group

Observable<String> values = Observable.just(
        "first",
        "second",
        "third",
        "forth",
        "fifth",
        "sixth"
);

values.groupBy(word -> word.charAt(0))
    .subscribe(
        group -> group.last()
            .subscribe(v -> System.out.println(group.getKey() + ": " + v))
    );