HelloWorld Klasse für Java-Einstieg

HelloWorld.java

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

public class HelloWorld {
    private JButton buttonOK;
    private JPanel panel1;

    public HelloWorld() {
        buttonOK.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                JOptionPane.showMessageDialog(null, "Mein erstes Click-Event! :-)");
            }
        });
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setContentPane(new HelloWorld().panel1);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setVisible(true);
    }
}