How to use multiple streams and in Java 8 with lambda expressions Map function

I have a list County, which only contains the unique county name, and a list txcrarray, which contains the city name, county name and population of the city

I need to use Java 8 with lambda expression and streams to get the maximum city name of each county from TXC array

This is my code so far:

List<String> largest_city_name = 
    counties.stream() 
            .map(a -> txcArray.stream()
                              .filter(b ->  b.getCounty().equals(a))
                              .mapToInt(c -> c.getPopulation())
                              .max())
            .collect( Collectors.toList());

I'm trying to find a way out Add another after max() Map statement to get the names of cities with the maximum population, but my new lambda expression does not exist in the txcrarray stream. It only recognizes it as int type and texascitiesclass type This is what I want to do

List<String> largest_city_name = 
     counties.stream() 
             .map(a -> txcArray.stream()
                               .filter( b ->  b.getCounty().equals(a))
                               .mapToInt(c->c.getPopulation())
                               .max()
                               .map(d->d.getName()))
             .collect( Collectors.toList());

Can someone tell me what I did wrong?

Solution

You don't need the county list at all Simply stream txcrarray and group by county:

Collection<String> largestCityNames = txcArray.stream()
        .collect(Collectors.groupingBy(
                City::getCounty,Collectors.collectingAndThen(
                        Collectors.maxBy(City::getPopulation),o -> o.get().getName())))
        .values();