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（只是將13.1的內容加以修改，具體的就是 implements Comparable；
然後我又在Rectangle類中重寫了toString()方法，下面還給了一個關於Rectangle類的測試類。）

public abstract class CompareGeometricObject 
    implements Comparable<CompareGeometricObject>
{
    private String color;
    private boolean filled;
    private
 
 java.util.Date dateCreated;

    protected CompareGeometricObject()
    {
        dateCreated = new java.util.Date();
    }

    protected CompareGeometricObject(String color, boolean filled)
    {
        this.color = color;
        this.filled = filled;
        dateCreated = new java.util.Date();
    }

    public
 
 String getColor()
    {
        return color;
    }

    public void setColor(String color)
    {
        this.color = color;
    }

    public boolean isFilled()
    {
        return filled;
    }

    public void setFilled(boolean filled)
    {
        this.filled = filled;
    }

    public java.util.Date getDateCreated
 
()
    {
        return dateCreated;
    }

    @Override
    public String toString()
    {
        return "Created on " + dateCreated + "\nColor : " + color + "\n and Filled : " + filled;
    }

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

    public static CompareGeometricObject max(CompareGeometricObject o1, CompareGeometricObject o2)
    {
        if (o1.compareTo(o2) > 0)
        {
            return o1;
        }
        else
            return o2;
    }

    public abstract double getArea();

    public abstract double getPerimeter();
}

public class CompareMyRectangle extends CompareGeometricObject
{
    private double width;
    private double height;

    public CompareMyRectangle()
    {
    }

    public CompareMyRectangle(double width, double height)
    {
        this(width, height, "white", false);
    }

    public CompareMyRectangle(double width, double height, String color, boolean filled)
    {
        super(color, filled);
        this.width = width;
        this.height = height;
    }

    public double getWidth()
    {
        return width;
    }

    public void setWidth(double width)
    {
        this.width = width;
    }

    public double getHeight()
    {
        return height;
    }

    public void setHeight(double height)
    {
        this.height = height;
    }

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

    @Override
    public double getPerimeter()
    {
        return 2 * (width + height);
    }

    @Override
    public String toString()
    {
        return super.toString() + "\nMy Rectangle Width : " + getWidth() + " Height : " + getHeight();
    }
}

import java.util.Scanner;

public class TestCompareRectangle
{
    public static void main(String [] args)
    {
        Scanner input = new Scanner(System.in);
        System.out.print("Create the first Rectangle (enter width, height): ");
        double firstWidth = input.nextDouble();
        double firstHeight = input.nextDouble();

        System.out.print("Crate the second Rectangle (enter width, height): ");
        double secondWidth = input.nextDouble();
        double secondHeight = input.nextDouble();

        CompareMyRectangle rectangle1 = new CompareMyRectangle(firstWidth, firstHeight);
        CompareMyRectangle rectangle2 = new CompareMyRectangle(secondWidth, secondHeight);

        System.out.println(CompareGeometricObject.max(rectangle1, rectangle2).toString());
        // 可以看出此處的 max 返回的是 CompareMyRectangle 的一個例項
    }
}