如何使用抽象方法的示例。用抽象类实现对长方形，正方形，圆形面积的计算，然后实现对随机产生的是个图形面积的累加（图形的形状和长、宽、半径均随机产生）-Abstract class implements a pair of rectangular, square, circular area is calculated, and then implement the randomly generated graphics area accumulate (the shape of the graph and the length, width, radius are randomly generated)