如何使用组合的方法编写一个计算圆环面积的Java程序？

Question

使用组合的方法编写一个计算圆环面积的Java程序。要求：先设计一个圆形类Circle，再基于类Circle使用组合的方法定义一个圆环类Ring。圆环可认为是由一大一小两个同心圆组合而成。最后编写一个测试类测试所编写的圆环类Ring。

Answer 1

public class Circle {
private double radius;
public Circle(double radius){
this.radius = radius;
}
public double getArea(){
return Math.PI * radius * radius;
}
}

public class Ring {
private Circle large;
private Circle small;
public Ring(Circle large,Circle small) throws Exception {
if(large.getArea()<small.getArea()) throw new Exception("The area of a large circle is smaller than that of a small circle");
this.large = large;
this.small = small;
}
public double getArea(){
return large.getArea() - small.getArea();
}
/**
* 测试方法 声明一个大圆一个小圆，将其组合成一个圆环，求圆环面积
* @param args
* @throws Exception
*/
public static void main(String[] args) throws Exception {
Circle l = new Circle(10);
Circle s = new Circle(5);
Ring r = new Ring(l,s);
System.out.println(r.getArea());
}
}