Trying to move graphic vertically

The program moves an image from left to right. Once it gets to the right side, I want it to go down. So is there a way can I call the repaint function again only with -y in the second column of gg.fillOval?

import java.awt.*;
import javax.swing.*;

public class moving extends JFrame implements Runnable{
    private int x = 0;
    private int y;
    private Image img;
    private Graphics gg;
   	
	public static void main(String[] args){
		Thread th = new Thread(new moving());
		th.start();
	}
	
	public moving(){
		this.setSize(500, 500);
		this.setTitle("Moving");
		this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
		this.setVisible(true);
		this.setResizable(false); 
	}
		
	public void run(){
		for(i = 0; i <= 300; ++i){
			repaint();
			++x;
			try { Thread.sleep(5); } catch(Exception e) { }
 		}
        }
	public void paint(Graphics g){
		img = createImage(500,500);
		gg = img.getGraphics();
		gg.setColor(Color.white);
		gg.fillRect(0,0,500,500);
		gg.setColor(Color.blue);
		gg.fillOval(100+x, 100, 40, 40);
		g.drawImage(img, 0, 0, this);
	}
	
}