SCJP 6 question

[samir vasani]

Dear all,

I am not getting the answer of the following question .

classA {}
class B extends A {}
class C extends A {}
class D extends B {}
Which three statements are true? (Choose three.)
A. The type List<A> is assignable to List.
B. The type List<B> is assignable to List<A>.
C. The type List<Object> is assignable to List<?>.
D. The type List<D> is assignable to List<? extends B>.
E. The type List<? extends A> is assignable to List<A>.
F. The type List<Object> is assignable to any List reference.
G. The type List<? extends B> is assignable to List<? extends A>.
Answer: CDG

Please explain me how this answer comes.
Thank in advance.

[bhanu chowdary]

C is obvious as you can assign anything to a List<?> generic type

samir vasani wrote:

The type List<D> is assignable to List<? extends B>

Anything which extends B can be assignable to List<? extends B>. Since class D is extending class B, List <D> can be assigned to List<? extends B>

 G. The type List<? extends B> is assignable to List<? extends A>  

Here you can assign any list that extends B to List<? extends A> as class B is extending class A.

If you feel the answer should be something else, please put your points here as to why you think, so that we can discuss.

[samir vasani]

bhanu chowdary wrote:C is obvious as you can assign anything to a List<?> generic type

samir vasani wrote:

The type List<D> is assignable to List<? extends B>

Anything which extends B can be assignable to List<? extends B>. Since class D is extending class B, List <D> can be assigned to List<? extends B>

 G. The type List<? extends B> is assignable to List<? extends A>  

Here you can assign any list that extends B to List<? extends A> as class B is extending class A.

If you feel the answer should be something else, please put your points here as to why you think, so that we can discuss.

Can you programmaticaly show this statement "The type List<? extends B> is assignable to List<? extends A>."

[bhanu chowdary]

samir vasani wrote:
Can you programmaticaly show this statement "The type List<? extends B> is assignable to List<? extends A>."

class A {}  
class B extends A {}  
class C extends A {}  
class D extends B {} 

public class GenClass {
	public static void main(String[] args) {
		List<A> list2 = new ArrayList<A>();
		List<? extends A> list = new ArrayList<B>();
		List<? extends B> list1 = new ArrayList<D>();
		list2 = list; //compiler error
		list = list1; //compiles fine
	}

}

[Riza Aktunc]

Shouldn't choice A be correct, too?

List<A> list=new ArrayList<A>();
List listA=list;

This compiles!

[bhanu chowdary]

Riza Aktunc wrote:

List<A> list=new ArrayList<A>();
List listA=list;

The above will generate a warning I suppose.

Welcome to the Ranch

[Riza Aktunc]

Thanks
I had assumed that just a warning would not mean "The type List<A> is not assignable to List. ". Am I wrong with this assumption?

[bhanu chowdary]

Riza Aktunc wrote:I had assumed that just a warning would not mean "The type List<A> is not assignable to List. ". Am I wrong with this assumption?

No not at all. But if I were asked to choose 3 options I would not choose option A.

[Tommy Delson]

Riza Aktunc wrote:Shouldn't choice A be correct, too?

List<A> list=new ArrayList<A>();
List listA=list;

This compiles!

Let's try this...

//Create a list
List list1 = new ArrayList<A>(); //Anything can assign to "list" since List is not Generic, but
List list2 = new ArrayList<B>(); //When you use it or get something out of the List you'll get
List list3 = new ArrayList<D>(); //a compiler error unless a cast to specific type is needed

//Add to the list, no problem of adding generic types to the list.
list.add(list1); 
list.add(list2);
list.add(list3);

//Instantiate B object
B b = new B();

//Retrieve an object...
b = list.get(1); //Compile error....a cast needed here
b = (B) list.get(1); //No problem since we have the cast to B type

If you still have doubt or not clear reread K & B book on Chapter 7, page 638 on "Generics and Legacy Code" section for
more insight.

This sort of generic stuffs will likely on the exam...

[Riza Aktunc]

Yes, I got this perfectly. I was aware that if the referance type is not generic, type casting is required during the access of the list element.
However, I was trying to state that this type casting need is not enough to say "The type List<A> is not assignable to List. ".
In fact, you stated that "Anything can assign to "list" since List is not Generic", too. Am I missing a point here?

Thanks anyway for the mind refreshing information about generics

[Riza Aktunc]

After all, just to clarify:

Can't I say " The type List<A> is assignable to List."?

[Tommy Delson]

Riza Aktunc wrote:After all, just to clarify:

Can't I say " The type List<A> is assignable to List."?

The question is so obvious "Which three statements are true?" that meant which is true based on the given scenario in this case type List<A> is assignable to List, but not true because of List is not generic and needed a cast to retrieve an object.

In programming the type List<A> is assignable to List, but it doesn't work 100% unless you cast it to specific type. The question asking which is true meaning that 100% working so, the given answer "The type List<A> is assignable to List." is not applied here.

You need to pay very close attention to this type of question otherwise, you won't get answer correctly.

Hope that clear your confusion...