import java.awt.Container;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JRootPane;
public class Main {
public static void main(String[] args) {
JFrame f = new JFrame();
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JRootPane root = f.getRootPane();
Container content = root.getContentPane();
content.add(new JButton("Hello"));
f.pack();
f.setVisible(true);
}
}