gistfile1.txt

public class FeedLoader { 
    public Observable<FeedResponse> loadFeedsList() {
        return feedWebService.getFeeds();
    }
}

public class FeedSubscriber {
    feedLoader = new FeedLoader();

    public void loadFeedsList() {
        feedLoader.loadFeedsList()
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(feeds -> {
                    getView().setToolbarPager(adapter);
                }, throwable -> {
                    getView().showConnectionError();
                });
    }
}

public class SubscriberTest {
@Mock
BlaFeedLoader feedLoader;

...
when(feedLoader.loadFeedsList())
                .thenReturn(Observable.<FeedResponse>error(new Throwable("Connection error")));

feedSubscriber.loadFeedList();

}