Switch Card Layout in Java

Question

Here's how to change card layouts from a menu item. I asked how to do it earlier but no luck. I have figured out the answer so here's what it does; 1. Builds your main frame when running the java file. Then in the menu bar it allows you to switch JPanels (For this example welcome is a different public class inside of a package.) 2. Now you can build as many public classes as you want and still be able to go to that JPanel.

import java.awt.BorderLayout;
import java.awt.CardLayout;
import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.IOException;

import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JPanel;

public class ArmyQuestions {

    CardLayout cards;
    JPanel cardPanel;

    public static void main(String[] args) throws IOException {

         //Use the event dispatch thread for Swing components
         EventQueue.invokeLater(new Runnable()
         {

            @Override
             public void run()
             {

                 new ArmyQuestions();         
             }
         });

    }

    public ArmyQuestions()
    { 

        JFrame mainFrame = new JFrame();

        //make sure the program exits when the frame closes
        mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mainFrame.setTitle("Army Questions");
        mainFrame.setSize(797,510);

        //This will center the JFrame in the middle of the screen
        mainFrame.setLocationRelativeTo(null);
        mainFrame.getContentPane().setLayout(new BorderLayout());

        //Adds a menu bar
        JMenuBar menuBar = new JMenuBar();
        mainFrame.getContentPane().add(menuBar, BorderLayout.NORTH);

        //Adds a menu option
        JMenu mnFile = new JMenu("File");
        menuBar.add(mnFile);

        //Adds an item to the menu option
        JMenuItem mntmNew = new JMenuItem("New");
        mnFile.add(mntmNew);
        mntmNew.addActionListener(new ActionListener() 
        {
            @Override
            public void actionPerformed(ActionEvent arg0) 
            {
                cards.show(cardPanel, "Welcome");
            }
        });

        //Adds cardpanel to getContentPane           
        cards = new CardLayout();
        cardPanel = new JPanel();
        cardPanel.setLayout(cards);
        mainFrame.getContentPane().add(cardPanel,BorderLayout.CENTER);

        //Adds a JPanel to your cardpanel 
        Welcome welcome = new Welcome();
        cardPanel.add(welcome, "Welcome");

        mainFrame.setVisible(true);


    }
}

Answer 1

Two things I see going on.

1. You have SuggestedQuesion_2 declared globally then you create a whole new one in you method. JPanel SuggestedQuestion_2 = new JPanel();
2. I see a CardLayout for your Welcome - Welcome.setLayout(new CardLayout(0, 0));, but not for you SuggestedQuestion_2. Yet you're trying to access SuggestedQuestions's CardLayout

You should learn how to post an SSCCE So it is easier for us to see the problem. Also, in trying to recreate the problem into a smaller, runnable version you sometimes figure out the solution yourself.

And please follow Java naming convention using lowercase letters first letter of reference variable