButton log = new JButton("Connect");
        JButton cancel = new JButton("Cancel");
        
        add(lname, new GridBagConstraints(0, 0, 1, 1, 0, 0, GridBagConstraints.NORTH, GridBagConstraints.BOTH, new Insets(5, 5, 5, 5), 0, 0));
        add(login, new GridBagConstraints(1, 0, 2, 1, 0, 0, GridBagConstraints.NORTH, GridBagConstraints.BOTH, new Insets(5, 5, 5, 5), 0, 0));
        add(lpass, new GridBagConstraints(0, 2, 1, 1, 0, 0, GridBagConstraints.CENTER, GridBagConstraints.HORIZONTAL, new Insets(5, 5, 5, 5), 0, 0));
        add(password, new GridBagConstraints(1, 2, 2, 1, 0, 0, GridBagConstraints.CENTER, GridBagConstraints.HORIZONTAL, new Insets(5, 5, 5, 5), 0, 0));
        add(log, new GridBagConstraints(0, 4, 1, 1, 0, 0, GridBagConstraints.NORTH, GridBagConstraints.SOUTH, new Insets(5, 5, 5, 5), 0, 0));
        add(cancel, new GridBagConstraints(1, 4, 1, 1, 0, 0, GridBagConstraints.NORTH, GridBagConstraints.SOUTH, new Insets(5, 5, 5, 5), 0, 0));
    
        log.addActionListener(new ActionListener() {
            
            @Override
            public void actionPerformed(ActionEvent arg0) {
                if (login.getText().equals("a") && password.getText().equals("ad")) {
                    mf.setPane(new MainPane());
                } else {
                    JOptionPane.showMessageDialog(null, "Не верно");
                }
                
            }
        });
    }
}
 
class MainPane extends JPanel {
    
    public MainPane() {
        setBackground(Color.RED);
    }
}

    
public void setPane(JPanel pa) {
        mainpanel.remove(current);
        current = pa;
        mainpanel.add(current, BorderLayout.CENTER);
        mainpanel.revalidate();
    }