Study Gude Chap 7 Q16

[Jacky Zhang]

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;

my comments: E replaced by Integer and ? replaced by Integer,
which meets the return collection type bound <Integer super Integer>
and the generic declaration <Integer extend Number>.
ArrayList<Integer> should be campatible with List<Integer>
so this is a correct answer.


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

my comments: same reasoning as A, this is a correct answer.


C. ArrayList<Integer> input = null;
List<Number> output = null;

my comments: E replaced by Integer and ? replaced by Number,
which meets the return collection type bound <Number super Integer>
and the generic declaration <Integer extends Number>.
so this is a correct answer.

D. List<Number> input = null;
ArrayList<Integer> output = null;

my comments: E replaced by Number and ? replaced by Integer,
which violates because Integer is not a super of Number.


E. List<Number> input = null;
List<Number> output = null;

my comments: E replaced by Number and ? replaced by Number,
which meets the return collection type bound <Number super Number>
and the generic declaration <Number extends Number>.
so this is a correct answer.


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

my comments: E replaced by Integer and ? replaced by Integer,
which meets the return collection type bound <Integer super Integer>
and the generic declaration <Integer extends Number>.
so this is a correct answer.

So my answer would be: ABCEF.

Is this correct?
[ September 18, 2006: Message edited by: Jacky Zhang ]

[wise owen]

K & B, SCJP 5 - Errata

[Jacky Zhang]

can we just take it as it is now and what would the answer be?
I ll post up an errata applied edition too.

[Jacky Zhang]

can anyone help me with this?
my exam day is getting close ...

[Meena R. Krishnan]

Q-16: method declaration s/b:
public static <E extends Number> List<E> process(List<E> nums)

for the above method declaration:

A. ArrayList<Integer> input = null;
ArrayList<Integer> output = null;

No. since the output is ArrayList while it s/b List.

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

Yes

C. ArrayList<Integer> input = null;
List<Number> output = null;

No, since the output is Number while it s/b something that extends Integer.

D. List<Number> input = null;
ArrayList<Integer> output = null;

No, same reason as A

E. List<Number> input = null;
List<Number> output = null;

Yes, <Number extends Number> for both input and output.

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

Yes
[ September 19, 2006: Message edited by: M Krishnan ]

[Jacky Zhang]

what does s/b stand for?
I really don't understand why option A is wrong.

as normally we can declare:

List<String> list=new ArrayList<String>();

so why when the return type is List<String>,
we can not return a ArrayList<String>?

[Jacky Zhang]

please someone help me...

[Meena R. Krishnan]

s/b --> "should be"

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

Why option A is wrong?

The method returns 'List' and not 'ArrayList'.

//A.
ArrayList<Integer> input = null;
ArrayList<Integer> output = null;
output = process(input)

Try placing the option A above the line 'output = process(input)' (as given above). You're actually assigning 'output' (which is of type ArrayList) with the value returned from the method (which is of type List ).

You can assign an ArrayList to a List and not the other way around. (ArrayList 'is a' List but not the other way ).
[ September 20, 2006: Message edited by: M Krishnan ]