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

public class GUI {
    public static void main(String[] args) {

        try {
            UIManager.setLookAndFeel(UIManager.getCrossPlatformLookAndFeelClassName());
        } catch (Exception e) {
            System.exit(1);
        }

        JFrame vindu = new JFrame("Min jframe");
        vindu.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel panel = new JPanel();
        vindu.add(panel);
        JButton knapp = new JButton("Exit");

        class Stopper implements ActionListener {
            @Override
            public void actionPerformed(ActionEvent e) {
                System.exit(0);
            }
        }

        knapp.addActionListener(new Stopper());
        panel.add(knapp);

        vindu.pack();
        vindu.setLocationRelativeTo(null);
        vindu.setVisible(true);
    }
}