private int shotTimer;
private boolean spaceDown;

public void shoot()
{
    if (shotTimer > 0 && --shotTimer > 0) return; // if timer running and not yet zero, exit method
    if (spaceDown != Greenfoot.isKeyDown("space")) // if change in state of trigger
    {
        spaceDown = !spaceDown; // record new trigger state
        if (spaceDown) // if new trigger state is down
        {
            ball shot = new ball();
            getWorld().addObject(shot, getX(), getY());
            shot.setRotation(getRotation());
        }
    }
}