Singleton with CardLayout will not show the card when another class calls Singleton.instance.show ()
public class MainWindow extends JPanel {
public static MainWindow instance = new MainWindow();
private CardLayout cards = new CardLayout();
public MainWindow() {
setLayout(cards);
add(new FirstPage(), Pages.FIRST.toString());
add(new SecondPage(), Pages.SECOND.toString());
add(new ThirdPage(), Pages.THIRD.toString());
}
public void showPage(Pages page) {
cards.show(this, page.toString());
}
}
The method showPage(page);
works fine if I call it in the constructor MainWindow
. But when I try to call MainWindow.instance.showPage(Pages.SECOND);
from ActionListener in FirstPage
, nothing happens. I have verified that the method is showPage(page)
working correctly. I have verified that the ActionEvent is fired and included in the correct if / else clause. What am I doing wrong, why is my second page not showing?
public class FirstPage extends JPanel {
private JButton showSecond = new JButton("Show Second");
private JButton showThird = new JButton("Show Third");
public FirstPage() {
insertButton(showSecond);
insertButton(showThird);
}
private void insertButton(JButton button) {
button.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
if (e.getSource() == showSecond) {
MainWindow.instance.showPage(Pages.SECOND);
} else {
MainWindow.instance.showPage(Pages.THIRD);
}
}
});
this.add(button);
}
}
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1 answer
This would suggest a reference issue. public static MainWindow instance = new MainWindow();
looks suspicious as you would have to create an instance first to MainWindow
initialize it, which assumes that you now have two instances MainWindow
, one on the screen and one that is not
Using it static
this way is a bad idea as it leads to problems like this. Instead, you must pass a controller link to the page. The controller will determine the actions each page can take (and if done correctly, it will determine how interface
)
Alternatively, you can separate the navigation from the pages into a separate mechanism, this means the pages don't care and can simply be rendered in whatever order you want or reused, where
Example # 1 - controller based pages
This example defines a simple controller that pages can call to navigate the pages.
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 - central controller example
This example decouples the controller from the pages, so the buttons are not part of the pages themselves. This frees up the pages / views to be what you need to be
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 - Based on a model
Or, you can use a model-driven approach (which is probably more preferable) that determines the order in which the components are displayed. for example
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