javafiddle/static/examples/hello-world-swing/Main.java

import java.awt.Font;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.SwingUtilities;

class Main implements Runnable {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Main());
    }

    public void run() {
        JFrame f = new JFrame("Swing example");
        JLabel l = new JLabel("Hello, world!");
        l.setFont(new Font("Serif", Font.PLAIN, 42));
        f.add(l);
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }
}