K & B Mock exam Generics doubt

[Swati Khanna]

 
import java.util.*;
public class BackLister{
    // Insert Here
     {
      List<T>output = new LinkedList<T>();
      for(T t :input)
         output.add(0,t);
      return output;
     }
 }
Which of the following can be inserted at //insert here to compile and run without error?
1. public static <T> List<T> backwards(List<T>input)
2. public static <T> List<T> backwards(ist<? extends T> input)
3. public static <T>List<? extends T> backwards(List<T> input)
4. public static <T> List<? super T> backwards (List<T>input)
The above four are all correct answers.
I am not able to understand why 3 & 4 are correct.
As we cannot write List<Object>= List<String>
So how can we specify the return type as List<? extends T> or List <? super T> when we are returning List<T> in the method. Please clarify.
   

[Amrit Kashyap]

hi swati,

3. public static <T>List<? extends T> backwards(List<T> input)
4. public static <T> List<? super T> backwards (List<T>input)

I think both the options are correct, since in 3 we are returing a list of T or or one of it's child class. And in the 4, we are returning a list of T or one of it's parent class.

We are not calling that function from anywhere in that fragement of code. I think your confusion is like this "How can we return a reference of parent class where a child class reference is expected" for option 4.

And option 3 is correct anyway since we can return reference of child class where parent class reference is expected.

[Swati Khanna]

My confusion is that in the method backwards we are returning List<T>
and the return type of the method is List<? extends T> or List<? extends super> in option 3 and 4.
So isn't it like are trying to assign a List<Object> to List<String>
or vica versa. I hope I make sense.

[Yogvinder Singh Ghotra]

First of all, i think List<? extends T> is different from List<Object>.

List<Object> can only be assigned List implementations with generic type Object where as List<? extends T> can take any implementation with generic type T or anything that extends T or implements T(in case of T being an interface).

Second, return type is List<? extends T> and you are returning List<T>
List<? extends T> t = ArrayList<T>(); is possible I think.

Similarly the case with <? super T>. Hope this helps!!!

[Xander Steinmann]

I think that you have a very interesting question... And I don't think that the last two implementations are very useful. Look at the classes Animal, Dog and Cat (extend Animals).

Now I input a List of Animals into the 3rd backwards method, and the return type is List<? extends Animal>. Now, what is my return type? A list of Animals? a list of Dogs? a list of Cats and Dogs? you can't tell, but you also can't put the outputted list in a List<Animal> (because it could be a List<Dog> )! You can only put the list in a List<? extends Animal> or an Object.

The same goes for the 4th backward method (is it a List<Object> or List<Animal>?). I think that it is more useful to use 2 specified generics instead of just one, like this:

static <T,S extends T> List<S> backwards(List<T> input) {     
  List<S>output = new LinkedList<S>();
  for(T t :input) 
    if(t instanceof S)
      output.add(0,(S)t); 
  return output; 
}

With this code you can input a List<Animal> and get a List<Dog>, List<Cat> or List<Beagle> in return (it's probably not perfect but illustrates the use of multiple generics).

[ September 19, 2007: Message edited by: Xander Steinmann ]
[ September 19, 2007: Message edited by: Xander Steinmann ]

[Swati Khanna]

Hi Yogvinder,

  
[B]Second, return type is List<? extends T> and you are returning List<T>
List<? extends T> t = ArrayList<T>(); is possible I think.[/B]  

So does 'nt this mean that you are trying to write
List<String> t = ArrayList<Object>();
as String extends Object where T is Object.

I am still not clear.

[Xander Steinmann]

So does 'nt this mean that you are trying to write
List<String> t = ArrayList<Object>();
as String extends Object where T is Object.
I am still not clear.

No, "List<? extends T> t" means that t is a list of Objects that extend T. If you use T=Object then it says:
List<? extends Object> t = new List<Object>();
This doesn't mean that you can fill in just anything for <? extends Object> (like String). List t has as type "? extends Object", not String. But you CAN say: "List<? extends Object> t = new ArrayList<String>();"

[Yogvinder Singh Ghotra]

Hi.. Swati..

Please go through the tutorial
http://java.boot.by/scjp-tiger/ch06s04.html

Here in java ranch, You can rad the thread
https://coderanch.com/t/265228/java-programmer-SCJP/certification/Generic-methods-wildcards

Hope this will help. If not, do tell me.