Combinig two generic List

Combinig two generic List

I want to union two generic list and want to produce new list , I am
providing my code in a simple format.
public class Example{
public static <E> List<E> union(List<? extends E> a,List<? extends E> b){
List<Object> es= new ArrayList<Object>();
for( E e:a){
es.add(e);
}
for( E e:b){
es.add(e);
}
return (List<E>) es;
}
public static void main(String[] args) {
List<Long> a=new ArrayList<Long>();
a.add(1L);
List<Integer> b=new ArrayList<Integer>();
b.add(2);
List<Number> list3=union(a, b);//Compilation Error due to mismatch of
return type
List<String> a1=new ArrayList<String>();
a1.add("H");
List<String> a2=new ArrayList<String>();
a1.add("=E");
List<String> a3=union(a1, a2);//no compilation error
}
}
My requirements is I should able to combine two integer and long list to
produce a number list and I should able to combine other type as well. The
problem is here is about the return type when I am trying combine the
integer and long list. What changes should i need to make to make my code
work.