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 may map each country to number of regions and then reduce result using sum:

countries.stream()
  .map(c -> c.getRegions() == null ? 0 : c.getRegions().size())
  .reduce(0, Integer::sum);