public static<R,U> List<R> myMap(Stream<U> s, Function<U,R> fnctn) {
    return s.parallel().reduce(new ArrayList<R>(),
                               (List<R> list, U elem) -> list.add(fnctn.apply(elem)),
                               (List<R> list1, List<R> list2)-> list1.addAll(list2));
}
List<String> words = Arrays.asList("word1", "word2", "word3");
System.out.println(myMap(words.stream(), s->Integer.valueOf(s.substring(s.length()-1))));