Pls help with this error in generics

[Ajay Divakaran]

import java.util.*;
class GenLearn
{
public static void main( String args[] )
{
ArrayList<Integer> input = new ArrayList<Integer>();
input.add(5);
List<Integer> output ;//= new ArrayList<Integer>();
output = process( input ); //error!!

}
public static <E extends Number> List<? super E> process( List<E> nums)
{
List<Integer> tp = new ArrayList<Integer>();
tp.add(6);
return tp; // error!!

}

[Burkhard Hassel]

Tricky.

output = process( input ); //error!!

because output is of type List<Integer> and process returns <? super Integer> when invoked with a List<Integer>.
You cannot assign like this;
List<Integer> li_int = null;
List<? super Integer> li_sup = null;
li_int = li_sup; // not allowed

Because li_sup could also refer to a List<Object>.




In the method:

public static <E extends Number> List<? super E> process( List<E> nums)
{...

return type depends on the type the list is invoked with.
You can easily invoke this method with a List<Float>.
So it is not allowed to return a List<Integer> because Integer is not a Float itself nor a supertype thereof.


Note that it doesn't depend on how you invoke the process method in your main method. It has to be consistant on all possible invokations.


As I said in the beginning:
Tricky.


Yours,
Bu.

[Tony Smith]

I don't think I have ever seen syntax like this: <E extends Number> List<? super E>. On both side of List? Is that legal? What does that mean?

[Burkhard Hassel]

legal? yes.

public static <E extends Number> List<? super E> process( List<E> nums) {
...
}

the first <E extends Number> says only that the method is generic and can be used for all types that are either Number itself or subtypes (Integer, Float...).
It does not belong to the return type.

The second <? super E> however belongs to the return type List that is parameterized.
So you won't expect to get lists back that contain String etc., only Lists of the kind how the list is invoked. Or supertypes.


Bu.