我在 JPanel 中遇到了 JComponent 的问题，但没有显示出来。
我部分使用 Netbeans 的 Java GUI Builder 来构建 GUI。我有一个使用 GUI Builder 构建的 MainFrame 类来处理菜单项。我有一个 MainPanel 类，它位于 MainFrame 类中（在菜单栏下）。
然后我手动将另一个 JPanel 添加到该 MainPanel 中，这样我就可以再次完全控制 GUI，而 Netbeans GUI 不会打扰我。
但是，当我向该 JPanel 添加一个 JButton 时，什么都没有出现。
MainFrame.java（包括 Netbeans 代码）：
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（包括 Netbeans 代码）：
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
}
黄色边框出现在 JPanel 周围。
任何线索为什么带有“测试”的 JButton 没有出现？
描述情况的图片（看没有按钮！）：
问候。