About Chapter 7: Generics and Collections

[Zhao zhenhua]

In self test number 16th. I find a problem,it seems that every answer is wrong!!,why?

16.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.
I try out every answer,but it compile wrong! Who can tell me why and explain how this work?

The compiler say:"Type mismatch: cannot convert from List<capture-of ? super Integer> to List<Number>" WHY
What it means "capture-of ?".I used following codes in eclipse3.3.

 
import java.util.*;
 
class Group {
public static void main(String[] args) {
List<Integer> input = null; 
List<Integer> output = null; 
output = process(input);
}
 
public static <E extends Number> List<? super E> process(List<E> nums) {
return null;
}
}   

[ June 03, 2007: Message edited by: Zhao zhenhua ]

[Chandra Bhatt]

See the K&B Errata. Link is available on the top of the SCJP forum.

The declared method is:


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


Thanks,

[Zhao zhenhua]

If the method just is what you say,the answer is right ,Thank you Chandra