Generics in Collection

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)