http://stackoverflow.com/questions/30831044/collect-grouped-observables-emissions-into-one-list

gistfile1.java

public class Main {
    public static void main(final String[] args) throws Exception {
        Observable.from(Arrays.asList(0, 1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12))
                .groupBy(item -> item % 4)
                .flatMap(Observable::toList)
                .filter(integers -> integers.size() > 2)
                .toList()
                .subscribe(result -> {
                    System.out.printf("%d results %s%n", result.size(), result);
                });
    }
}