Java Collectors grouping-by dynamic fields

Question

I have the following nested groupingBy block:

Map<String,Map<String,Long>> namesCountersMap =
            events.stream().collect(
                Collectors.groupingBy(
                    namesDAO::getName,
                    Collectors.groupingBy(
                        genericDAO::SOME_DYNAMIC_FIELD,
                        Collectors.counting())
                )
            );

There is a situation where I need to call this block 3 times, and the only thing I change is the inner groupingBy field ("SOME_DYNAMIC_FIELD").

Basically what i'm trying to do, is to use group-by and counting for a different field (at the second level) every time, and then merge the results.

Example:

{
      "NamesRecords": {
        "Sam": {
            "Cars": 4
            "Bags": 6 
            "Houses": 2 
        },

        "Bob": {
            "Cars": 2
            "Bags": 1 
            "Houses": 3 
        },

      }
}

As you can see, group-by at the 2nd level is done by 3 different fields, that's is why I need to duplicate this block of code three times.

If I pass a parameter of fieldToGroupBy to the function, is there a way I can use it dynamically? Or if you have any other ideas of how to avoid code duplication, I would love to hear.

Andreas · Accepted Answer · 2019-04-12 16:21:48Z

The first argument to Collectors.groupingBy is a Function<T,R>, so you can use a variable of that type.

It is simply a functional interface with a method for extracting the value to group by, and it can be implemented by a method reference, a lambda expression, an anonymous class, or any other type of class.

Function<namesDAO, String> classifier2 = genericDAO::SOME_DYNAMIC_FIELD;

Map<String,Map<String,Long>> namesCountersMap =
        events.stream().collect(
            Collectors.groupingBy(
                namesDAO::getName,
                Collectors.groupingBy(
                    classifier2,
                    Collectors.counting())
            )
        );

Now you can assign different values to classifier2, e.g.

Function<namesDAO, String> classifier2 = someDAO::getCars;

Function<namesDAO, String> classifier2 = otherDAO::getBags;

Function<namesDAO, String> classifier2 = dao -> dao.getHouses();

Pradeep Nooney · Accepted Answer · 2022-05-28 14:39:22Z

public class CarDetailsService {

    private final CarRepository carRepository;

    private final Map<String, Function<CarDTO, String>> carColumnMapper = new HashMap<>();

    public ApplicationDetailsServiceImpl(CarRepository carRepository) {
        this.carRepository = carRepository;

       //---- Initialise all the mappings ------- //

        carColumnMapper.put("BRAND", CarDTO::getBrandName);
        carColumnMapper.put("MILEAGE", CarDTO::getMileage);
    }

    public Map<String, List<CarDTO>> getListOfCars(String groupBy) {

        return carRepository.findAll()
                .stream()
                .map(toCarDTO)
                .collect(groupingBy(carColumnMapper.get(groupBy.toUpperCase())));
    }

    Function<CarDetails, CarDTO> toCarDTO = (carDetails) -> CarDTO
            .builder()
            .brand(carDetails.getBrand())
            .engineCapacity(carDetails.getEngineCapacity())
            .mileage(carDetails.getMileage())
            .fuel(carDetails.getFuel())
            .price(carDetails.getPrice())
            .build();
}

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