Netbeans Java中的预览设计不同于运行时设计

import javax.swing.*;
import java.awt.event.*;

public class MySwingApp extends JFrame {
    private JButton button;
    private JTextField textField;

    public MySwingApp() {
        button = new JButton("Click Me");
        textField = new JTextField(20);

        button.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                textField.setText("Button Clicked!");
            }
        });

        JPanel panel = new JPanel();
        panel.add(button);
        panel.add(textField);

        this.add(panel);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.pack();
        this.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                new MySwingApp();
            }
        });
    }
}