Sorting a list of lists in an specific order

Question

This question is about sorting lists.

I have a generic method to get Vehicles

List<Vehicle>  getRandomVehiclesList() {    
   return  fetchVehicles(); 
   //returns List <Car>, List<Bus>, etc
   };

Vehicles can be Car, Motorcycle, Bus, Truck, SUV, etc.

Car extends Vehicle
Bus extends Vehicle

Say I want to call 5 times getRandomVehiclesList() and then sort the resulted 5 lists in an specific order.

—————————————————————————————————

THE LIST NEEDS TO BE IN THIS ORDER

List<Car>
List<Truck>
List<SUV>
List<Bus>
List<Motorcycle>

—————————————————————————————————

but the results can be returned in this order or other random order.

List<Bus>
List<Motorcycle>
List<Truck>
List<Car>
List<SUV>

What is the best way to sort all the lists so they match the order shown above (Car,Truck,Suv,Bus,Motorcycle)

Can RxJava be used to simplify?

Answer 1

You can easily create your own Comparator<List<Vehicle>> taking advantage of Guava's Ordering.explicit comparator:

class VehicleListComparator implements Comparator<List<Vehicle>> {
    private final Ordering<Class<? extends Vehicle>> vehicleClassOrdering;

    public VehicleListComparator(Ordering<Class<? extends Vehicle>> vehicleClassOrdering) {
        this.vehicleClassOrdering = vehicleClassOrdering;
    }

    @Override
    public int compare(List<Vehicle> vehiclesList1, List<Vehicle> vehiclesList2) {
        Class<? extends Vehicle> left = getVehicleClass(vehiclesList1);
        Class<? extends Vehicle> right = getVehicleClass(vehiclesList2);
        return vehicleClassOrdering.compare(left, right);
    }

    private Class<? extends Vehicle> getVehicleClass(List<Vehicle> vehicleList) {
        if (vehicleList.isEmpty()) {
            throw new IllegalArgumentException();
        } else {
            return vehicleList.get(0).getClass();
        }
    }
}

You can then sort your list of vehicle lists using this comparator:

Ordering<Class<? extends Vehicle>> vehicleClassOrdering = Ordering.explicit(Arrays.asList(
        Car.class, Truck.class, SUV.class, Bus.class, Motorcycle.class));
Collections.sort(vehicleLists, new VehicleListComparator(vehicleClassOrdering));