Could u tell me why in this SCJP question?

843789

What will happen when you attempt to compile and run the following code?

class Trebble{
    int i = 99;
}
class Base extends Trebble{
    int i =100;
}
public class Central extends Base{
    public static void main(String argv[]){
	Central c = new Central();
	c.wynyard();
    }
    public void wynyard(){
	Trebble t = new Central();
	Base b = (Base) t;
	System.out.println(b.i);
    }
	
}

1 Compile time error
2 Compilation and output of 99
3 Compilation and output of 100
4 Compilation, but runtime error because an instance of Trebble cannot be cast to type Base

I chosed 2 but the right answer is 3.
Anyone can tell me why? Thanks!

843789

The value of i in class B is 100.

843789

Thank you for answering my question.
But I still don't know it.
Well, If it is written as follows:
Base b = new Base();
so I can understand that the i value is 100.
But the code is:

Trebble t = new Central();   //Now the i value is 99
Base b = (Base) t;        //Here  b is created, pointing to t. so the 
                         //b.i  is t.i, and t.i is 100

Perhaps I can run this code to see the result, but I really can't understand it. Do a member value of a class will be changed when the class is casted?
Could you tell me more about it? Thanks a lot.

3004

Which class' i to use could be determined by one of two things: The type of the reference, which is known at compile time, or the class of the object the reference points to, which is not known until runtime.

For non-private, non-final, non-static methods, it's the runtime class that determines this. For everything else--every member that's either static, private, final, or not a method (that is, it's a variable), it's the compile-time type of the reference that determines which class' version of that membe to access.

In this case, i is a variable, so it's "bound" at compile time by the type of the reference, which is B.

3004

Trebble t = new Central(); //Now the i value is 99

There are two i variables here--Trebble's and Base's. One of them is 99, and the other is 100.

Base b = (Base) t; //Here b is created,
pointing to t.

Not quite. It doesn't point to t, because t is just a variable. It points to the same object as t points to. And that object has two diferent i varaibles.

so the
//b.i is t.i,

No. b.i is Base's i because b is of type Base, and for variables, it's the compile-time referenc type that determine's whose version we see.

843789

Dear jverd ,
Thank you for your explanation.
I had run the test code as follows, which can proov that your explanation is right.

class Trebble{
    final int fi = 1;
    static int si = 2;

    private void pdo(){
	System.out.println("Trebble private pdo().");
    }
    final   void fdo(){
	System.out.println("Trebble final   fdo().");
    }
    static  void sdo(){
	System.out.println("Trebble static  sdo().");
    }
    void ndo(){
	System.out.println("Trebble none-p,f,s ndo().");
    }
}
class Base extends Trebble{
    final int fi = 10;
    static int si = 20;

    private void pdo(){
	System.out.println("Base private pdo().");
    }
    static  void sdo(){
	System.out.println("Base static  sdo().");
    }
    void ndo(){
	System.out.println("Base none-p,f,s ndo().");
    }
}
public class Central extends Base{
    final int fi = 100;
    static int si = 200;

    private void pdo(){
	System.out.println("Central private pdo().");
    }
    static  void sdo(){
	System.out.println("Central static  sdo().");
    }
    void ndo(){
	System.out.println("Central none-p,f,s ndo().");
    }
    public static void main(String argv[]){
	Trebble t = new Central();
	Base b = (Base) t;
	Central c = new Central();

	System.out.println("t.fi=" + t.fi + " t.si=" + t.si);
	System.out.println("b.fi=" + b.fi + " b.si=" + b.si);
	System.out.println("c.fi=" + c.fi + " c.si=" + c.si);
	t.fdo();  t.sdo();  t.ndo();
	b.fdo();  b.sdo();  b.ndo();
	c.pdo();  c.fdo();  c.sdo();  c.ndo();
    }
}

And the result is :
t.fi=1 t.si=2
b.fi=10 b.si=20
c.fi=100 c.si=200
Trebble final fdo().
Trebble static sdo().
Central none-p,f,s ndo().
Trebble final fdo().
Base static sdo().
Central none-p,f,s ndo().
Central private pdo().
Trebble final fdo().
Central static sdo().
Central none-p,f,s ndo().

I've learned a lot of things from it. Thanks a lot!