casting

[gaurav gupta sitm]

class uvs implements Runnable { public void run(){  }}
public class MainClass {  
     public static void main(String args[]){
          Runnable r1=new Runnable(){
               public void run(){
                    
     }};
          
Thread t1=(Thread)r1;     //Exception   
        
uvs u = new uvs();
Thread t2=(Thread)u;      //error

   }

}

why one gives error & other exception
i think both have same situation like

 
 __________________________________
/   Runnable             Runnable  \
|      |                     |     |
|      |                     |     |
|      |                     |     |
|   Thread t1 <---------- uvs u    |
\__________________________________/ 

 __________________________________
/   Runnable             Runnable  \
|      |                     |     |
|      |                     |     |
|      |                     |     |
|   Thread t1 <--------(unknown)r1 |
\__________________________________/ 

[Devaka Cooray]

gaurav gupta sitm wrote:

 __________________________________
/   Runnable             Runnable  \
|      |                     |     |
|      |                     |     |
|      |                     |     |
|   Thread t1 <-------(unknown)r1 |
\__________________________________/ 

The reference type of 'r1' is NOT unknown - it is Runnable.

The casting attempt for t1 does not give you a compile time error as you are casting within a single hierarchy root - you are casting from Runnable to Thread, and Thread is-a Runnable.

The other casting attempt for t2 issues a compile time error as Thread and uvs are not in the same hierarchy root - you are trying to cast from uvs to Thread, but neither uvs is-a Thread nor Thread is-a uvs.

[gaurav gupta sitm]

Devaka Cooray wrote:
The reference type of 'r1' is NOT unknown - it is Runnable.
The casting attempt for t1 does not give you a compile time error as you are casting within a single hierarchy root - you are casting from Runnable to Thread, and Thread is-a Runnable.

sir if i am following your concept then there will be exception ...
but K&B book says that there is Plain-Old Anonymous Inner Classes, Flavor Two.
Anonymous class would be an implementer of the interface Runnable
it's not instantiating a Runnable object, it's creating an instance of a new, anonymous, implementer of Runnable.
Declare a reference variable r1 of type Runnable that, obviously, will refer to an object from a class that implements the Runnable interface.

so there is a Anonymous class that implement Runnable
like : class uvs implements Runnable
so what is diffrence in root hierarchy

sir please clear my concept ....
Thanks

[Devaka Cooray]

Similar to what explained in the book, r1 does NOT instantiate the Runnable interface (it's not possible too). However, there is a reference type for r1 - that is Runnable. The reference type is what checked at the compile time. In case of the second cast, the variable 'u' has 'uvs' as its reference type. You are getting the error due to the fact that uvs and Thread are not in the same hierarchy root.

[gaurav gupta sitm]

Thanks sir my concept is clear now 100% emoticon('');
and also thanks for valuable Exam Lab simulator.