And here is the code:

package pingpong1;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Point;

public class PingPong extends Game {
	public static final int BATWIDTH=60;
	public static final int BATHEIGHT=20;
	private int batX=150, batY=200;
	
	public void init(){
	resize(600,600);
	setSleepTime(50);

	}
	public Color getBackgroundColor(){
	return Color.black;
	}

	public void paint(Graphics g) {
	g.setColor(Color.red); //To make the bat red
	Point mp=gp.getMousePosition(); //point mp will hold the coordinates of the mouse
	if(mp!=null)
	{
	//Then, if the mouse is in the game window, it's coordinates are copied to
	//batX and batY. If the mouse is out of the game window, batX and batY will not change.
	batX=mp.x;
	batY=mp.y;
	}
	//finaly, to draw the bat, I call the Garphics method fillRect, that draws and fills
	//a rectangle.
	g.fillRect(batX-BATWIDTH/2,batY-BATHEIGHT/2,BATWIDTH,BATHEIGHT);
	}
}