不允许从下限方法调用通用的上限方法

Question

请参见以下代码：

public class GenericsMethod {

    // As per  my understanding this will accept any list which extends Number
    public  double sum(List<? extends Number> list){
        double sum = 0;
        for(Number n : list){
            sum += n.doubleValue();
        }
        return sum;
    }

   // As per  my understanding this will accept any list which is extended by Integer, e.g            Number
    public Integer addIntegers(List<? super Integer> list){
       return sum(list); // does not allow me
    }

    public static void main(String[] args){
       //I am sending the same type of list to two different methods.

        List<Number> l = new ArrayList<>();
        l.add(1);
        l.add(2);
        l.add(4);
        l.add(5);
        System.out.println(m.sum(l));

        List<Number> numL = new ArrayList<>();
        numL.add(1);
        numL.add(2);
        numL.add(4);
        numL.add(5);
       System.out.println(m.addIntegers(numL));
    }
}

错误消息如下：

Error:(27, 15) java: method sum in class com.learning.Generics.GenericsMethod cannot be applied to given types;
  required: java.util.List<? extends java.lang.Number>
  found: java.util.List<capture#1 of ? super java.lang.Integer>
  reason: actual argument java.util.List<capture#1 of ? super java.lang.Integer> cannot be converted to java.util.List<? extends java.lang.Number> by method invocation conversion

请更正我的理解。我的问题是我将相同类型的列表发送给两种不同的方法。为什么不允许我通过addIntegers（）调用sum（）方法？