public double getRadius() {
        return this.radius;
    }
    
    /**
     * Sets the radius and sends a {@link DialLayerChangeEvent} to all 
     * registered listeners.
     *
     * @param radius  the radius (must be positive).
     * 
     * @see #getRadius()
     */
    public void setRadius(double radius) {
        if (radius <= 0) { 
            throw new IllegalArgumentException(
                    "The 'radius' must be positive.");
        }
        this.radius = radius;
        notifyListeners(new DialLayerChangeEvent(this));