Generics in Collection
generics, java
Solution
It's because what you are doing in the first case isn't type safe. You have declared `list` as a `List` of "some subclass of `Number`", and then tried to insert an `Integer` into it. There is absolutely no guarantee that `Integer` is compatible with the actual run-time type of the underlying list. The compiler stops you here because what you are doing doesn't make any sense.
Consider, as an extreme example:
List< ? extends Object > list = new ArrayList<Integer>();
list.add("Hello, World!");
If this were to work, you would have a `List<Integer>` with a `String` in it!
If you really want it to work, you have to tell the compiler you know what you are doing by casting:
((List<Integer>)list).add(6);
But even then you will still get a warning about type safety.
The second case works because the list is guaranteed to be "some superclass of Number". `Integer` is a subclass of `Number`, so it can be implicitly converted to any superclass (including `Number` itself), so there is no risk that the value is incompatible with the actual type of the list.
For further information, you may want to read up on the difference between covariance and contravariance.
Problem
Here is my program. I am not sure why I am getting a compile time error. ``` import java.util.ArrayList; import java.util.List; public class Test { public static void main(String[] args) { List< ? extends Number > list = new ArrayList<Integer>(); list.add(6); // Compile Time Error System.out.println(list); } } ``` But the following program works fine ``` import java.util.ArrayList; import java.util.List; public class Test { public static void main(String[] args) { List< ? super Number > list = new ArrayList<Number>(); list.add(6); System.out.println(list); } } ``` Error from Eclipse: Here is the error description from Eclipse: The method add(int, capture#1-of ? extends Number) in the type List is not applicable for the arguments (int)