JPanel not showing anything

Question

I have a problem with JComponent's inside a JPanel that are not showing up.

I am using Netbeans' Java GUI Builder partially to build the GUI. I got a MainFrame class which is build with the GUI Builder to take care of menu items. I got a MainPanel class which is inside the MainFrame class (under the menu bar).

And then I manually add another JPanel to that MainPanel such that I can have full control of the GUI again without Netbeans GUI bothering me.

However when I add a JButton to that JPanel then nothing is showing up.

MainFrame.java (including Netbeans code):

package gui;

import javax.swing.JFrame;

/**
 *
 * @author Frank
 */
public class MainFrame extends javax.swing.JFrame {

    /**
     * Creates new form MainFrame
     */
    public MainFrame() {
        initComponents();
        customInit();
    }
    
    private void customInit() {
        this.setTitle("Trading Card Game");
        this.setExtendedState(JFrame.MAXIMIZED_BOTH);
    }

    /**
     * This method is called from within the constructor to initialize the form. WARNING: Do NOT
     * modify this code. The content of this method is always regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">//GEN-BEGIN:initComponents
    private void initComponents() {

        mainPanel1 = new gui.MainPanel();
        jMenuBar1 = new javax.swing.JMenuBar();
        menuFile = new javax.swing.JMenu();
        menuFileQuit = new javax.swing.JMenuItem();
        menuPreview = new javax.swing.JMenu();
        menuPreviewStart = new javax.swing.JMenuItem();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        javax.swing.GroupLayout mainPanel1Layout = new javax.swing.GroupLayout(mainPanel1);
        mainPanel1.setLayout(mainPanel1Layout);
        mainPanel1Layout.setHorizontalGroup(
            mainPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 400, Short.MAX_VALUE)
        );
        mainPanel1Layout.setVerticalGroup(
            mainPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 279, Short.MAX_VALUE)
        );

        menuFile.setText("File");

        menuFileQuit.setText("Quit");
        menuFileQuit.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                menuFileQuitActionPerformed(evt);
            }
        });
        menuFile.add(menuFileQuit);

        jMenuBar1.add(menuFile);

        menuPreview.setText("Preview");

        menuPreviewStart.setText("Start");
        menuPreview.add(menuPreviewStart);

        jMenuBar1.add(menuPreview);

        setJMenuBar(jMenuBar1);

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(mainPanel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(mainPanel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
        );

        pack();
    }// </editor-fold>//GEN-END:initComponents

    private void menuFileQuitActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_menuFileQuitActionPerformed
        System.exit(0);
    }//GEN-LAST:event_menuFileQuitActionPerformed

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(MainFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(MainFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(MainFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(MainFrame.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                new MainFrame().setVisible(true);
            }
        });
    }
    // Variables declaration - do not modify//GEN-BEGIN:variables
    private javax.swing.JMenuBar jMenuBar1;
    private gui.MainPanel mainPanel1;
    private javax.swing.JMenu menuFile;
    private javax.swing.JMenuItem menuFileQuit;
    private javax.swing.JMenu menuPreview;
    private javax.swing.JMenuItem menuPreviewStart;
    // End of variables declaration//GEN-END:variables
}

MainPanel.java (including Netbeans code):

package gui;

import java.awt.BorderLayout;
import java.awt.Color;
import javax.swing.BorderFactory;
import javax.swing.JButton;
import model.Game;

/**
 *
 * @author Frank
 */
public class MainPanel extends javax.swing.JPanel {

    /**
     * Creates new form MainPanel
     */
    public MainPanel() {
        initComponents();
        initPanel();
    }
    
    private void initPanel() {
        this.setBorder(BorderFactory.createLineBorder(Color.yellow));
        this.add(new JButton("Test"), BorderLayout.CENTER);
        revalidate();
    }

    /**
     * This method is called from within the constructor to initialize the form. WARNING: Do NOT
     * modify this code. The content of this method is always regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">//GEN-BEGIN:initComponents
    private void initComponents() {

        setLayout(new java.awt.BorderLayout());
    }// </editor-fold>//GEN-END:initComponents
    // Variables declaration - do not modify//GEN-BEGIN:variables
    // End of variables declaration//GEN-END:variables
}

The yellow border is showing up around the JPanel though.

Any clue why the JButton with "Test" is not showing up?

Image describing the situation (look no button!):

Regards.

Answer 1

It is because you are overwriting your layout for mainPanel1.

I commented out these lines in MainFrame.java and the test button showed as expected.

//        javax.swing.GroupLayout mainPanel1Layout = new javax.swing.GroupLayout(mainPanel1);
//        mainPanel1.setLayout(mainPanel1Layout);
//        mainPanel1Layout.setHorizontalGroup(
//            mainPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
//            .addGap(0, 400, Short.MAX_VALUE)
//        );
//        mainPanel1Layout.setVerticalGroup(
//            mainPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
//            .addGap(0, 279, Short.MAX_VALUE)
//        );