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 achieve the triangle, circle and square of the relevant perimeter, area and volume calculation.