RxJava - fetch every item on the list

As an alternative to flatMapIterable you can do this with flatMap:

Observable.just(Arrays.asList(1, 2, 3)) //we create an Observable that emits a single array
            .flatMap(numberList -> Observable.fromIterable(numberList)) //map the list to an Observable that emits every item as an observable
            .flatMap(number -> downloadFoo(number)) //download smth on every number in the array
            .subscribe(...);


private ObservableSource<? extends Integer> downloadFoo(Integer number) {
   //TODO
}

Personally I think .flatMap(numberList -> Observable.fromIterable(numberList)) is easier to read and understand than .flatMapIterable(numberList -> numberList ).

The difference seems to be the order (RxJava2):

• Observable.fromIterable: Converts an Iterable sequence into an ObservableSource that emits the items in the sequence.
• Observable.flatMapIterable: Returns an Observable that merges each item emitted by the source ObservableSource with the values in an Iterable corresponding to that item that is generated by a selector.

Using method references this looks like:

Observable.just(Arrays.asList(1, 2, 3))
            .flatMap(Observable::fromIterable)
            .flatMap(this::downloadFoo)

Here's a small self contained example

public class Example {

    public static class Item {
        int id;
    }

    public static void main(String[] args) {
        getIds()
                .flatMapIterable(ids -> ids) // Converts your list of ids into an Observable which emits every item in the list
                .flatMap(Example::getItemObservable) // Calls the method which returns a new Observable<Item>
                .subscribe(item -> System.out.println("item: " + item.id));
    }

    // Simple representation of getting your ids.
    // Replace the content of this method with yours
    private static Observable<List<Integer>> getIds() {
        return Observable.just(Arrays.<Integer>asList(1, 2, 3));
    }

    // Replace the content of this method with yours
    private static Observable<Item> getItemObservable(Integer id) {
        Item item = new Item();
        item.id = id;
        return Observable.just(item);
    }
}

Please note that Observable.just(Arrays.<Integer>asList(1, 2, 3)) is a simple representation of Observable<ArrayList<Long>> from your question. You can replace it with your own Observable in your code.

This should give you the basis of what you need.

p/s : Use flatMapIterable method for this case since it belongs to Iterable as explained below:

/**
 * Implementing this interface allows an object to be the target of
 * the "for-each loop" statement. See
 * <strong>
 * <a href="{@docRoot}openjdk-redirect.html?v=8&path=/technotes/guides /language/foreach.html">For-each Loop</a>
 * </strong>
 *
 * @param <T> the type of elements returned by the iterator
 *
 * @since 1.5
 * @jls 14.14.2 The enhanced for statement
  */
 public interface Iterable<T>