Generics Question from K&B Study Guide.

[Tom Scott]

The following is part of a question from the K&B study guide.

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;


Answer A shown above was stated as being incorrect as 'The return type of process is definitely declared as a List, not an ArrayList'. The above confused me so I tried out the following code which seemed to compile fine. So is the answer above correct or (more likely) am I confused.

    
public class Main {
    
    /** Creates a new instance of Main */
    public Main() {
    }
    
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        ArrayList<Integer> input = null; 
        ArrayList<Integer> output = null; 
 
    }
    
    public static <E extends Number> List<? super E> process(List<E> nums) {
       System.out.println("Inside 'process'.");
       return new ArrayList();
    }
    
}

Thanks
Tom

[Tom Scott]

Gah - sorry all spent ages on this, now I see i missed the call to process()

[Tom Scott]

And looking at the compiler message I now see that the return type is a List<> and therefore does not 'fit' in an ArrayList<> reference.

Cheers