public static Double getDoubleAvg(List<ContactList> contactList, ToLongFunction fun) {
return contactList.stream().collect(Collectors.averagingDouble(t -> t.getContactHoliday()));
}
public static void main(){
..
Double avg = getDoubleAvg(contactList,new ToLongFunction<ContactList>() {
@Override
public long applyAsLong(ContactList e) {
return e.getMorningContact();
}
});
..
}