按地图上的键排序

Question

as i posted here

i have a class Car representing name and IDs of cars:

public class Car {
String name;
int ID;
}

and another class representing races in which i need to sort the cars by their order in race:

public class Race {
private Set<Car> cars = new TreeSet<>();
private Map<Integer, Integer> races = new TreeMap<>();//key represents the order in race, value represents the ID of a car, so i need to sort cars by the keys in races
...
public Collection getSortedCars() { ??? }
}

i need to sort cars by key from map races, i have this code:

class CompareCarsByWins implements Comparator<Car> {
    Map<Integer,Integer> wins;

    public CompareCarsByWins(Map<Integer,Integer> wins) {
        this.wins = wins;
    }

    public int compareTo (Car a, Car b) {

        int winsA = wins.get(a.getID());
        int winsB = wins.get(b.getID());
        return winsB - winsA;
    }
    // ..
}

-this is not comparing by key, but its comparing by value because car.getID() is equal to value from the map, and i need to compare it by key, but i dont know how to do it. In the previous thread i didnt find an answer so i hope now someone can help me. example input:

cars.add(new Car("bmw", 1));
cars.add(new Car("porsche", 2));
cars.add(new Car("audi", 3)); 
races.put(1,3); //audi is first
races.put(2,1); //bmw is second
races.put(3,2); //porsche is third

and i want to return of the method:

public Collection<Car> getSortedCars() {}

like this: List of cars containing: audi, bmw, porsche.

Answer 1

Maintain a second map that maps the values to keys, for instance:

public static <V,K> Map<V,K> InvertMap(Map<K,V> map)
{
    Map<V,K> inv = new HashMap<V, K>();
    for(Entry<K,V> entry: map.entrySet())
    {
        inv.put(entry.getValue(), entry.getKey());
    }
    return inv;
}

or

public static <V,K> K FindKeyForValue(Map<K,V> map, V v)
{
    Map<V,K> inv = new HashMap<V, K>();
    for(Entry<K,V> entry: map.entrySet())
    {
        if(entry.getValue().equals(v) || entry.getValue() == v)
            return entry.getKey();            
    }
    return null;
}

Or write a function that searches through the entrySet() for a matching value and returns the associated key.