此范例包含一个抽象类，其中定义了计算各种形状周长、面积以及体积的方法，同时定义其他3个类分别实现三角形、圆形与正方形的相关周长、面积以及体积的计算方法。-This example contains an abstract class, which defines the shape of the calculation of perimeter, area and volume methods, while the other three categories were defined to a