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

public class Temp extends JFrame{
    private JPanel panel1;
    private JButton button1;

    private Temp() {
        button1.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                setVisible(false);
                JOptionPane.showMessageDialog(null, "Hello World");
            }
        });
    }


    public static void main(String[] args) {
        JFrame tempWindow = new JFrame("TempWindow");
        tempWindow.setContentPane(new Temp().panel1);
        tempWindow.setLocationRelativeTo(null); // this line set the window in the center of the screen
        tempWindow.setDefaultCloseOperation(tempWindow.EXIT_ON_CLOSE);
        tempWindow.pack();
        tempWindow.setVisible(true);

    }

}