scjp generic question

[Payam Rastogi]

Hello

I'm completely new to java generics.

can any one help me with this question?




The following program contains two compilation errors, at what lines?

    

import java.util.ArrayList;
import java.util.List;
interface Chewable {}
class Meat implements Chewable {}
public class Tester {
public static List<? extends Chewable> printSize(List<? extends Chewable> list) { // Line 10
System.out.println(list.size());
return list;
}
public static void main(String[] args) {
List<? extends Chewable> list1 = new ArrayList<Meat>(); // Line 16
List<Chewable> list2 = new ArrayList<Chewable>(); // Line 17
List<Meat> list3 = new ArrayList<Meat>(); // Line 18
list1 = printSize(list1); // Line 20
list2 = printSize(list2); // Line 21
list3 = printSize(list3); // Line 22
}
}

thanks

Payam

[Raju Champaklal]

there isan error on line 10...16,17,18 look fine to me...20,21,22 look fine too....

[Raju Champaklal]

well...sorry my answers are wrong...sorry got them wrong....let me check why 21 and 22 are wrong

[Raju Champaklal]

Tester.java:17: incompatible types
found : java.util.List<capture#674 of ? extends Chewable>
required: java.util.List<Chewable>
list2 = printSize(list2); // Line 21
^
Tester.java:18: incompatible types
found : java.util.List<capture#876 of ? extends Chewable>
required: java.util.List<Meat>
list3 = printSize(list3); // Line 22

[Raju Champaklal]

well i think this is because list2 has a generic type of Chewable but we are assigning it a list of some other generic type...same is the case with list3......but if we had list2 and list3 without any geneircs then we would have got only warnings as then these would be non legacy codes...

am not sure...please someone confirm

[Payam Rastogi]

Hi Raju

it seems your answer is correct according to the question source:

http://www.javachamp.com/questions/proper-instantiation-of-java-generic-collection.xhtml

Raju, I am not sure that I got you right,

is the problem here in the return type of method printSize()?

Best.

Payam

[Raju Champaklal]

yup..you are returning a List<? extends Chewable> from the method....so you can assign to a list which has a generic of type <? extends Chewable> or list without any generics.....without any generic would work becaue generic codes are allowed with non generics codes but you get warnings in such cases

[Martin Sergeant]

In generics you must rememeber that if you have a List<class> reference it can only ever refer
to a List<class>, not a List which is a sub-type of class or a list which has the wild card character ?
(although for backward compatibilty as Raju says, it can refer to a an old-fasioned non-generic list,
but you will get a compiler warning).In this case the return of value of the method is a List<? extends Chewable>,
which is being assigned to a List<Chewable> and List<Meat>, which is illegal

Martin.