rxJava observeOn and subscribeOn explanation source: https://github.com/ReactiveX/RxJava/issues/2925#issuecomment-98649659

example1.java

Observable.just(1) // 1 will be emited in the IO thread pool
                  .subscribeOn(Schedulers.io())
                  .flatMap(...) // will be in the IO thread pool
                  .observeOn(Schedulers.computation())
                  .flatMap(...) // will be executed in the computation thread pool
                  .observeOn(AndroidSchedulers.mainThread())
                  .subscribe(); // will be executed in the Android main thread (if you're running your code on Android)

example2.java

// "main" represents thread where entire chain is executed. But (!) we are not switching back to main thread
//neverthelesss all "doOnSubscribe" are executed in main thread
// That happens because doOnSubscribe is executed immediately in the thread where entire chain is exected.
// doOnTerminated, doFinaly, doOnNext are deferred, that's why they are affected by observeOn
Observable.just(1) // 1 will be emited in the IO thread pool
                .doOnSubscribe(...) // io thread
                .doOnNext(...) // io thread
                .doOnTerminate(...) // io thread
                .doFinally(...) // io thread
                .subscribeOn(Schedulers.io()) 
                .flatMap(...) // io thread
                .doOnSubscribe(...) // !!! main thread
                .doOnNext(...) // io thread
                .doOnTerminate(...) //  io thread
                .doFinally(...) //io thread
                .observeOn(Schedulers.computation())
                .flatMap(...) // computation thread
                .doOnSubscribe(...) // !!! main
                .doOnNext(...) // computation thread
                .doOnTerminate(...) // computation thread
                .doFinally(...) //computation thread
                .observeOn(Schedulers.newThread())
                .doOnSubscribe(...) // !!! main
                .doOnNext(...) // new thread
                .doOnTerminate(...) //new thread
                .doFinally(...) //new thread
                .subscribe(...); // new thread

links.txt

https://github.com/ReactiveX/RxJava/issues/2925#issuecomment-98649659
https://github.com/ReactiveX/RxJava/issues/5459
http://akarnokd.blogspot.de/2016/03/subscribeon-and-observeon.html