Generics Again

[Costa lamona]

Hi, I thought that I have finished with Generics but, It is going again !!

public static <T> List<T> backwords (List<? super T> input)
 {
  List<T> output = new LinkedList<T> ();
  for (T t: input)
   output.add(0,t);
  return output;
 }
}

This code does not compile with error message like this

incompatible types
found: Object
required: T
for (T t: input)

Why this error ?, specially that
<? super T> means any T or super type of it.
and if you replaced <? super T> with <? extends T>, It compiles.

[Dan Polak]

Of course it is not possible

What will happen when I run your methot like this

List<Integer> i = backwords(new ArrayList<Object>());

???

[Burkhard Hassel]

Hi ranchers,

the parameter input is of type <? super T>
but in the for loop it is used as type <T>.

You cannot cast from <? super T> to <T>.

Here a variation of yours included, method foo:

static <T> List<T> backwords(List<? super T> input) {
List<T> output = new LinkedList<T>();
 for (T t: input) // no compile
 output.add(0,t);
return output;
}
static  List<Sub> shnoof(List<Super> input) {
List<Sub> out = new LinkedList<Sub>();
for(Super sup: input)
out.add(sup); // no compile
return out;
}
...
class Super {}
class Sub extends Super {}

Yours,
Bu.