How to create an executable Java file?

Although this question might have already been asked numerous times in SO, I'm unable to create an executable Java file (*.jar).

I have tried closely following the instructions in the highest-voted solution at How do I create executable Java program?, but when I double-clicked on the jar file, nothing happened (no pop-up window appeared). Please advise on what could be wrong here.

Thanks.

============UPDATE===================

For those who wish to know, here's the source code I used:

HelloWorldSwing.java

package start;

    import javax.swing.*;

    public class HelloWorldSwing {
        public static void main(String[] args) {
            //Create and set up the window.
            JFrame frame = new JFrame("HelloWorldSwing");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

            JLabel label = new JLabel("Hello World");
            frame.add(label);

            //Display the window.
            frame.pack();
            frame.setVisible(true);
        }
    }
    class Dummy {
        // just to have another thing to pack in the jar
    }

Manifest.mf

Manifest-Version: 1.0
Ant-Version: Apache Ant 1.8.4
Created-By: 1.8.0_172 (Oracle Corporation)
Main-Class: HelloWorldSwing

=========================================

Now, I've tried it with another *.java file, and the same problem recurs! Please see the code below for details:

Code:

SwingExample.java

import java.awt.FlowLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.WindowConstants;
import javax.swing.SwingUtilities;

public class SwingExample implements Runnable {

    @Override
    public void run() {
        // Create the window
        JFrame f = new JFrame("Hello, !");
        // Sets the behavior for when the window is closed
        f.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        // Add a layout manager so that the button is not placed on top of the label
        f.setLayout(new FlowLayout());
        // Add a label and a button
        f.add(new JLabel("Hello, world!"));
        f.add(new JButton("Press me!"));
        // Arrange the components inside the window
        f.pack();
        // By default, the window is not visible. Make it visible.
        f.setVisible(true);
    }

    public static void main(String[] args) {
        SwingExample se = new SwingExample();
        // Schedules the application to be run at the correct time in the event queue.
        SwingUtilities.invokeLater(se);
    }

}

Manifest.mf

Manifest-Version: 1.0
Ant-Version: Apache Ant 1.8.4
Created-By: 1.8.0_172 (Oracle Corporation)
Main-class: start.SwingExample

WTH is happening here???