Map<String, Set<Category>> categoriesByCar = new HashMap<>();
        for (Car car : cars)
            categoriesByCar.computeIfAbsent(car.name, c -> new HashSet<>()).addAll(car.categories);
        List<Car> groupedCars = new ArrayList<>(categoriesByCar.size());
        for (Map.Entry<String, Set<Category>> entry : categoriesByCar.entrySet())
            groupedCars.add(new Car(entry.getKey(), entry.getValue()));

import org.junit.Test;

import java.util.*;

import static io.elsci.peaksel.chromatogram.peak.PeakApplyTest.Category.ANOTHER_CATEGORY;
import static io.elsci.peaksel.chromatogram.peak.PeakApplyTest.Category.SOME_CATEGORY;

public class PeakApplyTest {
    static class Car {
        String name;
        Set<Category> categories = new HashSet<>();

        Car(String name, Category category) {
            this.name = name;
            this.categories.add(category);
        }
        Car(String name, Set<Category> categories) {
            this.name = name;
            this.categories.addAll(categories);
        }
    }
    enum Category{
        SOME_CATEGORY, ANOTHER_CATEGORY
    }
    @Test public void blah() {
        ArrayList<Car> cars = new ArrayList<Car>();
        cars.add(new Car("mazda", SOME_CATEGORY));
        cars.add(new Car("mazda", ANOTHER_CATEGORY));
        cars.add(new Car("bmv", SOME_CATEGORY));
        cars.add(new Car("gaz", SOME_CATEGORY));

        Map<String, Set<Category>> categoriesByCar = new HashMap<>();
        for (Car car : cars)
            categoriesByCar.computeIfAbsent(car.name, c -> new HashSet<>()).addAll(car.categories);
        List<Car> groupedCars = new ArrayList<>(categoriesByCar.size());
        for (Map.Entry<String, Set<Category>> entry : categoriesByCar.entrySet())
            groupedCars.add(new Car(entry.getKey(), entry.getValue()));
    }

}