Convert.java

import com.twitter.finagle.Service;
import com.twitter.util.Future;
import com.twitter.util.Promise;

import rx.Observable;
import rx.Observer;
import rx.Subscription;

public class Convert {
    static public <Res> Future<Res> fromObservable(final Observable<Res> observable) {
        final Observable<Res> single = observable.single();


        final Promise<Res> result = new Promise<Res>() {
            Promise<Res> self = this;
            Res singletonValue;
            final Subscription subscription = single.subscribe(new Observer<Res>() {

                @Override
                public void onCompleted() {
                    assert singletonValue != null : "At least one on next should have occured; the singletonValue should be set";
                    self.setValue(singletonValue);
                }

                @Override
                public void onError(Throwable e) {
                    self.setException(e);
                }

                @Override
                public void onNext(Res res) {
                    singletonValue = res;
                }
            });

            @Override
            public void cancel() {
                subscription.unsubscribe();
            }
        };
        return result;
    }
}