Interface question

[Muthaiah Ramanathan]

Ranchers,

This is a question in Devaka's mock exam. Why is this allowed at compile time?

 interface I {
}
class A {
}
final class B extends A {
}
//some where else you say
A a = new A();
B b = new B();
I i1 = (I) a;//--> This compiles - but why.
I i2 = (I) b;//--> This fails. I found out that its because, B is final. But I would love some explanations.

Thanks.

[Ankit Garg]

The cast between A and I compiles fine because A is non-final. So there can be a class which inherits from both A and I, so the compiler allows the cast. Since B is final, so there can't be a class which inherits from both B and I, so at runtime the cast between B and I can never succeed. This is why the compiler disallows the cast...

[Prasad Kharkar]

thats really a great answer
May I know how much studying and coding does it take to reach to such a perfection level?
please let me know

[Prithvi Sehgal]

Hello,

You should go through K&B thoroughly and additionally frequently consult JLS and java API docs and with lots
of mock questions practice, you will be able to reach what you desire. Write lots of code though.

Best Regards,

[Seetharaman Venkatasamy]

Prasad Kharkar wrote:May I know how much studying and coding does it take to reach to such a perfection level?

IMO, apart from studying and coding , also you need to browse through the *forum* like Javaranch regularly

[Ankit Garg]

Seetharaman Venkatasamy wrote:IMO, apart from studying and coding , also you need to browse through the *forum* like Javaranch regularly

+1 (from my experience )

[Prasad Kharkar]

well well well
I am going fine then
thanks for the posts
I'll be like you all experts in few months

[Muthaiah Ramanathan]

Oh good.

Thanks,

Muthaiah

[Anup Om]

Ankit Garg wrote:

The cast between A and I compiles fine because A is non-final. So there can be a class which inherits from both A and I, so the compiler allows the cast. Since B is final, so there can't be a class which inherits from both B and I, so at runtime the cast between B and I can never succeed. This is why the compiler disallows the cast...

 I i1 = (I) a;//--> This compiles - but why.     

I really don't understand this. Compiler can makeout that class A doesn't implement interface I. Why is it allowing the cast to compile?

[Andre Brito]

I guess that it's because there can be a class ( C ) that extends A and implements I... Note that I'm not saying that there's.

In compile time it will work, but in runtime it will throw a ClassCastExcpetion.

[Ankit Garg]

The answer would still be the same (Andre said the same thing too). There can be a class which extends A and implements I. So the compiler has to allow the cast. Suppose I have a method like this

void method(A a) {
    I i = (I)a;  //allowed
}

How can the compiler not allow the cast. This cast may actually be successful at runtime. We don't know what is the actual object passed to this method. The object might be of a class C which extends A and implements I. We have not created the class C right now but that doesn't mean we can't in the future. Or see it this way, suppose you are developing a library, then others might extend your classes. Lets take the java API for example. If I create this type of code

Thread td = ...;
Comparable s = (Comparable)td;

The Thread class doesn't implement Comparable. But I might create a Thread subclass which also implements Comparable. And someone else might use my comparable-thread class. So the cast will have to be allowed...