使用多种几何图形，如圆、矩形和三角形等，其中要包括计算面积的方法Area。请声明一个抽象类Shape，在Shape类中统一声明一个抽象方法Area-use a variety of geometric shapes, such as circular, triangular and rectangular, which would include the area of the calculation method Area. Please statement an abstract class Shape, the Shape class unified statement an abstract method Area