How It Paralleled It

gistfile1.java

            Log.i("BackgroundRealmActivity", "MainThread Id: " + Thread.currentThread().getId());

            PublishSubject<Integer> intSubject = PublishSubject.create();
            Observable<Integer> ints = intSubject.asObservable();

            intSubject.subscribe((Integer i) -> {
               Log.i("BackgroundRealmActivity emitted", Integer.toString(i));
            });

            ints.flatMap((Integer i) -> {
                return Observable.defer(() -> {
                    Log.i("BackgroundRealmActivity", "Integer: " + Integer.toString(i));
                    Log.i("BackgroundRealmActivity", "Thread: " + Thread.currentThread().getId());

                    // make it "look" like  I/O
                    try {
                        Thread.sleep(30);
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                    return Observable.just(i).subscribeOn(AndroidSchedulers.mainThread());
                }).subscribeOn(Schedulers.io());
            }).subscribe((Integer i) -> {
                Log.i("BackgroundRealmActivity", "Back on MainThread Id: " + Thread.currentThread().getId());
                ((TextView) findViewById(R.id.test_result)).setText(TEST_COMPLETE);
            });

            for(int i = 0; i < 9; i++) {
                intSubject.onNext(i);
            }

/*
Output:

I/BackgroundRealmActivity(24029): MainThread Id: 1
I/BackgroundRealmActivity emitted(24029): 0
I/BackgroundRealmActivity emitted(24029): 1
I/BackgroundRealmActivity emitted(24029): 2
I/BackgroundRealmActivity emitted(24029): 3
I/BackgroundRealmActivity emitted(24029): 4
I/BackgroundRealmActivity emitted(24029): 5
I/BackgroundRealmActivity emitted(24029): 6
I/BackgroundRealmActivity emitted(24029): 7
I/BackgroundRealmActivity emitted(24029): 8
I/BackgroundRealmActivity(24029): Integer: 0
I/BackgroundRealmActivity(24029): Thread: 85939
I/BackgroundRealmActivity(24029): Back on MainThread Id: 1
I/BackgroundRealmActivity(24029): Integer: 1
I/BackgroundRealmActivity(24029): Thread: 85940
I/BackgroundRealmActivity(24029): Back on MainThread Id: 1
I/BackgroundRealmActivity(24029): Integer: 3
I/BackgroundRealmActivity(24029): Thread: 85942
I/BackgroundRealmActivity(24029): Back on MainThread Id: 1
I/BackgroundRealmActivity(24029): Integer: 2
I/BackgroundRealmActivity(24029): Thread: 85941
I/BackgroundRealmActivity(24029): Integer: 4
I/BackgroundRealmActivity(24029): Thread: 85943
I/BackgroundRealmActivity(24029): Integer: 5
I/BackgroundRealmActivity(24029): Thread: 85944
I/BackgroundRealmActivity(24029): Integer: 6
I/BackgroundRealmActivity(24029): Thread: 85945
I/BackgroundRealmActivity(24029): Back on MainThread Id: 1
I/BackgroundRealmActivity(24029): Integer: 7
I/BackgroundRealmActivity(24029): Thread: 85946
I/BackgroundRealmActivity(24029): Back on MainThread Id: 1
I/BackgroundRealmActivity(24029): Integer: 8
I/BackgroundRealmActivity(24029): Thread: 85947
I/BackgroundRealmActivity(24029): Back on MainThread Id: 1
I/BackgroundRealmActivity(24029): Back on MainThread Id: 1
I/BackgroundRealmActivity(24029): Back on MainThread Id: 1
I/BackgroundRealmActivity(24029): Back on MainThread Id: 1