.flatMap() parts of the flow can become really complicated. Often, they contain nested calls to multiple .map(), Observable.just(), and other functions.

This makes the .flatMap() Function interfaces a prime candidate for simplification by method extraction. At the moment, consider the tweet retrieval part (the version that doesn't use for loop to filter tweets):

observeTwitterStream(configuration, filterQuery)
        .sample(2700, TimeUnit.MILLISECONDS)
        .map(StockUpdate::create)
        .flatMapMaybe(update -> Observable.fromArray(trackingKeywords)
                .filter(keyword -> 
                update.getTwitterStatus().toLowerCase()
                .contains(keyword.toLowerCase()))
                .map(keyword -> update)
                .firstElement()
        )

It has such a flow where the following bit can be extracted to make the bigger tweet retrieval flow simpler:

.flatMapMaybe(update -> Observable.fromArray(trackingKeywords)
                .filter(keyword -> update.getTwitterStatus()
                .toLowerCase().contains(keyword.toLowerCase()))
                .map(keyword -> update)
                .firstElement()
        )

We will do that by introducing a method:

skipTweetsThatDoNotContainKeywords(trackingKeywords)

So, the .flatMapMaybe() call becomes as follows:

.flatMapMaybe(skipTweetsThatDoNotContainKeywords(trackingKeywords))

The body of such a method will be as follows:

@NonNull
private Function<StockUpdate, MaybeSource<? extends StockUpdate>> skipTweetsThatDoNotContainKeywords(String[] trackingKeywords) {
    return update -> Observable.fromArray(trackingKeywords)
            .filter(keyword -> update.getTwitterStatus()
            .toLowerCase().contains(keyword.toLowerCase()))
            .map(keyword -> update)
            .firstElement();
}

Again, this can be done very easily using the IDE refactoring functionality.