Sorting PriorityQueue

Question

I am having a problem with PriorityQueues, as I am lead to believe it orders on priority however I am not sure what the priority is (I mean what the value is and where it comes from). A priorityQueue can be made with a comparator in the constructor and I have tried this but it does not work.

Queue class:

public JavaPriorityFlightQueue() {
    super();
    flights = new PriorityQueue(5, new SortQueueViaPriority());
}

Comparator:

import java.util.Comparator;

public class SortQueueViaPriority implements Comparator {

    public int compare(Object o1, Object o2){
        Flight f1 = (Flight) o1; 
        Flight f2 = (Flight) o2;

        if( f1 == null || f2 == null ){
            if( f1 == f2 ) return 0;
            else if( f2 == null) return +1;
                else return -1;
        }

    Integer i1 = (Integer) f1.getPriority();
    Integer i2 = (Integer) f2.getPriority();
    return i2.compareTo(i1);
    }
}

Priority is an int value which is part of the flight class. I test this.

JavaPriorityFlightQueue flightQueue = new JavaPriorityFlightQueue();

Flight flight1 = new Flight("0001",9);
Flight flight2 = new Flight("0002",7);
Flight flight3 = new Flight("0003",1);
Flight flight4 = new Flight("0004",2);
Flight flight5 = new Flight("0005",1);

However the PriorityQueue is not sorted, and when I check it the value 9 is never compared to anything and the result is nothing is sorted. the compare class SortQueueViaPriority is copy and pasted from another class where the class sorts perfectly.

Answer 1

I suggest you try the following example. If you use PriorityQueue as a queue, the entries are removed in order.

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

public class Main {
    public static void main(String... args) {
        PriorityQueue<Flight> flights = new PriorityQueue<Flight>(5, new SortQueueViaPriority());
        flights.add(new Flight("0001", 9));
        flights.add(new Flight("0002", 7));
        flights.add(new Flight("0003", 1));
        flights.add(new Flight("0004", 2));
        flights.add(new Flight("0005", 1));

        while (!flights.isEmpty())
            System.out.println(flights.remove());
    }
}

class SortQueueViaPriority implements Comparator<Flight> {
    @Override
    public int compare(Flight f1, Flight f2) {
        return Integer.compare(f2.getPriority(), f1.getPriority());
    }
}

class Flight {
    private final String name;
    private final int priority;

    Flight(String name, int priority) {
        this.name = name;
        this.priority = priority;
    }

    public int getPriority() {
        return priority;
    }

    @Override
    public String toString() {
        return "Flight{" +
                "name='" + name + '\'' +
                ", priority=" + priority +
                '}';
    }
}

prints

Flight{name='0001', priority=9}
Flight{name='0002', priority=7}
Flight{name='0004', priority=2}
Flight{name='0003', priority=1}
Flight{name='0005', priority=1}

Note: PriorityQueue sorts entries such that only the first element will be the smallest. If you iterate over the queue, you will see all the elements, but they may or may not be in order.

Answer 2

Issue is Iterator.As Documented in Java doc of PriorityQueue#iterator

Returns an iterator over the elements in this queue. The iterator does not return the elements in any particular order.

As toString uses iterator it will not get printed in order. Or if you use loop based on iterator then also it will be in order.

And in the Java doc of PriorityQueue

The queue retrieval operations poll, remove, peek, and element access the element at the head of the queue.

To get results in order you will have to use one of these methods.

Answer 3

Instead of Comparator just use Comparable interface.

Your Flight class should implement Comparable interface. Then you need to override the compareTo() method. In that method you can add your own logic for sorting based on the property you need.

Just like this way:

@Override
public int compareTo(Object obj) {
    // TODO Auto-generated method stub
    Flight f = (Flight)obj;
    if(this.a <f.a){
    return 1;
    }else{
        return -1;
    }
}