RxJava: How to convert List of objects to List of another objects

Question

I have the List of SourceObjects and I need to convert it to the List of ResultObjects.

I can fetch one object to another using method of ResultObject:

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Answer 1

The Observable.from() factory method allows you to convert a collection of objects into an Observable stream. Once you have a stream you can use the map operator to transform each emitted item. Finally, you will have to subscribe to the resulting Observable in order to use the transformed items:

// Assuming List<SourceObject> srcObjects
Observable<ResultsObject> resultsObjectObservable = Observable.from(srcObjects).map(new Func1<SourceObject, ResultsObject>() {
    @Override
    public ResultsObject call(SourceObject srcObj) {
        return new ResultsObject().convertFromSource(srcObj);
    }
});

resultsObjectObservable.subscribe(new Action1<ResultsObject>() { // at this point is where the transformation will start
    @Override
    public void call(ResultsObject resultsObject) { // this method will be called after each item has been transformed
        // use each transformed item
    }
});

The abbreviated version if you use lambdas would look like this:

Observable.from(srcObjects)
  .map(srcObj -> new ResultsObject().convertFromSource(srcObj))
  .subscribe(resultsObject -> ...);

Answer 2

As an extension to Noel great answer. Let's say transformation also depends on some server data that might change during subscription. In that case use flatMap + scan.

As a result when id list changes, transformations will restart anew. And when server data changes related to a specific id, single item will be retransformed as well.

fun getFarmsWithGroves(): Observable<List<FarmWithGroves>> {

        return subscribeForIds() //may change during subscription
                .switchMap { idList: Set<String> ->

                    Observable.fromIterable(idList)
                            .flatMap { id: String -> transformId(id) } //may change during subscription
                            .scan(emptyList<FarmWithGroves>()) { collector: List<FarmWithGroves>, candidate: FarmWithGroves ->
                                updateList(collector, candidate)
                            }
                }
    }

Answer 3

You can use map operator. For example if you have lists of integers and you want to convert to lists of doubles:

    List<Integer> li = new ArrayList<>();
    Observable.just(li).map( l -> {
        List<Double> lr = new ArrayList<Double>();
        for(Integer e:l) {
            lr.add(e.doubleValue());
        }
        return lr;
    });

But all it's a lot more natural if you can control the observable and change it to observe single elements instead of collections. Same code that converts single integer elements into double ones:

Observable.just(1,2,3).map( elem -> elem.doubleValue())