201. Filtering a map

Let’s consider the following map:

public class Car {
  private final String brand;
  private final String fuel;
  private final int horsepower;
  ...
}
Map<Integer, Car> cars = Map.of(
  1, new Car("Dacia", "diesel", 100),
  ...
  10, new Car("Lexus", "diesel", 300)
);

In order to stream a map, we can start from the entrySet() of the Map, values(), or keyset(), followed by a stream() call. For instance, if we want to express a pipeline as Map -> Stream -> Filter -> String that returns a List<String> containing all the electric brands, then we can rely on entrySet() as follows:

String electricBrands = cars.entrySet().stream()
  .filter(c -> "electric".equals(c.getValue().getFuel()))
  .map(c -> c.getValue().getBrand())
  .collect(Collectors.joining(", "));

However, as you can see, this stream pipeline doesn’t use the map’s keys. This means...