Doubt in Genrics on super keyword

[Arul Prasad]

in Lesson No 7 and for the question no 16 i have a doubt

question was ...

Given a method declared as:

public static <E extends Number> List<? super E> process(List<E> nums)
A programmer wants to use this method like this:

// INSERT DECLARATIONS HERE
output = process(input);
Which pairs of declarations could be placed at // INSERT DECLARATIONS HERE to allow
the code to compile? (Choose all that apply.)

A. ArrayList<Integer> input = null;
ArrayList<Integer> output = null;

B. ArrayList<Integer> input = null;
List<Integer> output = null;

C. ArrayList<Integer> input = null;
List<Number> output = null;

D. List<Number> input = null;
ArrayList<Integer> output = null;

E. List<Number> input = null;
List<Number> output = null;

F. List<Integer> input = null;
List<Integer> output = null;

G. None of the above.

Answer:
B, E, and F are correct.


but i 'm getting the compile error can any aone help me ...

import java.util.*;
class Base
{
public void sayHello()
{
System.out.println("Base Hello");
}
}
class Sub extends Base
{
public void sayHello()
{
System.out.println("sub Hello");
}
}
public class TestColl
{
public static void main(String a[])
{
List<Base> bList = null;
List<Base> subList = print(bList); //line 23
}
public static <E extends Base> List<? super E> print(List<E> list)
{
return null;
}
}

Error ..
------------
TestColl.java:23: incompatible types
found : java.util.List<capture of ? super Base>
required: java.util.List<Base>
List<Base> subList = print(bList);
^
1 error





It was compiled after the change in line 23 as

List<? super Base> subList = print(bList);

now i need to know about the correct answer and also the use of the super keyword ....

is there anyone help me

[wise owen]

Check this thread.