*13.7 (The Colorable interface) Design an interface named Colorable with a void method named howToColor(). Every class of a colorable object must implement the Colorable interface. Design a class named Square that extends GeometricObject and implements Colorable. Implement howToColor to display the message Color all four sides. Draw a UML diagram that involves Colorable, Square, and GeometricObject. Write a test program that creates an array of five GeometricObjects. For each object in the array, display its area and invoke its howToColor method if it is colorable.

下面是Colorable接口的代码：

public interface Colorable {
    void howToColor();
}

下面是Square类的代码：

public class Square extends GeometricObject implements Colorable {
    private double side;

    public Square(double side) {
        this.side = side;
    }

    public double getSide() {
        return side;
    }

    @Override
    public double getArea() {
        return side * side;
    }

    @Override
    public void howToColor() {
        System.out.println("Color all four sides.");
    }
}

下面是GeometricObject类的代码：

public abstract class GeometricObject {
    public abstract double getArea();
}

下面是测试程序的代码：

public class TestColorable {
    public static void main(String[] args) {
        GeometricObject[] geometricObjects = new GeometricObject[5];
        geometricObjects[0] = new Square(2);
        geometricObjects[1] = new Square(3);
        geometricObjects[2] = new Circle(2);
        geometricObjects[3] = new Circle(3);
        geometricObjects[4] = new Rectangle(2, 3);

        for (GeometricObject geometricObject : geometricObjects) {
            System.out.println("Area: " + geometricObject.getArea());
            if (geometricObject instanceof Colorable) {
                ((Colorable) geometricObject).howToColor();
            }
        }
    }
}

下面是UML图：

        +--------------+
        |  Colorable   |
        +--------------+
        | +howToColor()|
        +--------------+
                ^
                |
        +--------------+
        |    Square    |
        +--------------+
        | -side: double|
        +--------------+
                ^
                |
        +---------------+
        | GeometricObject|
        +---------------+
        | +getArea()     |
        +---------------+
                ^
                |
        +---------------+
        |     Circle    |
        +---------------+
        | -radius: double|
        +---------------+
                ^
                |
        +---------------+
        |   Rectangle   |
        +---------------+
        | -width: double |
        | -height: double|
        +---------------+