Java streams - dealing with multiple for loop and multiple if and else conditions

Question

Is it possible to achieve this requirement using Java streams? For example: I have a list of countries. I want to iterate through for loop and for each country I want to call a method to get all states of that particular country. Inside the inner for loop, I want to prepare a new list which contains states which starts with letter "A". Can you please advise whether it is possible in streams? If yes, how?

I tried using map and filters but it didn't help. Any help would be appreciated. Thanks

Answer 1

Yes, it is possible using flatMap:

@Getter
@Setter
private static class Country {
    private List<State> states;
}

@Getter
@Setter
private static class State {
    private String name;
}

private static List<String> filterStates(List<Country> countries, String start) {
    return countries.stream()
            .map(Country::getStates)
            .flatMap(List::stream)
            .map(State::getName)
            .filter(name -> name.startsWith(start))
            .collect(Collectors.toList());
}