Understanding RxJava method with code not in stream - rx-java2

Question

I have a function that looks like this:
public Flowable<Integer> max(int a, int b){
// *** Part 1 - start ***
int max = Math.max(a,b);
// *** Part 1 - end ***
return Flowable.defer(() -> {
// *** Part 2 start ***
return Flowable.just(max);
// *** Part 2 end ***
});
}
When I now subscribe like this:
EDITED:
max(3,4).subscribeOn(Schedulers.io()).subscribe();
Will the code from Part 1 run on Schedulers.io()?
What are problems that can happen when you write a function like this?
What code runs in which thread?
What is the difference for part 1 if its not in the stream? Or is it in the stream?

Answer 1

When I now subscribe like this:
That code doesn't subscribe, you have to call subscribe().
Will the code from Part 1 run on Schedulers.io()?
The code in max() runs as soon as it is invoked on some thread: it calculates the max and creates a Flowable capturing the bigger value.
What are problems that can happen when you write a function like this?
Part 1 executes on the caller thread which may not be what you wanted. At that point, RxJava isn't even involved.
What code runs in which thread?
max() runs on the caller thread and nothing else gets executed.
What is the difference for part 1 if its not in the stream? Or is it in the stream?
Part 1 is out of the stream. You have to put those computation into the stream via fromCallable for example, although such trivial operations may not be worth putting into a stream.
public Flowable<Integer> max(int a, int b){
return Flowable.fromCallable(() ->
// *** Part 1 - start ***
Math.max(a, b)
// *** Part 1 - end ***
);
}
max(3, 4)
.subscribeOn(Schedulers.io())
.subscribe(v -> {
System.out.println(Thread.currentThread());
System.out.println(v);
});

Answer 2

Will the code from Part 1 run on Schedulers.io()?
Ans: Part one will run on main thread or caller thread. As you didn't put it under the rxjava method.
What code runs in which thread?
Ans: part 2 runs on caller thread. It may be main thread or a new worker thread.
What is the difference for part 1 if its not in the stream? Or is it in the stream?
Ans: It is out of the stream. You may get output from it. But that didn't depend on rxjava.
Use All code inside the rxjava method. Otherwise stream flow may differ. This type of simple calculation can be used on main thread. use rxjava when working with database or rest api call.