Java PriorityQueue Comparator - How/When do you sort?

Question

I'm initialising a Priority Queue like:

strategy = new FuelPriority();
incoming = new PriorityQueue<Vehicle>(1, strategy);

The code for my Comparator class is:

public class FuelPriority implements Comparator<Object> {

public int compare(Object o1, Object o2) {

    Vehicle a1 = (Vehicle) o1;
    Vehicle a2 = (Vheicle) o2;

    return Integer.compare(a1.getFuelLevel(), a2.getFuelLevel());
  }
}

After running a simulation, the elements aren't ordered at all - they are random; I set a breakpoint in the compare method of my FuelPriority class, but it wasn't called at all. Am I missing something here?

Answer 1

Aside from the typo on your code, it works for me.

import java.util.Comparator;
import java.util.PriorityQueue;

public class StackOverflow
{
    public static void main(String[] args)
    {

        FuelPriority strategy = new FuelPriority();
        PriorityQueue<Vehicle> incoming = new PriorityQueue<Vehicle>(4, strategy);
        incoming.add(new Vehicle("car1", 10));
        incoming.add(new Vehicle("car2", 20));
        incoming.add(new Vehicle("car3", 15));
        incoming.add(new Vehicle("car4", 1));

        // to retrieve the elements in order
        while (!incoming.isEmpty()) {
            System.out.println(incoming.poll());
        }

    }

}

class FuelPriority
    implements Comparator<Object>
{

    public int compare(Object o1, Object o2)
    {

        Vehicle a1 = (Vehicle)o1;
        Vehicle a2 = (Vehicle)o2;

        return Integer.compare(a1.getFuelLevel(), a2.getFuelLevel());
    }
}

class Vehicle
{

    private String name;
    private int fuelLevel;

    public Vehicle(String name, int fuelLevel)
    {
        this.name = name;
        this.fuelLevel = fuelLevel;
    }
    public int getFuelLevel()
    {
        return fuelLevel;
    }

    @Override
    public String toString()
    {
        return name + "=" + fuelLevel;
    }
}

Answer 2

API says that PriorityQueue iterator is not guaranteed to traverse the elements of the priority queue in any particular order. It's only guaranteed that poll, remove, peek, and element access the element at the head of the queue (least element)