定义Point2D，及定义它的属性和方法；

定义子类Point3D，及定义它的属性和方法；在Point3D中的主函数main()中实例化两个Point2D的对象，并通过这两个对象调用它们的属性和方法，输出方法执行结果。

-Defined Point2D and define its properties and methods　defined subclass Point3D, and define its properties and methods　instantiated in the Point3D the main function main () Point2D object, and call them by these two objects properties and methods, the output results of the implementation.