Java Filters Finding the most expensive house in each City

user11376058 :

I have a list of houses in a number of cities. I am trying to use a filter in order to generate a list of the most expensive house in each city. I am unable to use conventional loops.

//This returns unique City Names
List unique = obList.stream()
    .map(x -> x.getCity())
    .distinct()
    .sorted()
    .collect(Collectors.toList());

//This returns the house with the highest price in that city
RAddress obReturn = obList.stream()
        .filter(x -> x.getCity().equalsIgnoreCase(sName))
        .sorted((x,y) -> (int)(y.getPrice() - x.getPrice()))
        .findFirst()
        .get();

I understand that combining these in some way is necessary to this problem but I cannot for the life of me figure out how...

Any and all help is appreciated.

Andy Turner :

Use the groupingBy collector to collect all the houses in each city together; then use a downstream maxBy collector to keep just the most expensive house in each city:

obList.stream()
    .collect(
        groupingBy(
            x -> x.getCity(),
            maxBy(comparing(x -> x.getPrice())))

This returns a Map<CityType, Optional<HouseType>> (where CityType and HouseType are whatever the types of cities and houses are, respectively.

If you want a Map<CityType, HouseType> (that is, without the Optional, since you know the value is always present), wrap the downstream collector:

collectingAndThen(maxBy(...), Optional::get)