*13.6 (The ComparableCircle class) Define a class named ComparableCircle that extends Circle and implements Comparable. Draw the UML diagram and implement the compareTo method to compare the circles on the basis of area. Write a test class to find the larger of two instances of ComparableCircle objects.

下面是 `ComparableCircle` 类的代码实现和测试类的代码实现。在 `ComparableCircle` 类中，我们实现了 `Comparable` 接口，并重写了 `compareTo` 方法，按照圆的面积进行比较。

public class ComparableCircle extends Circle implements Comparable<ComparableCircle> {

    public ComparableCircle(double radius) {
        super(radius);
    }

    public double getArea() {
        return super.getArea();
    }

    @Override
    public int compareTo(ComparableCircle o) {
        if (this.getArea() > o.getArea()) {
            return 1;
        } else if (this.getArea() < o.getArea()) {
            return -1;
        } else {
            return 0;
        }
    }
}

测试类中，我们创建了两个 `ComparableCircle` 对象，然后比较它们的面积大小，并输出结果。

public class TestComparableCircle {

    public static void main(String[] args) {
        ComparableCircle circle1 = new ComparableCircle(10);
        ComparableCircle circle2 = new ComparableCircle(5);

        if (circle1.compareTo(circle2) > 0) {
            System.out.println("circle1 is larger than circle2");
        } else if (circle1.compareTo(circle2) < 0) {
            System.out.println("circle2 is larger than circle1");
        } else {
            System.out.println("circle1 and circle2 are the same size");
        }
    }
}

下面是 `Circle` 类的代码实现，作为 `ComparableCircle` 类的父类：

public class Circle {

    private double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    public double getRadius() {
        return radius;
    }

    public void setRadius(double radius) {
        this.radius = radius;
    }

    public double getArea() {
        return Math.PI * radius * radius;
    }
}

`Circle` 类中实现了圆的半径和面积的计算。