RxJava vs Java 8 Parallelism Stream

Question

What are all the similarities and diferences between them, It looks like Java Parallel Stream has some of the element available in RXJava, is that right?

Answer 1

There is also a difference in threading.

Stream#parallel splits the sequence into parts, and each part is processed in the separate thread.

Observable#subscribeOn and Observable#observeOn are both 'move' execution to another thread, but don't split the sequence.

In other words, for any particular processing stage:

• parallel Stream may process different elements on different threads
• Observable will use one thread for the stage

E. g. we have Observable/Stream of many elements and two processing stages:

Observable.create(...)
    .observeOn(Schedulers.io())
    .map(x -> stage1(x))
    .observeOn(Schedulers.io())
    .map(y -> stage2(y))
    .forEach(...);

Stream.generate(...)
    .parallel()
    .map(x -> stage1(x))
    .map(y -> stage2(y))
    .forEach(...);

Observable will use no more than 2 additional threads (one per stage), so no two x'es or y's are accessed by different threads. Stream, on the countrary, may span each stage across several threads.