Featherweight Java code implementing a linked list

class List extends Object {
    List() { super(); }
    List append(List l) {
      return this; // dummy
    }
}

class Nil extends List {
    Nil() { super (); }
    
    List append(List l) {
      return l;
    }
}

class Elem extends List {
    Object a; List next;
    
    Elem(Object a, List next) {
      super();
      this.a = a;
      this.next = next;
    }
    
    List append(List l) {
      return new Elem(this.a, this.next.append(l));
    }
}