Question

I really need to solve a problem i’ve been stuck into for 3 days. Any help would be appreciated.

I have a Car class with brand, model and color instance variables.
I have a Preferences class with name, brand, model and color instance variables.
I have a preferences (Key = name, value = Car) HashMap that contains people’s names as keys and Car they would like to drive as values.
Finally i have a names ArrayList that contains all the names in the system.

I want somehow to group the names of the people according to the car they would like to drive.
For example i want the output to be something like:

Ford Focus, 2006, Blue : Nick,Bill,Jim
Opel Astra, 2008, Black : Joe, George
Mercedes CLK, 2009, Black : Steve, Chris, John

The first thing i thought of was, that i could hold all the duplicates in a list and then get the names of that list, but i figured out i can’t form groups of people according to their preferred car. So, i think the following piece of code doesn’t help at all.

Preferences[] p = new Preferences[preferences.size()];  

for (int i = 0; i < preferences.size(); i++) {
    p[i] = (Preferences) preferences.get(names.get(i));
    Car car = new Car(p[i].getBrand(), p[i].getModel(), p[i].getColor());
    if (!set.add(car)) { //duplicate
        System.out.println("Duplicate!");
        listOfDuplicates.add(car);
    }
}

Is there anything i can do?

Answer 1

Map<Car, List<String>> namesByCar = new HashMap<Car, List<String>>();
for (Map.Entry<String, Car> entry : preferences) {
    String name = entry.getKey();
    Car car = entry.getValue();
    List<String> names = namesByCar.get(car);
    if (names == null) {
        names = new ArrayList<String>();
        namesByCar.put(car, names);
    }
    names.add(name);
}

Here you go…

It would be even easier with Guava’s Multimap.