Inheritance , dynamic binding?

[Zeeman Chen]

I cannot understand what's going on behind the code.
the result is
3
MOVE
if I hadn't written :
v =d; // upcasting , safe
runtime error since d= (Dog)v; downcasting unsafe.

who can tell me what happend in memory after the following two
lines?
thanks in advance!
v= d;
d = (Dog)v;

 
class Vertebrate{
public int a = 1;
 void move1(){
System.out.println("MOVE");
}
 
}
class Mannal extends Vertebrate{
public int a = 2;
 void move2(){
System.out.println("walks");
}
}
class Dog extends Mannal{
 
public int a = 3;
 void move3(){
        System.out.println("walks on paws");
}
}
class TestI{
public static void main(String[] args){
Dog d = new Dog();
 
Vertebrate v = new Vertebrate();
v=d;
d= (Dog)v;
System.out.println(d.a);
d.move1();
 
}
} 

[Layne Lund]

One way to see for yourself how things work here is to rename move1(), move2(), and move3() all to move(). Then add some calls to move() from main() like this:

class TestI{
public static void main(String[] args){
Dog d = new Dog();
 Vertebrate v = new Vertebrate();
[B]v.move();
d.move();[/B]
v=d;
[B]v.move();
d.move();[/B]
d= (Dog)v;
[B]v.move();
d.move();[/B]
System.out.println(d.a);
d.move1();
 }
}