K & B Generics Quiz question.

[taichi chen]

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.)

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.

According to the book, B, E, and F are correct.

C is wrong because the return type evaluates to List<Integer>, and that can't be assigned to a variable of type List<Number>.

I can understand Why A and D are wrong but I am confused on why C is incorrect. can anyone please explain why C is wrong? thanks!

[Vishal Chawla]

question is incorrect... refer K &B errata

[taichi chen]

Thanks, Vishal... I got it.

[taichi chen]

The question should really be:
public static <E extends Number> List<E> process(List<E> nums)