import org.junit.Test; import java.util.HashMap; import java.util.Map; public class FlatmapTest { @Test public void test() { Map<Integer, Map<String, Integer>> map = new HashMap<>(); Map<String, Integer> map1 = new HashMap<>(); map1.put(“a”, 1); map1.put(“b”, 2); map.put(1, map1); map1 = new HashMap<>(); map1.put(“c”, 3); map1.put(“d”, 4); map.put(2, map1); Integer sum = map.values().stream() .map(Map::values) .flatMap(i -> i.stream()) .reduce(0, Integer::sum); System.out.println(sum); […]