204. Implementing distinctBy() for the Stream API

Let’s suppose that we have the following model and data:

public class Car {
  private final String brand;
  private final String fuel;
  private final int horsepower;
  ...
}
List<Car> cars = List.of(
  new Car("Chevrolet", "diesel", 350),
  ...
  new Car("Lexus", "diesel", 300)
);

We know that the Stream API contains the distinct() intermediate operation, which is capable of keeping only the distinct elements based on the equals() method:

cars.stream()
    .distinct()
    .forEach(System.out::println);

While this code prints the distinct cars, we may want a distinctBy() intermediate operation that is capable of keeping only the distinct elements based on a given property/key. For instance, we may need all the cars distinct by brand. For this, we can rely on the toMap() collector and the identity function, as follows:

cars.stream()
    .collect(Collectors.toMap...