/** Move this ball according to its position and speed and redraw. */
public void move() {
// remove from canvas at the current position
erase();
// compute new position
ySpeed += GRAVITY;
yPosition += ySpeed;
xPosition += 2;
// check if it has hit the ground
if (yPosition >= (groundPosition - diameter) && ySpeed > 0) {
yPosition = (int) (groundPosition - diameter);
ySpeed = -ySpeed + ballDegradation;
}
// draw again at new position
draw();
}
