generic, self test example

[dolly shah]

This is a K & B book example, from chap:7, page620.
I am not getting i anyways? Can anybody please explain?
I appreciate your help.



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.

[Haibo Jia]

Hi,

The answer K&R book provided also make me confused. I make following test code

import java.util.*;
class x{
  public static void main (String[] arg)
      {
       ArrayList<Integer> input = null;
       List<Integer> output=null; // choose different option to test
       output=process(input);
      }
public static <E extends Number> ArrayList<? super E> process(List<E> nums)
            {
              return null;
           }
 }    

All the right anwser he gives can't be compiled. Anyone can give an explanation? Thanks.

[Manfred Klug]

Hi dolly,

hava a look at the errata
The correct definition for process is:
public static <E extends Number> List<E> process(List<E> nums)

[dolly shah]

What errata is there? Your method signature is same as in the book.

[Chandramouli Ram]

What's in the book is:

public static <E extends Number> List<? super E> process(List<E> nums)

The corrected version is:

public static <E extends Number> List<E> process(List<E> nums)

Only with the corrected version, B, E & F are correct.