实现一个Circle类。这个类的每个对象将表示一个圆，存储它的半径和它的中心的坐标x和y。包含一个默认的构造函数，访问函数，一个area( )函数和一个circumference( )函数。-Implement a Circle class. Each object of this class will represent a circle, its radius and its storage center coordinates x and y. Contains a default constructor, access function, and an area () function and a circumference () function.