Hello world

//Hello world class
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import javax.swing.event.*;

public class HelloWorld extends JFrame{
    private JButton button;

    public HelloWorld(){
    super("Hello World");

    button = new JButton("Click here");
    add(button);

    HandlerClass handler = new HandlerClass();
    button.addActionListener(handler);

    }
   private class HandlerClass implements ActionListener{
       @Override
    public void actionPerformed(ActionEvent event){
        JOptionPane.showMessageDialog(null, String.format("Hello world", event.getActionCommand()));

    }
    }
}

//main class
import javax.swing.*;
import java.awt.*;


class MyFirst extends JFrame {
public static void main(String[] args) {
    Gui hello = new Gui();

    hello.setSize(100, 100);
    hello.setDefaultCloseOperation(hello.EXIT_ON_CLOSE);
    hello.setVisible(true);

}
}