充分利用了类的继承，组合，多态性，实现点，线，圈等基本图形元素，可以借鉴其设计方案。-Full advantage of the type of inheritance, composition, polymorphism, the realization of points, lines, circles and other basic graphic elements, can learn from their design.

包含设计画图板所需要的类，采用类的多态性-Paint