SCJP S&B self test question 7.16

[Andrew Shumway]

I've got a question about problem 16 in the self test exam for chapter 7 (Generics and Collections) in the Siera and Bates SCJP programmer for java 5 study guide.

The question reads

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);    

The answer key lists the following as legal:

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

AND

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

Now, I'm not claiming to know everying about the <? super E> type parameters in the question but I'm a bit stumped. It would seem that the return type could be any List with parameter Type of E or a super of E. However the following is in the list of answers that are wrong.

 
List<Integer> input = null;
List<Number> output = null;

So, to clear up the confusion I tried compiling the following:

 
import java.util.*;
 
public class Generics {
 
public static <E extends Number> List<? super E> process(List<E> list){
return null;
}
public static void main(String[] args) {
/*
List<Integer> input = null; // Case 1
List<Integer> output = null;// should work
*/
 
List<Integer> input = null;  // Case 2
List<Number> output = null; // should not work
output = process(input);    // ****Problem here***
}
}
   

The compiler complains about the call output = process(input); with the following error

Type mismatch: cannot convert from List<capture-of ? super Integer> to List<Integer>

This indicates that I'm wrong about the parameter type of the returned list and that it can not be a supertype of E. However... If you change the comments in the code so that you are using Case 1 the compiler gives the same error.

Any ideas

-- andrew

[Jim Yingst]

This has now been anwered here. Sorry about that.