public class Rect {
    double width;
    double height;
    String color;

    public Rect( ) {
        width=0;
        height=0;
        color="transparent";      
    }

    public Rect( double w,double h) {
        width=w;
        height=h;
        color="transparent";
    }

    double area()
    {
        return  width*height;
    } 
}

2

public class PRect extends Rect{
    double depth;

    public PRect(double w,double h,double d) {
        width=w;
        height=h;
        depth=d;
    }

    double area()
    {
        return  width*height*depth;
    }     
}

3

public class CRect extends Rect{ 
    String color;

    public CRect(double w,String c) {
        width=w;
        height=h;
        color=c;
    }

    double area()
    {
        return  width*height;
    }     
}

4

public class test {

    public test() { }

    public static void main(String[] args) {  
        Rect r1=new Rect(2,3);
        System.out.println("area of r1="+r1.area());

        PRect pr1=new PRect(2,3,4);
        System.out.println("area of pr1="+pr1.area());


        CRect cr1=new CRect(2,"RED");
        System.out.println("area of cr1="+cr1.area()+"  color = "+cr1.color);


        System.out.println("\n POLY_MORPHISM ");
        Rect r2=new Rect(1,2);
        System.out.println("area of r2="+r2.area());

        Rect pr2=new PRect(1,2,4);
        System.out.println("area of pr2="+pr2.area());


        Rect cr2=new CRect(1,"Blue");
        System.out.println("area of cr2="+cr2.area()+"  color = "+cr2.color); 

    }
}

我得到了输出：

area of r1=6.0
area of pr1=24.0
area of cr1=6.0  color = RED
POLY_MORPHISM 
area of r2=2.0
area of pr2=8.0
area of cr2=2.0  color = transparent***

为什么将cr2视为Rect(超类)并将“透明”颜色视为具有“蓝色”颜色的CRect(子类)？