Java Priority Queue Sorting

Question

I have a PriorityQueue called Incoming, that contains objects of type Vehicle.

You can call the method getFuelLevel() on all Vehicles.

What I want to do is to sort Incoming, so that the Vehicles with the least fuel are given a higher priority and are put at the front of the Queue.

I'm assuming I have to use a Comparator here but have no idea how to do it.

Answer 1

One thing that I always do when using a PriorityQueue with my own class is to make that class implement Comparable<Class>. With this, rather than needing to implement a Comparator, all you need to implement is the int compareTo(Class o) method in the class which returns "a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object."

In your case, this would return 1 if the Vehicles has less fuel than the Vehicles inputed, 0 if the two have the same, and -1 if the Vehicles has more fuel than the one inputed.

http://docs.oracle.com/javase/7/docs/api/java/lang/Comparable.html

Answer 2

The PriorityQueue class has a constructor that takes a Comparator as an argument. You can construct a PriorityQueue by providing your specific Comparator as

PriorityQueue<Vehicle> queue = new PriorityQueue<Vehicle>(initialCapacity, new Comparator<Vehicle> {
    int compare(Vehicle a, Vehicle b) {
        return a.getFuelLevel() - b.getFuelLevel();
    }
});