About Question enthuware.ocajp.i.v7.2.1250 :

[arjun gaur]

Answer is wrong..look at this code.there should be an Is-a relationship,and since the access is "default" so both classes should be in same package.
class A
{
int a;
}
class B extends A
{
public static void main(String[] args)
{
B b=new B();
A a=new A();

System.out.println(b.a);
}
}

[admin]

I have no idea what you mean. Can you explain why you think the given answer is wrong?

[Tony.Singarayar]

Code: Select all

class B {}
class B1 extends B {}
class B2 extends B {}
public class ExtendsTest{
  public static void main(String args[]){
     B b = new B();
     B1 b1 = new B1();
     B2 b2 = new B2();
     // insert statement here
  }
}

i think b1 = (B1) b; has the logic as b2 = (B2) b1


Why is b1 = (B1)b; correct over b2 = (B2)b1? can you please explain?

Thanks,
Tony Singarayar

[admin]

No, it doesn't have the same logic. It is possible for b to point to an object of clasd b1 but it is not possible for b1 to point to an object of class b2 at all.

Think of it this way - an animal reference can point to a Dog or a Cat, but a dog reference can never point to a Cat because a Dog is an Animal but a Cat can never be a Dog.

[padraig]

Hi, can you post a link to where this casting is explained?
Thanks

[admin]

This is quite fundamental. Any good Java book will do. Or just google.

[liviu_varlam]

why are there 2 casts required for the last line?

[admin]

Two casts are not needed. Actually, no cast is needed here. The explanation is only trying to illustrate that after casting b1 to B, you have to cast it again to B1 to make it work.
The explanation has now been updated to make it clear.
thank you for your feedback!
Paul.

[dmcler]

first answer is right, but wrong answer is highlighted green

[admin]

The question is asking which lines will fail. That is why option 3 is highlighted in green. It is the right option.