Generics

[Venkata Saraswathi]

    
class A{
}
class B extends A{
}

Is below statement will be true?
The type List<? extends B> is assignable to List<? extends A>.

The answer is given it is allowed. I didn't get how it will allow to compile?
I tried in the follwoing way

   
		List <? extends A> al = new ArrayList<A>();
	                List <? extends B> bl = new ArrayList<B>();
		a1 = b1;

 

[Deepak Bala]

Please quote the source of this question ?

? extends B cannot be assigned to ? extends A as you pointed out.

[Venkata Saraswathi]

Venkata Saraswathi wrote:

    
class A{
}
class B extends A{
}

Is below statement will be true?

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

The answer is given it is allowed. I didn't get how it will allow to compile?
I tried in the follwoing way

   
		List <? extends A> al = new ArrayList<A>();
	                List <? extends B> bl = new ArrayList<B>();
		a1 = b1;

 

[Bob Wheeler]

Deepak Bala wrote:
? extends B cannot be assigned to ? extends A as you pointed out.

I think a type of "? extends B" can be assigned to "? extends A".
First of all, what is a type of "? extends B"? Actually it is just B
and B can be assigned to ? extends B. Just check Venkata code example.
But beware, he mixed the b1 with bl. So here the corrected code:

  
		List <? extends A> a1 = new ArrayList<A>();
        List <? extends B> b1 = new ArrayList<B>();
        a1 = b1;
  

cheers
Bob

[Venkata Saraswathi]

Bob Wheeler wrote:"? extends B" can be assigned to "? extends A".

I didn't get your explanation and that wrong you mentioned in given code example.

Can you give a sample program whcih will compile successfully and clarify above statement?

Thanks,
--Venkata.

[Bob Wheeler]

Venkata Saraswathi wrote:

Bob Wheeler wrote:"? extends B" can be assigned to "? extends A".

I didn't get your explanation and that wrong you mentioned in given code example.

Can you give a sample program whcih will compile successfully and clarify above statement?

Thanks,
--Venkata.

In your code you made a typo. You wrote b1 (b followed by "one") and also bl (be followed by "l"). I changed that in my code
and that code compiled successfully.

Edit: small explanation in code:

   
List <? extends A> a1 = new ArrayList<A>();
List <? extends B> b1 = new ArrayList<B>(); // right side is type of <? extends B>
a1 = b1; // we can assign type of <? extends B> to <? extends A>
 

cheers
Bob

[Deepak Bala]

Bob Wheeler wrote:

Deepak Bala wrote:
? extends B cannot be assigned to ? extends A as you pointed out.

I think a type of "? extends B" can be assigned to "? extends A".
First of all, what is a type of "? extends B"? Actually it is just B
and B can be assigned to ? extends B. Just check Venkata code example.
But beware, he mixed the b1 with bl. So here the corrected code:

  
		List <? extends A> a1 = new ArrayList<A>();
        List <? extends B> b1 = new ArrayList<B>();
        a1 = b1;
  

cheers
Bob

I meant it the other way round. Thanks for correcting

[Venkata Saraswathi]

Hi Bob,

Thanks, How do you observe that?

[yen hoang]

I've tried the following code

  
class A
{
}
class B extends A
{
}
class C extends A
{
}
public class GenericDemo 
{
	public static void main(String[] arg)
	{
		List<? extends A> a= new LinkedList[b]<C>[/b]();
		List<? extends B> b= new LinkedList<B>();
		a=b;// compile and run without any warning
	}
}

Please explain for me why? Why can I assign list of B to list of C when both B and C extends A

[Eduardo Bueno]

You can do that because both B and C extends A. The type of 'a' is List<? extends A>, that's why it works. You're saying there "forget the instantiated object (LinkedList<C>()) and point to the other one (new LinkedList<B>()).