抽象类的使用

设计一个类层次，定义一个抽象类--形状，其中包括有求形状的面积的抽象方法。

----继承该抽象类定义三角型、矩形、圆。

----编写一个主程序将一个三角形、矩形、圆存入一个数组中，将数组中各类图形的面积输出。-kind of abstract designs a class hierarchy, the definition of an abstract category -- shape, These include the shape of the area for an abstract method. ---- Succession of the abstract category definition triangular, rectangular, round. ---- Prepare a main program will be a triangle, rectangle, circle into an array, various array of graphics output of the area.