java - SCJP 面向对象问题

Question

我在理解 K&B 的 SCJP 书中《面向对象》一章中的问题 9 时遇到问题。

问题:

public class Redwood extends Tree { 

public static void main (String [] args) { 
new Redwood ( ) . go ( ) ; 

} 

void go ( ) { 

go2 (new Tree ( ) , new Redwood ( ) ) ; 

go2 ( (Redwood) new Tree ( ) , new Redwood ( ] 

}  

void go2 (Tree tl, Redwood rl) { 

Redwood r2 = (Redwood) tl; 

Tree t2 = (Tree)rl; 
}
}


class Tree { } 

选项:

What is the result? (Choose all that apply.) 

A. An exception is thrown at runtime 

B. The code compiles and runs with no output 

C. Compilation fails with an error at line 8 

D. Compilation fails with an error at line 9 

E. Compilation fails with an error at line 12 

F. Compilation fails with an error at line 13 

书中给出的答案是A，因为Tree不能向下转换为Redwood。我只是无法理解这个概念。

Answer 1

>

class Tree{
   // super class
}

public class Redwood extends Tree{
   //child class
   public static void main (String [] args) { 
   new Redwood ( ) . go ( ) ; // Calling method go()
    } 

    void go ( ) { 

   go2 (new Tree ( ) , new Redwood ( ) ) ; 

   go2 ( (Redwood) new Tree ( ) , new Redwood ( )); // Problem is Here

   /*
   (Redwood)new Tree(),------>by this line Tree IS-A Redwood Which wrong


   According to Question
     Redwood IS-A Tree So relationship is
     (Tree) new Redwood();


   */

  } 

  void go2 (Tree tl, Redwood rl) { 

  Redwood r2 = (Redwood) tl; 

  Tree t2 = (Tree)rl; 
}