TopLevelWindow.java:

import java.awt.*;
import javax.swing.*;
// Create a simple GUI window
public class TopLevelWindow {
        private static void createWindow() {
        JFrame frame = new JFrame("Simple GUI");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JLabel textLabel = new JLabel("I'm a label in the window",SwingConstants.CENTER);
        textLabel.setPreferredSize(new Dimension(300, 100));
        frame.getContentPane().add(textLabel, BorderLayout.CENTER);       //Display the window.
        frame.setLocationRelativeTo(null);
        frame.pack();
        frame.setVisible(true);
        }
        public static void main(String[] args) {
            createWindow();
        }
}