// Class to represent a circle
public class Circle implements Shape {
   private double radius;

   public Circle(double radius) {
      this.radius = radius;
   }
   
   // returns the area of this circle
   public double area() {
      return Math.PI * radius * radius;
   }
   
   // compares this circle to other by area
   // pre: other != null
   public int compareTo(Shape other) {
      if (this.area() < other.area()) {
         return -1;
      } else if (this.area() > other.area()) {
         return 1;
      } else {
         return 0;
      }
   }
   
   public String toString() {
      return "circle of area " + String.format("%.3f", area());
   }   
}