casting rules-ques from jq+

[preeti dengri]

hi all,
please look to the following code
class A implements Runnable{}
class B extends A implements Observer{}
and the dec
A a=new A();
B b=new B();
which of the foll will compile and execute without throwing exceptions
1)Object o=a;Runnable r=o;
2)Object o=a;Runnable r=(Runnable) o;
3)Object o=a;Observer ob=(Observer)o;
4)Object o=b;Observer o2=o;
5)Object o=b;Runnable r=(Runnable)o;
ans are 2 and 5.why not 3 as well??
please explain because i think ,in case of 1) and 2)when a implements Runnable then what is the need of explicit casting.does assingning objects to another Object ref makes a diff???

[S Dave]

1)Object o=a;Runnable r=o;
o is a normal Object which must explicitly cast into a Runnable.
2)Object o=a;Runnable r=(Runnable) o;
Appropriately cast, hence compiles & executes.
3)Object o=a;Observer ob=(Observer)o;
Compiles due to appropriate casting, but since A does not implement Observer, fails to run.
4)Object o=b;Observer o2=o;
Must be cast to Observer.
5)Object o=b;Runnable r=(Runnable)o;
B does not implement Runnable

Please correct if any of the above reasonings is wrong.

[Kumar Mahesh]

1)Object o=a;Runnable r=o;
o is a normal Object which must explicitly cast into a Runnable.
2)Object o=a;Runnable r=(Runnable) o;
Appropriately cast, hence compiles & executes.
3)Object o=a;Observer ob=(Observer)o;
Compiles due to appropriate casting, but since A does not implement Observer, fails to run.
4)Object o=b;Observer o2=o;
Must be cast to Observer.
5)Object o=b;Runnable r=(Runnable)o;
B does not implement Runnable

I think 5 should be ok since B is extending A which implements Runnable .Correct me if i am wrong.

[Kumar Mahesh]

1)Object o=a;Runnable r=o;
o is a normal Object which must explicitly cast into a Runnable.
2)Object o=a;Runnable r=(Runnable) o;
Appropriately cast, hence compiles & executes.
3)Object o=a;Observer ob=(Observer)o;
Compiles due to appropriate casting, but since A does not implement Observer, fails to run.
4)Object o=b;Observer o2=o;
Must be cast to Observer.
5)Object o=b;Runnable r=(Runnable)o;
B does not implement Runnable

I think 5 should be ok since B is extending A which implements Runnable .Correct me if i am wrong.

[S Dave]

that's right. option 5 compiles and executes as B extends A which implements Runnable.

[nitin sharma]

hi dengri,
third option is wrong because it is the class A which implicitly extends extends class Object and implements runnable.
Now the point is class A implements Runnable not Object class.so once u assign the reference of class A to class Object then u have to explicitly typecast the Object class varaible to the Runnable type.Now it is the typecasting in the opposite direction.now see the figure below.
A-------extends Object---------implements Runnable.
this is probably what is going on in your mind.The actual thing is like this.
1) A------extends Object.
2) A--------implement Runnable.
Now see statement (1)once u assign an object of class A to object , it is assigned becuase of up the heirarchy order.
Now see statement (2)It is class A which implement's Runnable not the Class Object.so till now your Object class varaible has the reference of Class A,u have to come down from object to Runnable, therefore u need an explixit typecasting.
Remember it is the class A which implmenet's Runnable not the Object.

Now please tell me one thing.In the line given below whether the class A implicitly extends class Object or not.

class B extends A implements Observer{}

[preeti dengri]

hi nitin,
u cud guess right!! i was confused coz of assigning to an Object ref ,now it is clear.as far as ur ques about
In the line given below whether the class A implicitly extends class Object or not.
class B extends A implements Observer{}
it does implicitly extends Object.Correct???
but now see this one which does not involve a third ref to Object type
class Base {}
class Sub extends Base {}
class Sub2 extends Base {}
public class CEx{
public static void main(String argv[]){
Base b=new Base();
Sub s=(Sub) b;
}
}
why this code is giving runtime classCast Exception???
Acc to me a ref of superclass type can be explicitly casted to ref of subclass type.it is this what is happening.
please explain.
thank you