How to run the Jar well?

This problem has puzzled me for nearly three days...

As I don't know even this kind of thing would happen at the first place...

About how to display the picture, if i write the related codes like this:

aquariumImage = Toolkit.getDefaultToolkit().getImage("bubbles.gif");

it can be run well in the eclipse but in the jar.

so I change it into this by adding URL :

URL address = getClass().getResource("bubbles.gif");
  aquariumImage = Toolkit.getDefaultToolkit().getImage(address);

it then works perfectly.

 

 

import java.awt.*;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.net.URL;


public class Test extends Frame
...{

    private static final long serialVersionUID = 1L;

    static Image aquariumImage;

    public Test()
    ...{
        addWindowListener(new WindowAdapter()
        ...{
            public void windowClosing(WindowEvent e)
            ...{
                dispose();
                System.exit(0);
            }
        });
    }

    public static void main(String[] args)
    ...{
        Test test = new Test();
        test.go();
        test.setSize(600, 435);
        test.setVisible(true);

        Graphics g = test.getGraphics();
        while (!g.drawImage(aquariumImage, 0, 0, test));
    }

    public void go()
    ...{
        URL address = getClass().getResource("bubbles.gif"); 
        aquariumImage = Toolkit.getDefaultToolkit().getImage(address);
    }
}