Minimal "popup" example

gistfile1.java

import javax.swing.JFrame;
import javax.swing.JLabel;

public class frame
{
    public static void main(String[] args)
    {
        JFrame f = new JFrame();
        f.add(new JLabel("This is a window."));
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.pack();
        f.setVisible(true);
    }
}