带有CardLayout的Java Swing GUI：返回第1张时，窗口未在第二张卡中关闭

Question

I have a Swing GUI that uses a CardLayout with two cards:

• main_card

• aisle_card

When I go to the aisle_card , it properly switches to the aisle_card and closes the main_card:

public class Runner
{

    private static JFrame frame;
    private static JPanel cards;

    void createAndShowGUI()
    {
                //ItemsInAisleGUI gui = new ItemsInAisleGUI();
        // Create and set up the window.
        ItemsInAisleGUI gui = new ItemsInAisleGUI();

        frame = new JFrame("Shopping List");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        cards = new JPanel(new CardLayout());
        JPanel mainCard = buildMainPanel();

        //JPanel aisleCard = new ItemsInAisleGUI(mainCard).aisleCard;
        JPanel aisleCard = gui.createAndShowGUI();

        cards.add(mainCard, MAIN_CARD);
        cards.add(aisleCard, AISLE_CARD);
        // Display the window.
        frame.getContentPane().add(cards);
        frame.pack();
        frame.setVisible(true);
    }


    private JPanel buildMainPanel()
    {
        JPanel mainCard = new JPanel();
        mainCard.setLayout(new GridLayout());
        mainCard.setName(MAIN_CARD);
        JPanel topRowPanel = new JPanel();
        topRowPanel.setLayout(new GridLayout(12, 2));
        topRowPanel.setName("TopRowPanel");

        //Button for the Search by Aisle Card
        JButton aisleButton = new JButton("\u25BA Search by Aisle");
        aisleButton.setName(AISLE_BUTTON);
        aisleButton.addActionListener(new ActionListener()
        {
            @Override
            public void actionPerformed(ActionEvent e)
            {
                CardLayout cl = (CardLayout) (cards.getLayout());
                cl.show(cards, AISLE_CARD);
            }
        });

        //panels and labels of mainCard

        mainCard.add(topRowPanel);
        return mainCard;
    }

    public static void main(String[] args)
    {
        javax.swing.SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                Runner r = new Runner();
                r.createAndShowGUI();
            }
        });

    }

    /**
     * @return the frame containing the application view
     */
    static JFrame getFrame()
    {

        return frame;
    }

    /**
     * @return the card that is current visible
     */
    Component getVisibleCard()
    {
        for (int i = 0; i < cards.getComponentCount(); i++)
        {
            if (cards.getComponent(i).isVisible())
            {
                return cards.getComponent(i);
            }
        }
        return null;
    }

}

However, when I am in the aisle_card and want to go back, it does open the main_card but not close the aisle_card and the console shows a NPE:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at view.ItemsInAisleGUI$2.CloseFrame(ItemsInAisleGUI.java" referring to this line jFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

public class ItemsInAisleGUI {

private static JFrame jFrame;

    public ItemsInAisleGUI() {

    }

    public JPanel createAndShowGUI()
    {
        JPanel content = new JPanel();
        content.setLayout(new GridLayout(2, 1));
        content.setName("aislePanel");

        //panel stuff

        backButton = new JButton("\u25C4 Back to Item Search");
        backButton.setName("BackButton");


        //button for going back to the main screen (Search by Item Name)
        backButton.addActionListener(new ActionListener()
        {

            @Override
            public void actionPerformed(ActionEvent e)
            {
                Runner myrunner = new Runner();
                myrunner.createAndShowGUI();
                CloseFrame();
            }
            public void CloseFrame(){
            jFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            jFrame.setVisible(false);
            }
        });     

        topRowPanel.add(aisleNumberLabel);
        topRowPanel.add(aisleNumberText);
        topRowPanel.add(itemsInAisleLabel);
        topRowPanel.add(itemsInAisleText);
        topRowPanel.add(searchItemButton);
        topRowPanel.add(backButton);    
        content.add(topRowPanel);
        return content;
    }

    public static Container getContainer()
    {
        return jFrame;
    }

    /**
     * @param args
     */
    public static void main(String[] args)
    {

    }

}

Any suggestions? Thank you.

Answer 1

First, your static variables are not necessary. Also, you have some misconceptions about what is happening:

it does open the main_card but not close the aisle_card

This is false. It actually doesn't up the main_card that you are thinking about. It is opening a NEW gui entirely as can be seen in your code here:

public void actionPerformed(ActionEvent e)
        {
            Runner myrunner = new Runner();
            myrunner.createAndShowGUI(); // Creates completely new GUI
            CloseFrame();
        }

I don't know exactly the structure of your app, but the following should compile and should at least give you a basis on how CardLayouts work:

import javax.smartcardio.Card;
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class Runner {

    private final JFrame frame;

    private final JPanel home;
    private final JPanel second;

    public static void main(String[] args) {
        Runner runner = new Runner();
        runner.show();
    }

    public Runner() {
        this.frame = new JFrame();
        this.frame.setSize(new Dimension(500, 500));
        this.frame.setLocationRelativeTo(null);

        JPanel contentPanel = new JPanel(new CardLayout());
        frame.setContentPane(contentPanel);

        home = buildMainPanel();
        second = buildSecondPanel();

        contentPanel.add("home", home);
        contentPanel.add("second", second);

    }

    public JPanel buildMainPanel() {
        JPanel something = new JPanel(null);
        something.setBounds(0,0,500,500);
        something.setBackground(Color.black);

        JButton flipButton = new JButton("Second");
        flipButton.setBounds(0,0,100,80);
        flipButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                CardLayout layout = (CardLayout) frame.getContentPane().getLayout();
                layout.show(frame.getContentPane(), "second");
            }
        });

        something.add(flipButton);
        return something;
    }

    public JPanel buildSecondPanel() {
        JPanel something = new JPanel(null);
        something.setBounds(0,0,500,500);
        something.setBackground(Color.blue);

        JButton flipButton = new JButton("Second");
        flipButton.setBounds(0,0,100,80);
        flipButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                CardLayout layout = (CardLayout) frame.getContentPane().getLayout();
                layout.show(frame.getContentPane(), "home");
            }
        });

        something.add(flipButton);
        return something;
    }

    public void show() {
        frame.setVisible(true);
    }
}

If you compile this and hit the button, you can see that the background flips from black to blue, meaning the different cards are being displayed. Hope this helps to get you on the right track.