Как показать значение текстового поля одной карты на другой карте в CardLayout Swap

Вы объявляете и инициализируете JPanel cards дважды в своем коде, один раз как переменную экземпляра и второй раз внутри конструктора класса CardLayoutLoginTest. Поскольку у вас пошаговая штука, лучше добавить свои JPanel, которые действуют как Cards, один за другим к CardLayout. Поскольку, если LoginFails вам не понадобятся, просто когда вход в систему действителен, добавьте то, что вы хотите добавить в CardLayout.

Используйте setLocationByPlatform(true) ; вместо setLocationRelativeTo(null);. Первый лучше, как объяснил @Andrew Thompson в одном из своих сообщений, Как лучше всего позиционировать Swing Графический интерфейс Я изменил ваш код, посмотрите и дайте мне знать, если чего-то не хватает.

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

public class CardTest
{   
    private JFrame frame;
    public static final String CARD_LOGIN =  "Card Login"; 
    public static final String CARD_WELCOME = "Card Welcome";
    public static JPanel cards;
    public CardLogin cardLogin = null;

    public CardTest()
    {
        frame = new JFrame("Card LOGIN");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocationByPlatform(true);

        cards = new JPanel();
        cards.setLayout(new CardLayout(20, 20));

        cardLogin = new CardLogin(this);
        cards.add(cardLogin, CARD_LOGIN);       

        frame.getContentPane().add(cards);
        frame.pack();
        frame.setVisible(true);
    }

    public void swapView(String key) 
    {
        CardLayout cardLayout = (CardLayout) cards.getLayout();
        cardLayout.show(cards, key);
    }

    public static void main(String... args)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                new CardTest();
            }
        });
    }
}

class CardLogin extends JPanel 
{ 
    private ActionListener action; 
    CardTest cardLayoutLoginTest;
    /*
     *  Made JTextField an instance variable so that
     * ActionListener can access it or you can make 
     * it final.
     */
    private JTextField tfUsername= null; 
    private String username = null;

    public CardLogin(CardTest cardLayoutLoginTest) 
    { 
        this.cardLayoutLoginTest = cardLayoutLoginTest;
        init(); 
    } 

    private void init() 
    { 

        JPanel panel = new JPanel(new GridBagLayout());
        GridBagConstraints gc = new GridBagConstraints();

        gc.fill = GridBagConstraints.HORIZONTAL;

        JLabel lbUsername = new JLabel("Username: ");
        gc.gridx = 0;
        gc.gridy = 0;
        gc.gridwidth = 1;
        panel.add(lbUsername, gc);

        tfUsername = new JTextField(20);
        gc.gridx = 1;
        gc.gridy = 0;
        gc.gridwidth = 2;
        panel.add(tfUsername, gc);

        JLabel lbPassword = new JLabel("Password: ");
        gc.gridx = 0;
        gc.gridy = 1;
        gc.gridwidth = 1;
        panel.add(lbPassword, gc);

        JPasswordField pfPassword = new JPasswordField(20);
        gc.gridx = 1;
        gc.gridy = 1;
        gc.gridwidth = 2;
        panel.add(pfPassword, gc);

        final JButton loginButton = new JButton("Login"); 

        action = new ActionListener() 
        { 
            public void actionPerformed(ActionEvent ae) 
            {

                    // Here need code to update text filed of welcome card
                /*
                 * Here we are first checking if there is any text inside
                 * the JTextField for USERNAME, if found we will send it to the
                 * next JPanel which will be serving as a new Card.
                 */ 
                if (tfUsername.getDocument().getLength() > 0)   
                {
                    username = tfUsername.getText();
                    CardWelcome cardWelcome = new CardWelcome(cardLayoutLoginTest.cardLogin);
                    CardTest.cards.add(cardWelcome, cardLayoutLoginTest.CARD_WELCOME);
                    cardLayoutLoginTest.swapView(cardLayoutLoginTest.CARD_WELCOME);
                }
            } 
        }; 

        loginButton.addActionListener(action); 

        JPanel bp = new JPanel();
        bp.add(loginButton);

        /*set size of the frame*/
        setSize( 640, 480);

        add(panel, BorderLayout.CENTER);
        add(bp, BorderLayout.PAGE_END);

    } 

    public String getUserName()
    {
        return username;
    }
} 

class CardWelcome extends JPanel 
{ 
    private JTextField textField;
    private CardLogin cardLogin;

    public CardWelcome(CardLogin cl) 
    { 
        cardLogin = cl;
        init(); 
    } 

    private void init() 
    { 
        setLayout(new GridLayout(1, 1)); 
        JLabel userLabel = new JLabel("Welcome "); 
        textField = new JTextField(); 
        textField.setText(cardLogin.getUserName());
        System.out.println("UserName : " + cardLogin.getUserName());

        add(userLabel); 
        add(textField); 
    } 
}

Небольшой пример программы для обновления JLabel во время выполнения:

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

public class UpdateLabel extends JFrame
{
    private int count = 0;
    public UpdateLabel()
    {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationByPlatform(true);

        final JPanel contentPane = new JPanel();
        contentPane.setLayout(new BorderLayout());

        final JLabel label = new JLabel("JLabel " + count);
        JButton button = new JButton("UPDATE JLABEL");
        button.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent ae)
            {
                count++;
                label.setText("JLabel " + count);
                contentPane.revalidate(); // sometimes you require to do this and the below line.
                contentPane.repaint();
            }
        });

        contentPane.add(label, BorderLayout.CENTER);
        contentPane.add(button, BorderLayout.PAGE_END);

        setContentPane(contentPane);
        pack();
        setVisible(true);
    }

    public static void main(String... args)
    {
        Runnable runnable = new Runnable()
        {
            public void run()
            {
                new UpdateLabel();
            }
        };
        SwingUtilities.invokeLater(runnable);
    }
}

вы повторно создаете CardLayout экземпляр каждый раз, когда JButton's Action срабатывает, создавайте его только один раз, например

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

public class OnTheFlyImageTest extends JFrame {

    private static final long serialVersionUID = 1L;
    private JPanel cardPanel;
    private CardLayout cardLayout;

    public OnTheFlyImageTest() {
        JPanel cp = new JPanel(new BorderLayout());
        cp.setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));
        cardLayout = new CardLayout(5, 5);
        cardPanel = new JPanel(cardLayout);
        cp.add(cardPanel);
        for (int i = 0; i < 5; i++) {// Create random panels for testing.
            String name = "ImagePanel" + (i + 1);
            String image = (i & 1) == 0 ? "foo.gif" : "bar.gif";
            Color clr = (i & 1) == 0 ? Color.red : Color.blue;
            ImagePanel imgPanel = new ImagePanel(name, image, clr);
            cardPanel.add(imgPanel, name);
            cardLayout.addLayoutComponent(imgPanel, name);
        }
        JPanel buttonPanel = new JPanel(new GridLayout(1, 2, 5, 5));
        JButton prevButton = new JButton("< Previous");
        prevButton.setActionCommand("Previous");
        prevButton.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                cardLayout.previous(cardPanel);
            }
        });
        buttonPanel.add(prevButton);
        JButton nextButton = new JButton("Next >");
        nextButton.setActionCommand("Next");
        nextButton.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                cardLayout.next(cardPanel);
            }
        });
        buttonPanel.add(nextButton);
        JPanel temp = new JPanel(new BorderLayout());
        temp.add(buttonPanel, BorderLayout.LINE_END);
        cp.add(temp, BorderLayout.SOUTH);
        setContentPane(cp);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setTitle("Test");
        pack();
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                new OnTheFlyImageTest().setVisible(true);
            }
        });
    }
}

class ImagePanel extends JPanel {

    private static final long serialVersionUID = 1L;
    private String imgString;
    private JLabel imgLabel;

    public ImagePanel(String name, String imgString, Color backGround) {
        setName(name);
        this.imgString = imgString;
        setLayout(new BorderLayout());
        setBackground((backGround));
        // Ensure size is correct even before any image is loaded.
        setPreferredSize(new Dimension(400, 300));
    }

    @Override
    public void setVisible(boolean visible) {
        if (visible) {
            System.err.println(getName() + ": Loading and adding image");
            ImageIcon icon = new ImageIcon(imgString);
            imgLabel = new JLabel(icon);
            add(imgLabel);
        }
        super.setVisible(visible);
        if (!visible) { // Do after super.setVisible() so image doesn't "disappear".
            System.err.println(getName() + ": Removing image");
            if (imgLabel != null) { // Before display, this will be null
                remove(imgLabel);
                imgLabel = null; // Hint to GC that component/image can be collected.
            }
        }
    }
}