这道题不会写，不太明白

package application.test;

public class Circle {

    private double x;
    private double y;
    private double r;

    public double getX() {
        return x;
    }

    public void setX(double x) {
        this.x = x;
    }

    public double getY() {
        return y;
    }

    public void setY(double y) {
        this.y = y;
    }

    public double getR() {
        return r;
    }

    public void setR(double r) {
        this.r = r;
    }

    public Circle() {
        super();
        this.x = 0.0;
        this.y = 0.0;
        this.r = 1.0;
    }

    public Circle(double x, double y, double r) {
        super();
        this.x = x;
        this.y = y;
        this.r = r;
    }

    @Override
    public String toString() {
        return "Circle [x=" + x + ", y=" + y + ", r=" + r + "]";
    }

    public double getArea() {
        return 3.14 * this.r * this.r;
    }

    public double getCircumference() {
        return 3.14 * this.r * 2;
    }

    public static void main(String[] args) {
        Circle c1 = new Circle();
        System.out.println(c1.getArea());
        System.out.println(c1.getCircumference());

        Circle c2 = new Circle(1, 2, 3.1);
        System.out.println(c2.getArea());
        System.out.println(c2.getCircumference());
    }

}