一个计算各种形状的面积和周长的程序。
父类Shape2D为一个抽象类，其中包含计算周长和计算面积两个方法。其他形状圆(Circle)，三角形(Triangle)和长方形(Rectangle)均继承自Shape2D类。而Square继承自Rectangle类。编写测试类来计算这些形状的面积。(A program for calculating the area and perimeter of various shapes.
The parent class Shape2D is an abstract class, which includes two methods for calculating perimeter and calculating area. Other shape circles (Circle), triangles (Triangle) and rectangles (Rectangle) are inherited from the Shape2D class. And Square inherits from the Rectangle class. Write test classes to calculate the area of these shapes.)