当另一个类调用Singleton.instance.show（）时，具有CardLayout的Singleton不会显示卡片。

当另一个类调用Singleton.instance.show（）时，具有CardLayout的Singleton不会显示卡片。

它将建议一个参考问题。

public static MainWindow instance = newMainWindow();

看起来很可疑，因为您必须先创建一个

MainWindow

first
实例才能进行初始化，这建议您现在有两个实例

MainWindow

，一个在屏幕上，另一个不在屏幕上。

static

以这种方式使用不是一个好主意，因为它会导致类似这样的问题。相反，您应该将控制器的引用传递给页面。控制器将定义每个页面可以执行的 *** 作（如果 *** 作正确，则将其定义为

interface

）

另外，您可以将导航与页面分开，形成单独的机制，这意味着页面不在乎，可以简单地以您想要的任何顺序显示或在其他地方重复使用

Example＃1-基于控制器的页面

本示例定义了一个简单的控制器，页面可以调用该控制器来实现页面导航

import java.awt.BorderLayout;import java.awt.CardLayout;import java.awt.EventQueue;import java.awt.FlowLayout;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import java.util.ArrayList;import java.util.List;import javax.swing.JButton;import javax.swing.JComponent;import javax.swing.Jframe;import javax.swing.JLabel;import javax.swing.JPanel;import javax.swing.UIManager;import javax.swing.UnsupportedLookAndFeelException;public class CardLayoutExample {    public static void main(String[] args) {        new CardLayoutExample();    }    public CardLayoutExample() {        EventQueue.invokeLater(new Runnable() { @Override public void run() {     try {         UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());     } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {     }     Jframe frame = new Jframe("Testing");     frame.setDefaultCloseOperation(Jframe.EXIT_ON_CLOSE);     frame.setLayout(new BorderLayout());     frame.add(new Wizard());     frame.pack();     frame.setLocationRelativeTo(null);     frame.setVisible(true); }        });    }    public interface NavigationController {        public void nextPage();        public void previousPage();        public void lastPage();        public void firstPage();    }    public interface Page {        public NavigationController getNavigationController();        public JComponent getView();        public String getName();    }    public class Wizard extends JPanel implements NavigationController {        private List<Page> pages;        private Page currentPage;        private CardLayout cardLayout;        public Wizard() { cardLayout = new CardLayout(); pages = new ArrayList<>(25); setLayout(cardLayout); pages.add(new FirstPage("Page01", this)); pages.add(new SecondPage("Page02", this)); pages.add(new ThirdPage("Page03", this)); for (Page page : pages) {     add(page.getView(), page.getName()); } firstPage();        }        @Override        public void nextPage() { int index = pages.indexOf(currentPage); index++; if (index < pages.size()) {     cardLayout.next(this);     currentPage = pages.get(index); }        }        @Override        public void previousPage() { int index = pages.indexOf(currentPage); index--; if (index >= 0) {     cardLayout.previous(this);     currentPage = pages.get(index); }        }        @Override        public void lastPage() { Page page = pages.get(pages.size() - 1); showPage(page);        }        @Override        public void firstPage() { Page page = pages.get(0); showPage(page);        }        protected void showPage(Page page) { cardLayout.show(this, page.getName()); currentPage = page;        }    }    public abstract class AbstractPage extends JPanel implements Page, ActionListener {        private NavigationController navigationController;        private JPanel buttons;        private String name;        public AbstractPage(String name, NavigationController navigationController) { this.name = name; this.navigationController = navigationController; setLayout(new BorderLayout()); buttons = new JPanel(new FlowLayout(FlowLayout.RIGHT)); add(buttons, BorderLayout.SOUTH);        }        protected void insertButton(JButton button) { button.addActionListener(this); buttons.add(button);        }        @Override        public NavigationController getNavigationController() { return navigationController;        }        @Override        public JComponent getView() { return this;        }        @Override        public String getName() { return super.getName();         }    }    public class FirstPage extends AbstractPage implements Page {        private JButton next = new JButton("Next >");        public FirstPage(String name, NavigationController controller) { super(name, controller); JLabel label = new JLabel("First page"); label.setHorizontalAlignment(JLabel.CENTER); add(label); insertButton(next);        }        @Override        public void actionPerformed(ActionEvent e) { if (e.getSource() == next) {     getNavigationController().nextPage(); }        }    }    public class SecondPage extends AbstractPage implements Page {        private JButton next = new JButton("Next >");        private JButton previous = new JButton("< Previous");        public SecondPage(String name, NavigationController controller) { super(name, controller); JLabel label = new JLabel("Second page"); label.setHorizontalAlignment(JLabel.CENTER); add(label); insertButton(previous); insertButton(next);        }        @Override        public void actionPerformed(ActionEvent e) { if (e.getSource() == next) {     getNavigationController().nextPage(); } else if (e.getSource() == previous) {     getNavigationController().previousPage(); }        }    }    public class ThirdPage extends AbstractPage implements Page {        private JButton previous = new JButton("< Previous");        public ThirdPage(String name, NavigationController controller) { super(name, controller); JLabel label = new JLabel("Third page"); label.setHorizontalAlignment(JLabel.CENTER); add(label); insertButton(previous);        }        @Override        public void actionPerformed(ActionEvent e) { if (e.getSource() == previous) {     getNavigationController().previousPage(); }        }    }}

Example＃2-中央控制器的例子

此示例将控制器与页面分开，以便按钮不属于页面本身。这样可以释放页面/视图，使其成为您需要的任何内容

import java.awt.BorderLayout;import java.awt.CardLayout;import java.awt.Component;import java.awt.EventQueue;import java.awt.GridBagConstraints;import java.awt.GridBagLayout;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import java.util.ArrayList;import java.util.HashMap;import java.util.List;import java.util.Map;import javax.swing.JButton;import javax.swing.Jframe;import javax.swing.JLabel;import javax.swing.JPanel;import javax.swing.UIManager;import javax.swing.UnsupportedLookAndFeelException;import javax.swing.border.EmptyBorder;public class CardLayoutExample2 {    public static void main(String[] args) {        new CardLayoutExample2();    }    public CardLayoutExample2() {        EventQueue.invokeLater(new Runnable() { @Override public void run() {     try {         UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());     } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {     }     Jframe frame = new Jframe("Testing");     frame.setDefaultCloseOperation(Jframe.EXIT_ON_CLOSE);     frame.setLayout(new BorderLayout());     frame.add(new WizardPane());     frame.pack();     frame.setLocationRelativeTo(null);     frame.setVisible(true); }        });    }    public class WizardPane extends JPanel {        private List<String> pages;        private String currentPage;        private JButton first;        private JButton previous;        private JButton next;        private JButton last;        private CardLayout cardLayout;        private JPanel contentPane;        public WizardPane() { setLayout(new BorderLayout()); cardLayout = new CardLayout(); pages = new ArrayList<>(3); contentPane = new JPanel(cardLayout); contentPane.setBorder(new EmptyBorder(4, 4, 4, 4)); pages.add("Page01"); pages.add("Page02"); pages.add("Page03"); contentPane.add(new FirstPage(), "Page01"); contentPane.add(new SecondPage(), "Page02"); contentPane.add(new ThirdPage(), "Page03"); JPanel actionsPane = new JPanel(new GridBagLayout()); actionsPane.setBorder(new EmptyBorder(4, 4, 4, 4)); GridBagConstraints gbc = new GridBagConstraints(); gbc.gridx = 0; gbc.gridy = 0; actionsPane.add((first = new JButton("<< First")), gbc); gbc.gridx++; gbc.weightx = 1; gbc.anchor = GridBagConstraints.WEST; actionsPane.add((previous = new JButton("< Previous")), gbc); gbc.gridx++; gbc.anchor = GridBagConstraints.EAST; actionsPane.add((next = new JButton("Next >")), gbc); gbc.gridx++; gbc.weightx = 0; actionsPane.add((last = new JButton("Last >>")), gbc); add(contentPane); add(actionsPane, BorderLayout.SOUTH); NavigationHandler handler = new NavigationHandler(); first.addActionListener(handler); previous.addActionListener(handler); next.addActionListener(handler); last.addActionListener(handler); gotoFirstPage();        }        protected void gotoFirstPage() { currentPage = pages.get(0); cardLayout.show(contentPane, currentPage);        }        protected void gotoPreviousPage() { int index = pages.indexOf(currentPage); index--; if (index >= 0) {     currentPage = pages.get(index);     cardLayout.show(contentPane, currentPage); }        }        protected void gotonextPage() { int index = pages.indexOf(currentPage); index++; if (index < pages.size()) {     currentPage = pages.get(index);     cardLayout.show(contentPane, currentPage); }        }        protected void gotoLastPage() { currentPage = pages.get(pages.size() - 1); cardLayout.show(contentPane, currentPage);        }        protected class NavigationHandler implements ActionListener { @Override public void actionPerformed(ActionEvent e) {     if (e.getSource() == first) {         gotoFirstPage();     } else if (e.getSource() == previous) {         gotoPreviousPage();     } else if (e.getSource() == next) {         gotonextPage();     } else if (e.getSource() == last) {         gotoLastPage();     } }        }    }    public class FirstPage extends JPanel {        public FirstPage() { setLayout(new BorderLayout()); JLabel label = new JLabel("Page One"); label.setHorizontalAlignment(JLabel.CENTER); add(label);        }    }    public class SecondPage extends JPanel {        public SecondPage() { setLayout(new BorderLayout()); JLabel label = new JLabel("Page Two"); label.setHorizontalAlignment(JLabel.CENTER); add(label);        }    }    public class ThirdPage extends JPanel {        public ThirdPage() { setLayout(new BorderLayout()); JLabel label = new JLabel("Page Three"); label.setHorizontalAlignment(JLabel.CENTER); add(label);        }    }}

Example＃3-基于模型

或者，您可以使用基于模型的方法（可能更可取），该方法定义了组件的显示顺序。例如