ObservableFromExample.java

List<Integer> coolList = new ArrayList<Integer>();
    coolList.add(1);
    coolList.add(2);
    coolList.add(3);

    Observable.from(coolList).map(new Func1<Integer, Long>() {
        @Override
        public Long call(Integer integer) {
            //runs on the IO background thread because we specify `Schedulers.io()`
            return integer * 10000000000L;
        }
    })
            //this schedulers tells all previous observables to run on the IO scheduler
            .subscribeOn(Schedulers.io())
            //this schedulers tells the subscriber to run on the main thread
            .observeOn(AndroidSchedulers.mainThread())
            .subscribe(new Subscriber<Long>() {
                //all these methods run on the main thread because of the `AndroidSchedulers.mainThread()`
                @Override
                public void onCompleted() {
                    System.out.println("This went perfectly! this is only called once, and cannot be called if onError is called");

                }

                @Override
                public void onError(Throwable e) {
                    System.out.println("There was an error! this is only called once, and cannot be called if onCompleted is called");

                }

                @Override
                public void onNext(Long l) {
                    System.out.println("Emitted number: " + l);
                }
            });