MapOnce.java

class MapOnce<T, R> {
    private final PublishSubject<R> subject = PublishSubject.create();

    private MapOnce(Observable<T> source, Function<T, R> func) {
        source.subscribe((T data) -> {
            R result = func.apply(data);
            subject.onNext(result);
        });
    }

    static public <T,R> Observable<R> apply(Observable<T> source, Function<T, R> func) {
        return (new MapOnce(source, func)).subject.asObservable();
    }
}