Question from KS&BB about Generic Methods?

[Shivey Upadhyay]

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.

Correct Answer is:
B
E
F
<E extends Number> is defined as type of Method! and the return type is <? super E> so, the output should be of type Number or Object, hoe it is Integer. And Similarly as argument of Type is defined as <E> it should be of Type Number.
So, i was thinking the answer is E. I am really Confused with this Generic typed Class and Methods.
KS&BB describes Thread as toughest Chapter, but i suppose its the toughest Chapter on Exam!
Thanks,
Shivey

[Anil Pradhan]

Hello Shivey,

There is an error in that Question. The correct method declaration is
public static <E extends Number> List<E> process(List<E> nums)

Please look into K&B Errata if you come across any unusual things.
Hope this helps.

[Dean Jones]

Can someone explain why this is an error. I don't understand. It seems to be fine what is given in K&B book.