Java Stream API - count items of a nested list

Question

Let\'s assume that we have a list of countries: List and each country has a reference to a list of its regions: List

Answer 1

You could even use flatMap() like:

countries.stream().map(Country::getRegions).flatMap(List::stream).count();

where,

map(Country::getRegions) = returns a Stream>
flatMap(List::stream) = returns a Stream