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Edit this page to describe your Submission.

Which Categories Best Fit Your Submission and Why?

In order of what I think fits my contributions best (best fit first).

This will be a bit murky as the categories are not defined in much detail anywhere. My contributions are to an API and its implementation, so a lot of these categories overlap, imho (API usability, API portability and API quality are quite strongly correlated, if you ask me).

So take your pick below...

  • Best new feature

    I added lots of new features to RxJava, mostly in the form of many new operators, but also by improving on schedulers and generics.

  • Best contribution to code quality

    I cleaned up a lot of warnings in the RxJava project. I helped make it a lot more typesafe. I added co-/contravariance which could be considered an API quality improvement. I wrote a lot of tests against RxJava features. I created and implemented an RxJava scheduler specifically for testing behavior of scheduled actions.

  • Best portability enhancement

    By contributing to making RxJava more typesafe and more generic, and by contributing to the discussions about Scala integration, I helped make RxJava more portable to other (typesafe) programming languages like Java, Scala and Kotlin.

    Also, the whole RxJava project foundation is about porting Rx.NET functionality over to the JVM world.

  • Best usability enhancement

    As RxJava is an API, this is about API usability. I think I contributed a lot to API usability by making the API more typesafe and introducing co- and contravariance everywhere.

Describe your Submission


I stumbled upon RxJava in March when looking for Rx for the JVM (for Scala, to be more specific). I was trying to write a little Android game in Scala and got lost in callback hell quickly - so I was looking for a way out via Rx.

Even at that time, RxJava looked very promising. However, it was not only missing features that I needed, but it also turned out to be implemented in a not very typesafe way (a lot of raw types and objects). This was due to being mostly used from dynamic languages at Netflix, of course, but it hurt a lot when used from Scala.

So I decided to improve all that, and thanks to a very friendly, quick and "reactive" Ben Christiansen, I was on board quickly.


My personal goals while contributing were the following, in order of importance to me:

  • Help make the library typesafe and clean it up a bit
  • Help add features, mostly those that I was missing in order to create a simple Swing demo (inspired by Pong in Elm)
  • Help add schedulers to make concurrency work well
  • Help make RxJava more popular
  • Help improve integration with Scala
  • Help improve Android support


With the exception of the last goal from above, my contributions helped achieve these goals, in my honest opinion. Of course I wasn't doing all this alone. I have to thank Ben and many others for a lot of help here.

My first contribution was starting a discussion about typesafety in the RxJava Google Group. This developed into a large reworking of the RxJava core to make it a lot more typesafe. I haven't contributed much to the reworking itself as other people were way better qualified to do this; however, I got the ball rolling here.

Then, I added a lot of features to RxJava in the form of implementations of Rx operators, helped with the Rx schedulers and added a module for Swing integration.

All in all, I added the following operators to RxJava (up to now):

  • average
  • combineLatest
  • count
  • delay
  • distinct
  • distinctUntilChanged
  • first
  • firstOrDefault
  • interval
  • mapWithIndex
  • sample
  • skipWhile
  • skipWhileWithIndex
  • sum
  • timestamp

In my own view, my main contribution was the large rework of the codebase to generalize the types to become covariant (observables, function return types and a few small other types) or contravariant (observers and function parameter types) where possible. This is an important cornerstone for improving Scala integration, too.

I haven't contributed code to the Scala integration because this was taken up by the people from Typesafe themselves. However, I keep contributing to the respective discussions.

I haven't gotten started yet with Android support, but thankfully, in the meantime, the people from SoundCloud have.

I also recently held a talk about RxJava at Herbstcampus 2013 (in Nuremberg). The (German) slides and sample code for this talk can be found at my Github repo "rxjava-samples".

So, that's all there is to this submission: A few contributions to improve RxJava. However, I'm very proud that Erik Meijer likes them.

Provide Links to Github Repos for your Submission