Observable.java

static <T> Observable<T> just(T t) {
    return Observable.create(
            s -> {
                s.onNext(t);
                s.onCompleted();
            }
    );
}

static <T> Observable<T> never() {
    return Observable.create(
            s -> {}
    );
}

static <T> Observable<T> empty() {
    return Observable.create(
            s -> {
                s.onCompleted();
            }
    );
}

static Observable<Integer> range(Integer from, Integer to) {
    return Observable.create(
            s -> {
                IntStream.range(from, to).forEach( i -> s.onNext(i));
                s.onCompleted();
            }
    );
}