public abstract class Shape implements Comparable<Shape> {
  protected double area;
  
  protected Shape(double area) {
    this.area = area;
  }

  public abstract double area();
  
  public int compareTo(Shape other) {
    double difference = area() - other.area();
    if (difference < 0) {
      return -1;
    } else if (difference > 0) {
      return 1;
    } else {
      return 0;
    }
  }
}